We have discovered that the nt!NtNotifyChangeDirectoryFile system call discloses portions of uninitialized pool memory to user-mode clients, due to output structure alignment holes.
On our test Windows 10 32-bit workstation, an example layout of the output buffer is as follows:
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00000000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ff ff ................
00000010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ?? ?? ................
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Where 00 denote bytes which are properly initialized, while ff indicate uninitialized values copied back to user-mode. The output data is returned in a list of FILE_NOTIFY_INFORMATION structures [1]. If we map the above shadow bytes to the structure definition, it turns out that the uninitialized bytes correspond to the alignment hole between the end of the FileName string and the beginning of the adjacent FILE_NOTIFY_INFORMATION structure, if that string is of an odd length (and therefore not 4-byte aligned).
The issue can be reproduced by running the attached proof-of-concept program on a system with the Special Pools mechanism enabled for ntoskrnl.exe. Then, it is clearly visible that bytes at the aforementioned offsets are equal to the markers inserted by Special Pools, and would otherwise contain leftover data that was previously stored in that memory region:
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00000000: 10 00 00 00 04 00 00 00 02 00 00 00 62 00[91 91]............b...
00000010: 00 00 00 00 05 00 00 00 02 00 00 00 63 00 ?? ?? ............c...
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00000000: 10 00 00 00 04 00 00 00 02 00 00 00 62 00[3d 3d]............b.==
00000010: 00 00 00 00 05 00 00 00 02 00 00 00 63 00 ?? ?? ............c...
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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.
References:
[1] FILE_NOTIFY_INFORMATION structure, https://msdn.microsoft.com/pl-pl/library/windows/desktop/aa364391(v=vs.85).aspx
