Posted by Brandon Azad, Project Zero
I recently found myself wishing for a single online reference providing a brief summary of the high-level exploit flow of every public iOS kernel exploit in recent years; since no such document existed, I decided to create it here.
This post summarizes original iOS kernel exploits from local app context targeting iOS 10 through iOS 13, focusing on the high-level exploit flow from the initial primitive granted by the vulnerability to kernel read/write. At the end of this post, we will briefly look at iOS kernel exploit mitigations (in both hardware and software) and how they map onto the techniques used in the exploits.
This isn't your typical P0 blog post: There is no gripping zero-day exploitation, or novel exploitation research, or thrilling malware reverse engineering. The content has been written as a reference since I needed the information and figured that others might find it useful too. You have been forewarned.
A note on terminology
Unfortunately, there is no authoritative dictionary called "Technical Hacking Terms for Security Researchers", which makes it difficult to precisely describe some of the high-level concepts I want to convey. To that end, I have decided to ascribe the following terms specific meanings for the context of this post. If any of these definitions are at odds with your understanding of these terms, feel free to suggest improved terminology and I can update this post. :)
Exploit primitive: A capability granted during an exploit that is reasonably generic.
A few examples of common exploit primitives include: n-byte linear heap overflow, integer increment at a controlled address, write-what-where, arbitrary memory read/write, PC control, arbitrary function calling, etc.
A common exploit primitive specific to iOS kernel exploitation is having a send right to a fake Mach port (struct ipc_port) whose fields can be directly read and written from userspace.
Exploit strategy: The low-level, vulnerability-specific method used to turn the vulnerability into a useful exploit primitive.
For example, this is the exploit strategy used in Ian Beer's async_wake exploit for iOS 11.1.2:
An information leak is used to discover the address of arbitrary Mach ports. A page of ports is allocated and a specific port from that page is selected based on its address. The IOSurfaceRootUserClient bug is triggered to deallocate the Mach port, yielding a receive right to a dangling Mach port at a known (and partially controlled) address.
The last part is the generic/vulnerability-independent primitive that I interpret to be the end of the vulnerability-specific exploit strategy.
Typically, the aim of the exploit strategy is to produce an exploit primitive which is highly reliable.
Exploit technique: A reusable and reasonably generic strategy for turning one exploit primitive into another (usually more useful) exploit primitive.
One example of an exploit technique is Return-Oriented Programming (ROP), which turns arbitrary PC control into (nearly) arbitrary code execution by reusing executable code gadgets.
An exploit technique specific to iOS kernel exploitation is using a fake Mach port to read 4 bytes of kernel memory by calling pid_for_task() (turning a send right to a fake Mach port into an arbitrary kernel memory read primitive).
Exploit flow: The high-level, vulnerability-agnostic chain of exploit techniques used to turn the exploit primitive granted by the vulnerability into the final end goal (in this post, kernel read/write from local app context).
Public iOS kernel exploits from app context since iOS 10
This section will give a brief overview of iOS kernel exploits from local context targeting iOS 10 through iOS 13. I'll describe the high-level exploit flow and list the exploit primitives and techniques used to achieve it. While I have tried to track down every original (i.e., developed before exploit code was published) public exploit available either as source code or as a sufficiently complete writeup/presentation, I expect that I may have missed a few. Feel free to reach out and suggest any that I have missed and I can update this post.
For each exploit, I have outlined the vulnerability, the exploit strategy (specific to the vulnerability), and the subsequent exploit flow (generic). The boundary between which parts of the exploit are specific to the vulnerability and which parts are generic enough to be considered part of the overall flow is subjective. In each case I've highlighted the particular exploitation primitive granted by the vulnerability that I consider sufficiently generic.
mach_portal - iOS 10.1.1
By Ian Beer of Google Project Zero (@i41nbeer).
The vulnerability: CVE-2016-7644 is a race condition in XNU's set_dp_control_port() which leads to a Mach port being over-released.
Exploit strategy: Many Mach ports are allocated and references to them are dropped by racing set_dp_control_port() (it is possible to determine when the race has been won deterministically). The ports are freed by dropping a stashed reference, leaving the process holding receive rights to dangling Mach ports filling a page of memory.
Subsequent exploit flow: A zone garbage collection is forced by calling mach_zone_force_gc() and the page of dangling ports is reallocated with an out-of-line (OOL) ports array containing pointers to the host port. mach_port_get_context() is called on one of the dangling ports to disclose the address of the host port. Using this value, it is possible to guess the page on which the kernel task port lives. The context value of each of the dangling ports is set to the address of each potential ipc_port on the page containing the kernel task port, and the OOL ports are received back in userspace to give a send right to the kernel task port.
In-the-wild iOS Exploit Chain 1 - iOS 10.1.1
Discovered in-the-wild by Clément Lecigne (@_clem1) of Google's Threat Analysis Group. Analyzed by Ian Beer and Samuel Groß (@5aelo) of Google Project Zero.
The vulnerability: The vulnerability is a linear heap out-of-bounds write of IOAccelResource pointers in the IOKit function AGXAllocationList2::initWithSharedResourceList().
Exploit strategy: The buffer to be overflowed is placed directly before a recv_msg_elem struct, such that the out-of-bounds write will overwrite the uio pointer with an IOAccelResource pointer. The IOAccelResource pointer is freed and reallocated with a fake uio struct living at the start of an OSData data buffer managed by IOSurface properties. The uio is freed, leaving a dangling OSData data buffer accessible via IOSurface properties.
Subsequent exploit flow: The dangling OSData data buffer slot is reallocated with an IOSurfaceRootUserClient instance, and the data contents are read via IOSurface properties to give the KASLR slide, the address of the current task, and the address of the dangling data buffer/IOSurfaceRootUserClient. Then, the data buffer is freed and reallocated with a modified version of the IOSurfaceRootUserClient, such that calling an external method on the modified user client will return the address of the kernel task read from the kernel's __DATA segment. The data buffer is freed and reallocated again such that calling an external method will execute the OSSerializer::serialize() gadget, leading to an arbitrary read-then-write that stores the address of the kernel task port in the current task's list of special ports. Reading the special port from userspace gives a send right to the kernel task port.
extra_recipe - iOS 10.2
By Ian Beer.
