Windows Kernel 64-bit stack memory disclosure in win32k!SfnINLPHELPINFOSTRUCT (via user-mode callback)(CVE-2018-0810)

We have discovered that a user-mode callback invoked by the win32k!SfnINLPHELPINFOSTRUCT 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 the following stack trace: ``` #0 0xfffff80002684830 ((00067830) ntoskrnl.exe!memcpy+00000000) #1 0xfffff8000297dbf0 ((00360bf0) ntoskrnl.exe!KeUserModeCallback+000000a4) #2 0xfffff9600018c282 ((0014c282) win32k.sys!SfnINLPHELPINFOSTRUCT+000001c2) #3 0xfffff9600010e45b ((000ce45b) win32k.sys!xxxSendMessageToClient+000001f7) #4 0xfffff9600010dca3 ((000cdca3) win32k.sys!xxxSendMessageTimeout+000002b6) #5 0xfffff960001bb557 ((0017b557) win32k.sys!xxxSendHelpMessage+0000007f) #6 0xfffff960000ec960 ((000ac960) win32k.sys!xxxRealDefWindowProc+00000700) #7 0xfffff960000f3b54 ((000b3b54) win32k.sys!xxxWrapRealDefWindowProc+0000003c) #8 0xfffff960000e5847 ((000a5847) win32k.sys!NtUserfnNCDESTROY+00000027) #9 0xfffff960000f302e ((000b302e) win32k.sys!NtUserMessageCall+0000012e) #10 0xfffff8000268d093 ((00070093) ntoskrnl.exe!KiSystemServiceCopyEnd+00000013) ``` At that point of execution, 0x88 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: ``` 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 00 00 00 00 ................ 00000040: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ 00000050: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ 00000060: 00 00 00 00 00 00 00 00 00 00 00 00 ff ff ff ff ................ 00000070: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ 00000080: 00 00 00 00 00 00 00 00 ?? ?? ?? ?? ?? ?? ?? ?? ................ ``` 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 0x6c-0x6f. We have determined that these bytes originally come from a smaller structure of size 0x28, which is passed down to win32k!SfnINLPHELPINFOSTRUCT through the 4th argument, and copied into offset 0x60 of the overall memory area passed to ring-3. More specifically, we have found that the nested structure is most likely of type HELPINFO, and the uninitialized bytes correspond to the 4 bytes of padding between the iCtrlId and hItemHandle fields, inserted by the compiler to align the latter to an 8-byte boundary. ``` typedef struct tagHELPINFO { UINT cbSize; int iContextType; int iCtrlId; HANDLE hItemHandle; DWORD dwContextId; POINT MousePos; } HELPINFO, *LPHELPINFO; ``` The origin of the leaked bytes is the stack frame of the win32k!xxxSendHelpMessage 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.