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

CVE-2018-0810


We have discovered that a user-mode callback invoked by the win32k!fnHkINLPMSLLHOOKSTRUCT function (via KeUserModeCallback) leads to the disclosure of uninitialized stack memory to user-mode clients. 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 two different stack traces, shown below:

--- cut ---
   #0  0xfffff800026b9830 ((00067830) ntoskrnl.exe!memcpy+00000000)
   #1  0xfffff800029b2bf0 ((00360bf0) ntoskrnl.exe!KeUserModeCallback+000000a4)
   #2  0xfffff9600020f238 ((0014f238) win32k.sys!fnHkINLPMSLLHOOKSTRUCT+00000088)
   #3  0xfffff9600013343e ((0007343e) win32k.sys!xxxHkCallHook+0000029a)
   #4  0xfffff96000150c8b ((00090c8b) win32k.sys!xxxCallHook2+000004b7)
   #5  0xfffff960001be4c6 ((000fe4c6) win32k.sys!xxxMoveEventAbsolute+000000ca)
   #6  0xfffff960001be6f5 ((000fe6f5) win32k.sys!xxxMoveEvent+000000a1)
   #7  0xfffff960001bff4a ((000fff4a) win32k.sys!xxxMouseEventDirect+0000028e)
   #8  0xfffff960001c032f ((0010032f) win32k.sys!xxxSendInput+000000e3)
   #9  0xfffff960001a36aa ((000e36aa) win32k.sys!NtUserSendInput+00000112)
   #10  0xfffff800026c2093 ((00070093) ntoskrnl.exe!KiSystemServiceCopyEnd+00000013)
--- cut ---
   #0  0xfffff800026b9830 ((00067830) ntoskrnl.exe!memcpy+00000000)
   #1  0xfffff800029b2bf0 ((00360bf0) ntoskrnl.exe!KeUserModeCallback+000000a4)
   #2  0xfffff9600020f238 ((0014f238) win32k.sys!fnHkINLPMSLLHOOKSTRUCT+00000088)
   #3  0xfffff9600013343e ((0007343e) win32k.sys!xxxHkCallHook+0000029a)
   #4  0xfffff96000150c8b ((00090c8b) win32k.sys!xxxCallHook2+000004b7)
   #5  0xfffff960001bbc54 ((000fbc54) win32k.sys!xxxButtonEvent+000004f8)
   #6  0xfffff960001bc9fc ((000fc9fc) win32k.sys!xxxDoButtonEvent+000001a8)
   #7  0xfffff960001c0b14 ((00100b14) win32k.sys!ProcessQueuedMouseEvents+000002c8)
   #8  0xfffff960001bff83 ((000fff83) win32k.sys!xxxMouseEventDirect+000002c7)
   #9  0xfffff960001c032f ((0010032f) win32k.sys!xxxSendInput+000000e3)
   #10  0xfffff960001a36aa ((000e36aa) win32k.sys!NtUserSendInput+00000112)
   #11  0xfffff800026c2093 ((00070093) ntoskrnl.exe!KiSystemServiceCopyEnd+00000013)
--- cut ---

At that point of execution, 0x40 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 ff ff ff ff 00 00 00 00 00 00 00 00 ................
--- cut ---

Where 00 denote bytes which are properly initialized, while ff indicate uninitialized values copied back to user-mode. As shown above, there are 4 bytes leaked at offsets 0x34-37. We have determined that these bytes originally come from a smaller structure of size 0x20, which is passed down to win32k!fnHkINLPMSLLHOOKSTRUCT through the 3rd argument, and copied into offset 0x20 of the overall memory area passed to ring-3.

More specifically, we have found that the nested structure is allocated in the stack frame of the caller of win32k!xxxCallHook2 (so win32k!xxxMoveEventAbsolute and win32k!xxxButtonEvent in our case), and is most likely of type MSLLHOOKSTRUCT. If true, the uninitialized bytes correspond to the 4 bytes of padding between the time and dwExtraInfo fields, inserted by the compiler to align the latter to an 8-byte boundary.

--- cut ---
  typedef struct tagMSLLHOOKSTRUCT {
    POINT     pt;
    DWORD     mouseData;
    DWORD     flags;
    DWORD     time;
    ULONG_PTR dwExtraInfo;
  } MSLLHOOKSTRUCT, *PMSLLHOOKSTRUCT, *LPMSLLHOOKSTRUCT;
--- cut ---

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