The vulnerability: CVE-2017-2370 is a linear heap buffer overflow reachable from unprivileged contexts in XNU's mach_voucher_extract_attr_recipe_trap() due to an attacker-controlled userspace pointer used as the length in a call to copyin().
Exploit strategy: The vulnerable Mach trap is called to create a kalloc allocation and immediately overflow out of it with controlled data, corrupting the ikm_size field of a subsequent ipc_kmsg object. This causes the ipc_kmsg, which is the preallocated message for a Mach port, to believe that it has a larger capacity than it does, overlapping it with the first 240 bytes of the subsequent allocation. By registering the Mach port as the exception port for a userspace thread and then crashing the thread with controlled register state, it is possible to repeatedly and reliably overwrite the overlapping part of the subsequent allocation, and by receiving the exception message it is possible to read those bytes. This gives a controlled 240-byte out-of-bounds read/write primitive off the end of the corrupted ipc_kmsg.
Subsequent exploit flow: A second ipc_kmsg is placed after the corrupted one and read in order to determine the address of the allocations. Next an AGXCommandQueue user client is reallocated in the same slot and the virtual method table is read to determine the KASLR slide. Then the virtual method table is overwritten such that a virtual method call on the AGXCommandQueue invokes the OSSerializer::serialize() gadget, producing a 2-argument arbitrary kernel function call primitive. Calling the function uuid_copy() gives an arbitrary kernel read/write primitive.
Yalu102 - iOS 10.2
The vulnerability: CVE-2017-2370 (same as above).
Exploit strategy: The vulnerable Mach trap is called to create a kalloc allocation and immediately overflow out of it with controlled data, overwriting the contents of an OOL port array and inserting a pointer to a fake Mach port in userspace. Receiving the message containing the OOL ports yields a send right to the fake Mach port whose contents can be controlled directly.
Subsequent exploit flow: The fake Mach port is converted into a clock port and clock_sleep_trap() is used to brute force a kernel image pointer. Then the port is converted into a fake task port to read memory via pid_for_task(). Kernel memory is scanned backwards from the leaked kernel image pointer until the kernel text base is located, breaking KASLR. Finally, a fake kernel task port is constructed.
Notes: The exploit does not work with PAN enabled.
ziVA - iOS 10.3.1
By Adam Donenfeld (@doadam) of Zimperium.
The vulnerability: Multiple vulnerabilities in AppleAVE2 due to external methods sharing IOSurface pointers with userspace and trusting IOSurface pointers read from userspace.
Exploit strategy: An IOSurface object is created and an AppleAVE2 external method is called to leak its address. The vtable of an IOFence pointer in the IOSurface is leaked using another external method call, breaking KASLR. The IOSurface object is freed and reallocated with controlled data using an IOSurface property spray. Supplying the leaked pointer to an AppleAVE2 external method that trusts IOSurface pointers supplied from userspace allows hijacking a virtual method call on the fake IOSurface; this is treated as a oneshot hijacked virtual method call with a controlled target object at a known address.
Subsequent exploit flow: The hijacked virtual method call is used with the OSSerializer::serialize() gadget to call copyin() and overwrite 2 sysctl_oid structs. The sysctls are overwritten such that reading the first sysctl calls copyin() to update the function pointer and arguments for the second sysctl and reading the second sysctl uses the OSSerializer::serialize() gadget to call the kernel function with 3 arguments. This 3-argument arbitrary kernel function call primitive is used to read and write arbitrary memory by calling copyin()/copyout().
Notes: iOS 10.3 introduced the initial form of task_conversion_eval(), a weak mitigation that blocks userspace from accessing a right to the real kernel task port. Any exploit after iOS 10.3 needs to build a fake kernel task port instead.
async_wake - iOS 11.1.2
By Ian Beer.
The vulnerability: CVE-2017-13861 is a vulnerability in IOSurfaceRootUserClient::s_set_surface_notify() that causes an extra reference to be dropped on a Mach port. CVE-2017-13865 is a vulnerability in XNU's proc_list_uptrs() that leaks kernel pointers by failing to fully initialize heap memory before copying out the contents to userspace.
Exploit strategy: The information leak is used to discover the address of arbitrary Mach ports. A page of ports is allocated and a specific port from that page is selected based on its address. The port is deallocated using the IOSurfaceRootUserClient bug, yielding a receive right to a dangling Mach port at a known (and partially controlled) address.
Subsequent exploit flow: The other ports on that page are freed and a zone garbage collection is forced so that the page is reallocated with the contents of an ipc_kmsg, giving a fake Mach port with controlled contents at a known address. The reallocation converted the port into a fake task port through which arbitrary kernel memory can be read using pid_for_task(). (The address to read is updated without reallocating the fake port by using mach_port_set_context().) Relevant kernel objects are located using the kernel read primitive and the fake port is reallocated again with a fake kernel task port.
Notes: iOS 11 removed the mach_zone_force_gc() function which allowed userspace to prompt the kernel to perform a zone garbage collection, reclaiming all-free virtual pages in the zone map for use by other zones. Exploits for iOS 11 and later needed to develop a technique to force a zone garbage collection. At least three independent techniques have been developed to do so, demonstrated in async_wake, v0rtex, and In-the-wild iOS exploit chain 3.
In-the-wild iOS Exploit Chain 2 - iOS 10.3.3
Discovered in-the-wild by Clément Lecigne. Analyzed by Ian Beer and Samuel Groß.
The vulnerability: CVE-2017-13861 (same as above).
Exploit strategy: Two Mach ports, port A and port B, are allocated as part of a spray. The vulnerability is triggered to drop a reference on port A, and the ports surrounding A are freed, leading to a dangling port pointer. Zone garbage collection is forced by calling mach_zone_force_gc() and the page containing port A is reallocated with an OOL ports spray containing a pattern such that port A's ip_context field overlaps a pointer to port B. Calling mach_port_get_context() gives the address of port B. The vulnerability is triggered again with port B, leading to a receive right to a dangling Mach port at a known address.
