Windows Kernel 64-bit stack memory disclosure in win32k!fnHkINLPMOUSEHOOKSTRUCTEX (via user-mode callback) 

CVE-2018-0810


We have discovered that a user-mode callback invoked by the win32k!fnHkINLPMOUSEHOOKSTRUCTEX function (via KeUserModeCallback) leads to the disclosure of uninitialized stack memory to user-mode clients, due to compiler-introduced structure padding. The vulnerability affects Windows 7 64-bit; other versions of Windows have not been tested.

The act of copying uninitialized kernel memory has been detected under a number of different stack traces. One example is shown below:

--- cut ---
   #0  0xfffff80002684830 ((00067830) ntoskrnl.exe!memcpy+00000000)
   #1  0xfffff8000297dbf0 ((00360bf0) ntoskrnl.exe!KeUserModeCallback+000000a4)
   #2  0xfffff9600018efe3 ((0014efe3) win32k.sys!fnHkINLPMOUSEHOOKSTRUCTEX+000000a3)
   #3  0xfffff960000d12c9 ((000912c9) win32k.sys!xxxCallCtfHook+00000155)
   #4  0xfffff960000d2af6 ((00092af6) win32k.sys!xxxScanSysQueue+000017da)
   #5  0xfffff9600010ce4f ((000cce4f) win32k.sys!xxxRealInternalGetMessage+00000453)
   #6  0xfffff9600011a4c5 ((000da4c5) win32k.sys!NtUserRealInternalGetMessage+00000095)
   #7  0xfffff8000268d093 ((00070093) ntoskrnl.exe!KiSystemServiceCopyEnd+00000013)
--- cut ---

At that point of execution, 0x50 bytes are copied from kernel to user-mode. The layout of the i/o structure passed down to the user-mode callback that we're seeing is as follows:

--- cut ---
  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 00 00 00 00 00 00 00 00 ff ff ff ff ................
  00000040: 00 00 00 00 00 00 00 00 00 00 00 00 ff ff ff ff ................
--- cut ---

Where 00 denote bytes which are properly initialized, while ff indicate uninitialized values copied back to user-mode. As shown above, there are 8 bytes leaked at offsets 0x3c-0x3f and 0x4c-4f. We have determined that these bytes originally come from a smaller structure of size 0x28, which is passed down to win32k!fnHkINLPMOUSEHOOKSTRUCTEX through the 3rd argument, and copied into offset 0x28 of the overall memory area passed to ring-3. More specifically, we have found that the nested structure is most likely of type MOUSEHOOKSTRUCTEX, and the uninitialized bytes correspond to the 4 bytes of padding between the wHitTestCode and dwExtraInfo fields, and to 4 bytes at the end of the structure, to align it to an 8-byte boundary of 40 (0x28) bytes.

--- cut ---
  typedef struct tagMOUSEHOOKSTRUCT {
    POINT     pt;
    HWND      hwnd;
    UINT      wHitTestCode;
    ULONG_PTR dwExtraInfo;
  } MOUSEHOOKSTRUCT, *PMOUSEHOOKSTRUCT, *LPMOUSEHOOKSTRUCT;

  typedef struct tagMOUSEHOOKSTRUCTEX {
    MOUSEHOOKSTRUCT MOUSEHOOKSTRUCT;
    DWORD           mouseData;
  } MOUSEHOOKSTRUCTEX, *PMOUSEHOOKSTRUCTEX, *LPMOUSEHOOKSTRUCTEX;
--- cut ---

In all observed cases, the origin of the leaked stack bytes is the stack frame of the win32k.sys!xxxScanSysQueue function.

A proof-of-concept program is not provided for this issue, but it has been observed and confirmed at normal system runtime, and is quite evident in the code.

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.

This bug is subject to a 90 day disclosure deadline. After 90 days elapse or a patch has been made broadly available, the bug report will become visible to the public.



Found by: mjurczyk