Subsequent exploit flow: After another zone garbage collection, the dangling port B is reallocated with a segmented OOL memory spray such that calling mach_port_get_context() can identify which 4 MB segment of the spray reallocated port B. That segment is freed and port B is reallocated with pipe buffers, giving a controlled fake Mach port at a known address. The fake port is converted into a clock port and clock_sleep_trap() is used to brute force KASLR. The fake port is next converted into a fake task port and a 4-byte kernel read primitive is established using pid_for_task(). Finally, the fake port is converted into a fake kernel task port.
v0rtex - iOS 10.3.3
The vulnerability: CVE-2017-13861 (same as above).
Exploit strategy: Mach ports are sprayed and a reference on one port is dropped using the vulnerability. The other ports on the page are freed, leaving a receive right to a dangling Mach port.
Subsequent exploit flow: A zone garbage collection is forced using mach_zone_force_gc() and the page containing the dangling port is reallocated with an OSString buffer via an IOSurface property spray. The OSString buffer contains a pattern that initializes critical fields of the port and allows the index of the OSString containing the port to be determined by calling mach_port_get_context() on the fake port. The OSString containing the fake port is freed and reallocated as a normal Mach port. mach_port_request_notification() is called to put the address of a real Mach port in the fake port's ip_pdrequest field, and the OSString's contents are read via IOSurface to get the address. mach_port_request_notification() is used again to get the address of the fake port itself.
The string buffer is freed and reallocated such that mach_port_get_attributes() can be used as a 4-byte arbitrary read primitive, with the target address to read updateable via mach_port_set_context(). (This is analogous to the pid_for_task() technique, but with slightly different constraints.) Starting at the address of the real Mach port, kernel memory is read to find relevant kernel objects. The string buffer is freed and reallocated again with a fake task port sufficient to remap the string buffer into the process's address space. The fake port is updated via the mapping to yield a 7-argument arbitrary kernel function call primitive using iokit_user_client_trap(), and kernel functions are called to generate a fake kernel task port.
Incomplete exploit for CVE-2018-4150 bpf-filter-poc - iOS 11.2.6
Vulnerability analysis and POC by Chris Wade (@cmwdotme) at Corellium. Exploit by littlelailo (@littlelailo).
The vulnerability: CVE-2018-4150 is a race condition in XNU's BPF subsystem which leads to a linear heap buffer overflow due to a buffer length being increased without reallocating the corresponding buffer.
Exploit strategy: The race is triggered to incorrectly increase the length of the buffer without reallocating the buffer itself. A packet is sent and stored in the buffer, overflowing into a subsequent OOL ports array and inserting a pointer to a fake Mach port in userspace. Receiving the message containing the OOL ports yields a send right to the fake Mach port whose contents can be controlled directly.
Subsequent exploit flow: The fake Mach port is converted into a clock port and clock_sleep_trap() is used to brute force a kernel image pointer. Then the port is converted into a fake task port to read memory via pid_for_task(). Kernel memory is scanned backwards from the leaked kernel image pointer until the kernel text base is located, breaking KASLR. The final part of the exploit is incomplete, but construction of a fake kernel task port at this stage would be straightforward and deterministic using existing code.
Notes: The exploit does not work with PAN enabled.
multi_path - iOS 11.3.1
By Ian Beer.
The vulnerability: CVE-2018-4241 is an intra-object linear heap buffer overflow in XNU's mptcp_usr_connectx() due to incorrect bounds checking.
Exploit strategy: The kernel heap is groomed to place a 2048-byte ipc_kmsg struct at a 16 MB aligned address below the mptses structs (the object containing the overflow) associated with a few multipath TCP sockets. The vulnerability is used to overwrite the lower 3 bytes of the mpte_itfinfo pointer in the mptses struct with zeros and the socket is closed. This triggers a kfree() of the corrupted pointer, freeing the ipc_kmsg struct at the 16 MB alignment boundary. The freed ipc_kmsg slot is reallocated with sprayed pipe buffers. The vulnerability is triggered again to overwrite the lower 3 bytes of the mpte_itfinfo pointer in another mptses struct with zeros and the socket is closed, causing another kfree() of the same address. This frees the pipe buffer that was just allocated into that slot, leaving a dangling pipe buffer.
Subsequent exploit flow: The slot is reallocated again with a preallocated ipc_kmsg. A userspace thread is crashed to cause a message to be stored in the preallocated ipc_kmsg buffer overlapping the pipe buffer; reading the pipe in userspace yields the contents of the ipc_kmsg struct, giving the address of the dangling pipe buffer/ipc_kmsg. The pipe is written to change the contents of the ipc_kmsg struct such that receiving the message yields a send right to a fake Mach port inside the pipe buffer. The exception message is received and the pipe is rewritten to convert the fake port into a kernel read primitive using pid_for_task(). Relevant kernel objects are located and the fake port is converted into a fake kernel task port.
multipath_kfree - iOS 11.3.1
The vulnerability: CVE-2018-4241 (same as above).
Exploit strategy: The kernel heap is groomed to place preallocated 4096-byte ipc_kmsg structs near the mptses structs for a few multipath TCP sockets. The vulnerability is triggered twice to corrupt the lower 2 bytes of the mpte_itfinfo pointer in two mptses structs, such that closing the sockets results in kfree()s of the two corrupted pointers. Each pointer is corrupted to point 0x7a0 bytes into an ipc_kmsg allocation, creating 4096-byte holes spanning 2 messages. A Mach port containing one of the partially-freed ipc_kmsg structs (with the ipc_kmsg header intact but the message contents freed) is located by using mach_port_peek() to detect a corrupted msgh_id field. Once the port is found, the hole is reallocated by spraying preallocated ipc_kmsg structs and a message is placed in each. Filling the hole overlaps the original (partially freed) ipc_kmsg's Mach message contents with the ipc_kmsg header of the replacement, such that receiving the message on the original port reads the contents of the replacement ipc_kmsg header. The header contains a pointer to itself, disclosing the address of the replacement ipc_kmsg allocation. The vulnerability is triggered a third time to free the replacement message, leaving a partially freed preallocated ipc_kmsg at a known address.
Subsequent exploit flow: The hole in the corrupted ipc_kmsg is reallocated by spraying AGXCommandQueue user clients. A message is received on the Mach port in userspace, copying out the contents of the AGXCommandQueue object, from which the vtable is used to determine the KASLR slide. Then the corrupted ipc_kmsg is freed and reallocated by spraying more preallocated ipc_kmsg structs with a slightly different internal layout allowing more control over the contents. A message is placed in each of the just-sprayed ipc_kmsg structs to modify the overlapping AGXCommandQueue and hijack a virtual method call; the hijacked virtual method uses the OSSerializer::serialize() gadget to call copyout(), which is used to identify which of the sprayed AGXCommandQueue user clients overlaps the slot from the corrupted ipc_kmsg. The contents of each of the just-sprayed preallocated ipc_kmsg structs is updated in turn to identify which port corresponds to the corrupted ipc_kmsg. The preallocated port and user client port are used together to build a 3-argument arbitrary kernel function call primitive by updating the contents of the AGXCommandQueue object through an exception message sent to the preallocated port.
empty_list - iOS 11.3.1
By Ian Beer.
The vulnerability: CVE-2018-4243 is a partially controlled 8-byte heap out-of-bounds write in XNU's getvolattrlist() due to incorrect bounds checking.
Exploit strategy: Due to significant triggering constraints, the vulnerability is treated as an 8-byte heap out-of-bounds write of zeros off the end of a kalloc.16 allocation. The kernel heap is groomed into a pattern of alternating blocks for the zones of kalloc.16 and ipc.ports, and further grooming reverses the kalloc.16 freelist. The vulnerability is repeatedly triggered after freeing various kalloc.16 allocations until a kalloc.16 allocation at the end of a block is overflowed, corrupting the first 8 bytes of the first ipc_port on the subsequent page. The corrupted port is freed by calling mach_port_set_attributes(), leaving the process holding a receive right to a dangling Mach port.
Subsequent exploit flow: A zone garbage collection is forced and the dangling port is reallocated with an OOL ports array containing a pointer to another Mach port overlapping the ip_context field, so that the address of the other port is retrieved by calling mach_port_get_context(). The dangling port is then reallocated with pipe buffers and converted into a kernel read primitive using pid_for_task(). Using the address of the other port as a starting point, relevant kernel objects are located. Finally, the fake port is converted into a fake kernel task port.
In-the-wild iOS Exploit Chain 3 - iOS 11.4
Discovered in-the-wild by Clément Lecigne. Analyzed by Ian Beer and Samuel Groß.
The vulnerability: The vulnerability is a double-free reachable from AppleVXD393UserClient::DestroyDecoder() (the class name varies by hardware) due to failing to clear a freed pointer.
Exploit strategy: The target 56-byte allocation is created and freed, leaving the dangling pointer intact. The slot is reallocated with an OSData buffer using an IOSurface property spray. The vulnerable method is called again to free the buffer, leaving a dangling OSData buffer. The slot is reallocated again with an OOL ports array containing a single target Mach port pointer and the contents are read in userspace via IOSurface properties, yielding the address of the port. The vulnerable method is called once more to free the OOL ports and the slot is reallocated with another OSData buffer containing two pointers to the Mach port. The holding port holding the OOL descriptor is destroyed, dropping two references to the Mach port. This leaves the process with a receive right to a dangling Mach port at a known address.
Subsequent exploit flow: A zone garbage collection is performed and the dangling port is reallocated with a segmented OOL memory spray such that calling mach_port_get_context() can identify which segment of the spray reallocated the port. That segment is freed and the dangling port is reallocated with pipe buffers, giving a controlled fake Mach port at a known address. The fake port is converted into a clock port and clock_sleep_trap() is used to brute force KASLR. The fake port is next converted into a fake task port and a kernel read primitive is established using pid_for_task(). Finally, the fake port is converted into a fake kernel task port.
Spice - iOS 11.4.1
Vulnerability analysis and POC by Luca Moro (@JohnCool__) at Synacktiv. Exploit by Siguza, Viktor Oreshkin (@stek29), Ben Sparkes (@iBSparkes), and littlelailo.
The vulnerability: The "LightSpeed" vulnerability (possibly CVE-2018-4344) is a race condition in XNU's lio_listio() due to improper state management that results in a use-after-free.
Exploit strategy: The vulnerable function is called in a loop in one thread to repeatedly trigger the vulnerability by allocating a buffer from kalloc.16 and racing to free the buffer twice. Another thread repeatedly sends a message containing an OOL ports array allocated from kalloc.16, immediately sprays a large number of kalloc.16 allocations containing a pointer to a fake Mach port in userspace via IOSurface properties, and receives the OOL ports. When the race is won, the double-free can cause the OOL ports array to be freed, and the subsequent spray can reallocate the slot with a fake OOL ports array. Receiving the OOL ports in userspace gives a receive right to a fake Mach port whose contents can be controlled directly.
Subsequent exploit flow: A second Mach port is registered as a notification port on the fake port, disclosing the address of the second port in the fake port's ip_pdrequest field. The fake port is modified to construct a kernel read primitive using mach_port_get_attributes(). Starting from the disclosed port pointer, kernel memory is read to find relevant kernel objects. The fake port is converted into a fake user client port providing a 7-argument arbitrary kernel function call primitive using iokit_user_client_trap(). Finally, a fake kernel task port is constructed.
Notes: The exploit does not work with PAN enabled.
The analysis was performed on the implementation in the file pwn.m, since this seems to provide the most direct comparison to the other exploit implementations in this list.
treadm1ll - iOS 11.4.1
Vulnerability analysis and POC by Luca Moro. Exploit by Tihmstar (@tihmstar).
The vulnerability: The "LightSpeed" vulnerability (same as above).
Exploit strategy: The vulnerable function is called in a loop in one thread to repeatedly trigger the vulnerability by allocating a buffer from kalloc.16 and racing to free the buffer twice. Another thread sends a fixed number of messages containing an OOL ports array allocated from kalloc.16. When the race is won, the double-free can cause the OOL ports array to be freed, leaving a dangling OOL ports array pointer in some messages. The first thread stops triggering the vulnerability and a large number of IOSurface objects are created. Each message is received in turn and a large number of kalloc.16 allocations containing a pointer to a fake Mach port in userspace are sprayed using IOSurface properties. Each spray can reallocate a slot from a dangling OOL ports array with a fake OOL ports array. Successfully receiving the OOL ports in userspace gives a receive right to a fake Mach port whose contents can be controlled directly.
Subsequent exploit flow: A second Mach port is registered as a notification port on the fake port, disclosing the address of the second port in the fake port's ip_pdrequest field. The fake port is modified to construct a kernel read primitive using pid_for_task(). Starting from the disclosed port pointer, kernel memory is read to find relevant kernel objects. The fake port is converted into a fake user client port providing a 7-argument arbitrary kernel function call primitive using iokit_user_client_trap(). Finally, a fake kernel task port is constructed.
Notes: The exploit does not work with PAN enabled.
Chaos - iOS 12.1.2
The vulnerability: CVE-2019-6225 is a use-after-free due to XNU's task_swap_mach_voucher() failing to comply with MIG lifetime semantics that results in an extra reference being added or dropped on an ipc_voucher object.
Exploit strategy: A large number of ipc_voucher objects are sprayed and the vulnerability is triggered twice to decrease the reference count on a voucher and free it. The remaining vouchers on the page are freed and a zone garbage collection is forced, leaving a dangling ipc_voucher pointer in the thread's ith_voucher field.
Subsequent exploit flow: The dangling voucher is reallocated by an OSString buffer using an IOSurface property spray. thread_get_mach_voucher() is called to obtain a send right to a newly allocated voucher port for the voucher, which causes a pointer to the voucher port to be stored in the fake voucher overlapping the OSString buffer; reading the OSString property discloses the address of the voucher port. The OSString overlapping the fake voucher is freed and reallocated with a large spray that both forces the allocation of controlled data containing a fake Mach port at a hardcoded address and updates the fake voucher's iv_port pointer to point to the fake Mach port. thread_get_mach_voucher() is called again to obtain a send right to the fake port and to identify which OSString buffer contains the fake Mach port. This leaves the process with a send right to a fake Mach port in an IOSurface property buffer at a known address (roughly equivalent to a dangling Mach port). A kernel read primitive is built by reallocating the OSString buffer to convert the fake port into a fake task port and calling pid_for_task() to read arbitrary memory. Relevant kernel objects are located and the fake port is converted into a fake map port to remap the fake port into userspace, removing the need to reallocate it. Finally the fake port is converted into a fake kernel task port.
Notes: The A12 introduced PAC, which limits the ability to use certain exploitation techniques involving code pointers (e.g. vtable hijacking). Also, iOS 12 introduced a mitigation in ipc_port_finalize() against freeing a port while it is still active (i.e. hasn't been destroyed, for example because a process still holds a right to it). This changed the common structure of past exploits whereby a port would be freed while a process still held a right to it. Possibly as a result, obtaining a right to a fake port in iOS 12+ exploits seems to occur later in the flow than in earlier exploits.
voucher_swap - iOS 12.1.2
By Brandon Azad (@_bazad) of Google Project Zero.
The vulnerability: CVE-2019-6225 (same as above).
Exploit strategy: The kernel heap is groomed to put a block of ipc_port allocations directly before a block of pipe buffers. A large number of ipc_voucher objects are sprayed and the vulnerability is triggered to decrease the reference count on a voucher and free it. The remaining vouchers on the page are freed and a zone garbage collection is forced, leaving a dangling ipc_voucher pointer in the thread's ith_voucher field.
Subsequent exploit flow: The dangling voucher is reallocated with an OOL ports array containing a pointer to a previously-allocated ipc_port overlapping the voucher's iv_refs field. A send right to the voucher port is retrieved by calling thread_get_mach_voucher() and the voucher's reference count is increased by repeatedly calling the vulnerable function, updating the overlapping ipc_port pointer to point into the pipe buffers. Receiving the OOL ports yields a send right to a fake Mach port whose contents can be controlled directly. mach_port_request_notification() is called to insert a pointer to an array containing a pointer to another Mach port in the fake port's ip_requests field. A kernel read primitive is built using pid_for_task(), and the address of the other Mach port is read to compute the address of the fake port. Relevant kernel objects are located and a fake kernel task port is constructed.
machswap2 - iOS 12.1.2
By Ben Sparkes.
The vulnerability: CVE-2019-6225 (same as above).
Exploit strategy: A large number of ipc_voucher objects are sprayed and the vulnerability is triggered twice to decrease the reference count on a voucher and free it. The remaining vouchers on the page are freed and a zone garbage collection is forced, leaving a dangling ipc_voucher pointer in the thread's ith_voucher field.
Subsequent exploit flow: The dangling voucher is reallocated by an OSString buffer containing a fake voucher using an IOSurface property spray. thread_get_mach_voucher() is called to obtain a send right to a newly allocated voucher port for the voucher, which causes a pointer to the voucher port to be stored in the fake voucher overlapping the OSString buffer; reading the OSString property discloses the address of the voucher port. Pipe buffers containing fake task ports are sprayed to land roughly 1 MB after the disclosed port address. The OSString overlapping the fake voucher is freed and reallocated to update the fake voucher's iv_port pointer to point to point into the pipe buffers. thread_get_mach_voucher() is called again to retrieve the updated voucher port, yielding a send right to a fake Mach port at a known address whose contents can be controlled directly. The fake port is converted into a fake task port and a kernel read primitive is established using pid_for_task(). Relevant kernel objects are located and a fake kernel task port is constructed.
Notes: The author developed two versions of this exploit: one for pre-PAN devices, and one for PAN-enabled devices. The exploit presented here is for PAN-enabled devices.
In-the-wild iOS Exploit Chain 5 - iOS 12.1.2
Discovered in-the-wild by Clément Lecigne. Analyzed by Ian Beer and Samuel Groß.
The vulnerability: CVE-2019-6225 (same as above).
Exploit strategy: A large number of ipc_voucher objects are sprayed and the vulnerability is triggered to decrease the reference count on a voucher and free it. The remaining vouchers on the page are freed and a zone garbage collection is forced, leaving a dangling ipc_voucher pointer in the thread's ith_voucher field.
Subsequent exploit flow: The dangling voucher is reallocated by an OOL memory spray. A large number of Mach ports are allocated and then thread_get_mach_voucher() is called to obtain a send right to a newly allocated voucher port for the voucher, which causes a pointer to the voucher port to be stored in the fake voucher overlapping the OOL ports array. More ports are allocated and then the OOL memory spray is received, disclosing the address of the voucher port for the fake voucher. The dangling voucher is reallocated again with another OOL memory spray that updates the voucher's iv_port pointer to the subsequent page. The Mach ports are destroyed and a zone garbage collection is forced, leaving the fake voucher holding a pointer to a dangling port. The dangling port is reallocated with pipe buffers. Finally, thread_get_mach_voucher() is called, yielding a send right to a fake Mach port at a known address whose contents can be controlled directly. The fake port is converted into a fake task port and a kernel read primitive is established using pid_for_task(). Relevant kernel objects are located and the fake port is converted into a fake kernel task port.
In-the-wild iOS Exploit Chain 4 - iOS 12.1.3
Discovered in-the-wild by Clément Lecigne. Analyzed by Ian Beer and Samuel Groß. Also reported by an anonymous researcher.
The vulnerability: CVE-2019-7287 is a linear heap buffer overflow in the IOKit function ProvInfoIOKitUserClient::ucEncryptSUInfo() due to an unchecked memcpy().
Exploit strategy: The kernel heap is groomed to place holes in kalloc.4096 before an OOL ports array and holes in kalloc.6144 before an OSData buffer accessible via IOSurface properties. The vulnerability is triggered with the source allocated from kalloc.4096 and the destination allocated from kalloc.6144, causing the address of a target Mach port to be copied into the OSData buffer. The OSData buffer is then read, disclosing the address of the target port. The heap is groomed again to place holes in kalloc.4096 before an OOL memory buffer and in kalloc.6144 before an OOL ports array. The vulnerability is triggered again to insert a pointer to the target port into the OOL ports array. The target port is freed and a zone garbage collection is forced, leaving a dangling port pointer in the OOL ports array. The dangling port is reallocated with pipe buffers and the OOL ports are received, giving a receive right to a fake Mach port at a known address whose contents can be controlled directly.
Subsequent exploit flow: The fake port is converted into a fake clock port and clock_sleep_trap() is used to brute force KASLR. The fake port is converted into a fake task port and a kernel read primitive is established using pid_for_task(). Relevant kernel objects are located and the fake port is converted into a fake kernel task port.
Attacking iPhone XS Max - iOS 12.1.4
The vulnerability: The vulnerability is a race condition in XNU's UNIX domain socket bind implementation due to the temporary unlock antipattern that results in a use-after-free.
Exploit strategy: Sockets are sprayed and the vulnerability is triggered to leave a pointer to a dangling socket pointer in a vnode struct. The sockets are closed, a zone garbage collection is forced, and the sockets are reallocated with controlled data via an OSData spray (possibly an IOSurface property spray). The fake socket is constructed to have a reference count of 0. The use after free is triggered to call socket_unlock() on the fake socket, which causes the fake socket/OSData buffer to be freed using kfree(). This leaves a dangling OSData buffer accessible using unspecified means.
Subsequent exploit flow: The dangling OSData buffer is reallocated with an OOL ports array and the OSData buffer is freed, leaving a dangling OOL ports array. Kernel memory is sprayed to place a fake Mach port at a hardcoded address (or an information leak is used) and the OOL ports array is reallocated with another OSData buffer, inserting a pointer to the fake Mach port into the OOL ports array. The OOL ports are received, yielding a send or receive right to the fake Mach port at a known address. The fake port is converted into a fake kernel task port by unspecified means.
Notes: The only reference for this exploit is a BlackHat presentation, hence the uncertainties in the explanations above.
The authors developed two versions of this exploit: one for non-PAC devices, and one for PAC-enabled devices. The exploit presented here is for PAC-enabled devices. The non-PAC exploit is substantially simpler (hijacking a function pointer used by socket_lock()).
SockPuppet - iOS 12.2 and iOS 12.4
The vulnerability: CVE-2019-8605 is a use-after-free due to XNU's in6_pcbdetach() failing to clear a freed pointer.
Exploit strategy: Safe arbitrary read, arbitrary kfree(), and arbitrary Mach port address disclosure primitives are constructed over the vulnerability.
The arbitrary read primitive: The vulnerability is triggered multiple times to create a number of dangling ip6_pktopts structs associated with sockets. The dangling ip6_pktopts are reallocated with an OSData buffer spray via IOSurface properties such that ip6po_minmtu is set to a known value and ip6po_pktinfo is set to the address to read. The ip6po_minmtu field is checked via getsockopt(), and if correct, getsockopt(IPV6_PKTINFO) is called to read 20 bytes of data from the address pointed to by ip6po_pktinfo.
The arbitrary kfree() primitive: The vulnerability is triggered multiple times to create a number of dangling ip6_pktopts structs associated with sockets. The dangling ip6_pktopts are reallocated with an OSData buffer spray via IOSurface properties such that ip6po_minmtu is set to a known value and ip6po_pktinfo is set to the address to free. The ip6po_minmtu field is checked via getsockopt(), and if correct, setsockopt(IPV6_PKTINFO) is called to invoke kfree_addr() on the ip6po_pktinfo pointer.
The arbitrary Mach port address disclosure primitive: The vulnerability is triggered multiple times to create a number of dangling ip6_pktopts structs associated with sockets. The dangling ip6_pktopts are reallocated with an OOL ports array spray containing pointers to the target port. The ip6po_minmtu and ip6po_prefer_tempaddr fields are read via getsockopt(), disclosing the value of the target port pointer. The port is checked to be of the expected type using the arbitrary read primitive.
Subsequent exploit flow: The Mach port address disclosure primitive is used to disclose the address of the current task. Two pipes are created and the addresses of the pipe buffers in the kernel are found using the kernel read primitive. Relevant kernel objects are located and a fake kernel task port is constructed in one of the pipe buffers. The arbitrary kfree() primitive is used to free the pipe buffer for the other pipe, and the pipe buffer is reallocated by spraying OOL ports arrays. The pipe is then written to insert a pointer to the fake kernel task port into the OOL ports array, and the OOL ports are received, yielding a fake kernel task port.
Notes: Unlike most other exploits on this list which are structured linearly, SockPuppet is structured hierarchically, building on the same primitives throughout. This distinct structure is likely due to the power and stability of the underlying vulnerability: the bug directly provides both an arbitrary read and an arbitrary free primitive, and in practice both primitives are 100% safe and reliable because it is possible to check that the reallocation is successful. However, this structure means that there is no clear temporal boundary in the high-level exploit flow between the vulnerability-specific and generic exploitation. Instead, that boundary occurs between conceptual layers in the exploit code.
The SockPuppet bug was fixed in iOS 12.3 but reintroduced in iOS 12.4.
AppleAVE2Driver exploit - iOS 12.4.1
The vulnerability: CVE-2019-8795 is a memory corruption in AppleAVE2Driver whereby improper bounds checking leads to processing of out-of-bounds data, eventually resulting in a controlled virtual method call or arbitrary kfree(). CVE-2019-8794 is a kernel memory disclosure in AppleSPUProfileDriver due to uninitialized stack data being shared with userspace.
Exploit strategy: The KASLR slide is discovered using the AppleSPUProfileDriver vulnerability. OSData buffers containing fake task ports are sprayed using IOSurface properties. The vulnerability is triggered to free an OSData buffer at a hardcoded address, leaving a dangling OSData buffer accessible via IOSurface properties.
Subsequent exploit flow: The dangling OSData buffer is reallocated with an OOL ports array and the OSData buffer is freed, leaving a dangling OOL ports array. The OOL ports array is reallocated with another OSData buffer, inserting pointers to the fake task ports sprayed earlier into the OOL ports array. The OOL ports are received, yielding send rights to the fake task ports, and pid_for_task() is used to read pointers to relevant kernel objects. The OSData buffer is freed and reallocated to convert one of the fake ports into a fake kernel task port.
Notes: iOS versions up to 13.1.3 were vulnerable, but the exploit presented here targeted iOS 12.4.1.
The author developed two versions of this exploit: one for non-PAC devices, and one for PAC-enabled devices. The exploit presented here is for PAC-enabled devices.
oob_timestamp - iOS 13.3
By Brandon Azad.
The vulnerability: CVE-2020-3837 is a linear heap out-of-bounds write of up to 8 bytes of timestamp data in IOKit's IOAccelCommandQueue2::processSegmentKernelCommand() due to incorrect bounds checking.
Exploit strategy: The kernel map is groomed to lay out two 96 MB shared memory regions, an 8-page ipc_kmsg, an 8-page OOL ports array, and 80 MB of OSData buffers sprayed via IOSurface properties. The number of bytes to overflow is computed based on the current time and the overflow is triggered to corrupt the ipc_kmsg's ikm_size field, such that the ipc_kmsg now has a size of between 16 pages and 80 MB. The port containing the ipc_kmsg is destroyed, freeing the corrupted ipc_kmsg, the OOL ports array, and some of the subsequent OSData buffers. More OSData buffers are sprayed via IOSurface to reallocate the OOL ports array containing a pointer to a fake Mach port at a hardcoded address that is likely to overlap one of the 96 MB shared memory regions. The OOL ports are received, producing a receive right to a fake Mach port at a known address whose contents can be controlled directly.
Subsequent exploit flow: A kernel memory read primitive is constructed using pid_for_task(). Relevant kernel objects are located and a fake kernel task port is constructed.
Notes: iOS 13 introduced zone_require, a mitigation that checks whether certain objects are allocated from the expected zalloc zone before they are used. An oversight in the implementation led to a trivial bypass when objects are allocated outside of the zalloc_map.
References: oob_timestamp exploit code.
tachy0n (unc0ver 5.0.0) - iOS 13.5
The vulnerability: The "LightSpeed" vulnerability (see "Spice" above; reintroduced in iOS 13).
Exploit strategy: (Analysis pending.) The vulnerable function is called in a loop in one thread to repeatedly trigger the vulnerability by allocating a buffer from kalloc.16 and racing to free the buffer twice. Preliminary results suggest that the freed kalloc.16 slot is reallocated by OSData buffers sprayed via IOSurface properties.
Subsequent exploit flow: (Analysis pending.)
Notes: The unc0ver exploit was released as an obfuscated binary; a more complete analysis of the exploit strategy and exploit flow will be released after the tachy0n exploit code is published.
While iOS 12 patched the LightSpeed vulnerability, the patch did not address the root cause and created a memory leak. This memory leak was fixed in iOS 13, but the change also reintroduced the old (vulnerable) behavior. This is a regression, not a variant: the original LightSpeed POC does trigger on iOS 13.
References: LightSpeed, a race for an iOS/macOS sandbox escape, unc0ver-v5.0.0.ipa.
iOS kernel exploit mitigations
Next we will look at some current iOS kernel exploit mitigations. This list is not exhaustive, but it briefly summarizes some of the mitigations that exploit developers may encounter up through iOS 13.
Kernel Stack Canaries - iOS 6
iOS 6 introduced kernel stack canaries (or stack cookies) to protect against stack buffer overflows in the kernel.
None of the exploits in this list are affected by the presence of stack canaries as they do not target stack buffer overflow vulnerabilities.
Kernel ASLR - iOS 6
Kernel Address Space Layout Randomization (Kernel ASLR or KASLR) is a mitigation that randomizes the base address of the kernelcache image in the kernel address space. Before Kernel ASLR was implemented, the addresses of kernel functions and objects in the kernelcache image were always located at a fixed address.
Bypassing or working around KASLR is a standard step of all modern iOS kernel exploits.
Kernel Heap ASLR - iOS 6
Since iOS 6 the base addresses for various kernel heap regions have been randomized. This seeks to mitigate exploits that hardcode addresses at which objects will be deterministically allocated.
Working around kernel heap randomization is a standard step of modern iOS kernel exploits. Usually this involves heap spraying, in which the kernel is induced to allocate large amounts of data to influence the shape of the heap even when exact addresses are not known. Also, many vulnerabilities can be leveraged to produce an information leak, disclosing the addresses of relevant kernel objects on the heap.
W^X / DEP - iOS 6
iOS 6 also introduced substantial kernel address space hardening by ensuring that kernel pages are mapped either as writable or as executable, but never both (often called "write xor execute" or W^X). This means that page tables no longer map kernel code pages as writable, and the kernel heap and stack are no longer mapped as executable. (Ensuring that non-code data is not mapped as executable is often called Data Execution Prevention, or DEP.)
Modern public iOS exploits do not attempt to bypass W^X (e.g. by modifying page tables and injecting shellcode); instead, exploitation is achieved by modifying kernel data structures and performing code-reuse attacks instead. This is largely due to the presence of a stronger, hardware-enforced W^X mitigation called KTRR.
PXN - iOS 7
Apple's A7 processor was the first 64-bit, ARMv8-A processor in an iPhone. Previously, iOS 6 had separated the kernel and user address space so that user code and data pages were inaccessible during normal kernel execution. With the move to 64-bit, the address spaces were no longer separated. Thus, the Privileged Execute-Never (PXN) bit was set in page table entries to ensure that the kernel could not execute shellcode residing in userspace pages.
Similarly to W^X, PXN as a protection against jumping to userspace shellcode is overshadowed by the stronger protection of KTRR.
PAN - iOS 10
Privileged access-never (PAN) is an ARMv8.1-A security feature introduced with the Apple A10 processor that prevents the kernel from accessing virtual addresses that are also accessible to userspace. This is used to prevent the kernel from dereferencing attacker-supplied pointers to data structures in userspace. It is similar to the Supervisor Mode Access Prevention (SMAP) feature on some Intel processors.
While PAN has been bypassed before, modern public iOS kernel exploits usually work around PAN by spraying data into the kernel and then learning the address of the data. While the most reliable techniques involve disclosing the address of the data inserted into the kernel, techniques exist to work around PAN generically, such as spraying enough data to overwhelm the kernel map randomization and force a fixed, hardcoded address to be allocated with the controlled data. Other primitives exist for establishing shared memory mappings between userspace and the kernel, which can also be used to work around PAN.
KTRR - iOS 10
KTRR (possibly Kernel Text Readonly Region, part of Kernel Integrity Protection) is a custom hardware security mitigation introduced on the Apple A10 processor (ARMv8.1-A). It is a strong form of W^X protection enforced by the MMU and the memory controller over a single span of contiguous memory covering the read-only parts of the kernelcache image and some sensitive data structures like top-level page tables and the trust cache. It has also been referred to by Apple as Kernel Integrity Protection (KIP) v1.
While KTRR has been publicly bypassed twice before, modern public iOS kernel exploits usually work around KTRR by not manipulating KTRR-protected memory.
APRR - iOS 11
APRR (possibly standing for Access Protection Rerouting or Access Permission Restriction Register) is a custom hardware feature on Apple A11 and later CPUs that indirects virtual memory access permissions (usually specified in the page table entry for the page) through a special register, allowing access permissions for large groups of pages to be changed atomically and per-core. It works by converting the bits in the PTE that typically directly specify the access permissions into an index into a special register containing the true access permissions; changing the register value swaps protections on all pages mapped with the same access permissions index. APRR is somewhat similar to the Memory Protection Keys feature available on newer Intel processors.
APRR on its own does not provide any security boundaries, but it makes it possible to segment privilege levels inside a single address space. It is heavily used by PPL to create a security boundary within the iOS kernel.
PPL - iOS 12
PPL (Page Protection Layer) is the software layer built on APRR and dependent on KTRR that aims to put a security boundary between kernel read/write/execute and direct page table access. The primary goal of PPL is to prevent an attacker from modifying user pages that have been codesigned (e.g. using kernel read/write to overwrite a userspace process's executable code). This necessarily means that PPL must also maintain total control over the page tables and prevent an attacker from mapping sensitive physical addresses, including page tables, page table metadata, and IOMMU registers.
As of May 2020, PPL has not been publicly bypassed. That said, modern iOS kernel exploits are so far unaffected by PPL.
PAC - iOS 12
Pointer Authentication Codes (PAC) is an ARMv8.3-A security feature that mitigates pointer tampering by storing a cryptographic signature of the pointer value in the upper bits of the pointer. Apple introduced PAC with the A12 and significantly hardened the implementation (compared to the ARM standard) in order to defend against attackers with kernel read/write, although for most purposes it is functionally indistinguishable. Apple's kernel uses PAC for control flow integrity (CFI), placing a security boundary between kernel read/write and kernel code execution.
Despite numerous public bypasses of the iOS kernel's PAC-based CFI, PAC in the kernel is still an effective exploit mitigation: it has severely restricted exploitability of many bugs and killed some exploit techniques. For example, exploits in the past have used a kernel execute primitive in order to build a kernel read/write primitive (see e.g. ziVA); that is no longer possible on A12 without bypassing PAC first. Furthermore, extensive use of PAC-protected pointers in IOKit has made it significantly harder to turn many bugs into useful primitives. Given the long history of serious security issues in IOKit, this is a substantial win.
zone_require - iOS 13
zone_require is a software mitigation introduced in iOS 13 that adds checks that certain pointers are allocated from the expected zalloc zones before using them. The most common zone_require checks in the iOS kernelcache are of Mach ports; for example, every time an ipc_port is locked, the zone_require() function is called to check that the allocation containing the Mach port resides in the ipc.ports zone (and not, for example, an OSData buffer allocated with kalloc()).
Since fake Mach ports are an integral part of modern techniques, zone_require has a substantial impact on exploitation. Vulnerabilities like CVE-2017-13861 (async_wake) that drop a reference on an ipc_port no longer offer a direct path to creating a fake port. While zone_require has been publicly bypassed once, the technique relied on an oversight in the implementation that is easy to correct.
Changelog
2020/07/09
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An entry was added for tachy0n (unc0ver 5.0.0) - iOS 13.5.
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2020/06/19
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The entry on MachSwap was replaced with machswap2, since the latter works on PAN-enabled devices.
An entry was added for AppleAVE2Driver exploit - iOS 12.4.1.
The description for PAN was updated to clarify that it was introduced with the A10 processor, not iOS 10.
The description for PPL was updated to clarify that it primarily protects userspace processes, as the kernel's code is protected by KTRR.
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2020/06/11
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Original post published.
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