We have discovered that the nt!NtGdiGetPhysicalMonitorDescription system call discloses portions of uninitialized kernel stack memory to user-mode clients, on Windows 7 to Windows 10.
This is caused by the fact that the syscall copies a whole stack-based array of 256 bytes (128 wide-chars) to the caller, but typically only a small portion of the buffer is used to store the requested monitor description, while the rest of it remains uninitialized. This memory region contains sensitive information such as addresses of executable images, kernel stack, kernel pools and stack cookies.
The attached proof-of-concept program demonstrates the disclosure by spraying the kernel stack with a large number of 0x41 ('A') marker bytes, and then calling the affected system call. An example output is as follows:
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00000000: 47 00 65 00 6e 00 65 00 72 00 69 00 63 00 20 00 G.e.n.e.r.i.c. .
00000010: 4e 00 6f 00 6e 00 2d 00 50 00 6e 00 50 00 20 00 N.o.n.-.P.n.P. .
00000020: 4d 00 6f 00 6e 00 69 00 74 00 6f 00 72 00 00 00 M.o.n.i.t.o.r...
00000030: 74 00 6f 00 72 00 2e 00 64 00 65 00 76 00 69 00 t.o.r...d.e.v.i.
00000040: 63 00 65 00 64 00 65 00 73 00 63 00 25 00 3b 00 c.e.d.e.s.c.%.;.
00000050: 47 00 65 00 6e 00 65 00 72 00 69 00 63 00 20 00 G.e.n.e.r.i.c. .
00000060: 4e 00 6f 00 6e 00 2d 00 50 00 6e 00 50 00 20 00 N.o.n.-.P.n.P. .
00000070: 4d 00 6f 00 6e 00 69 00 74 00 6f 00 72 00 00 00 M.o.n.i.t.o.r...
00000080: 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 AAAAAAAAAAAAAAAA
00000090: 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 AAAAAAAAAAAAAAAA
000000a0: 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 AAAAAAAAAAAAAAAA
000000b0: 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 AAAAAAAAAAAAAAAA
000000c0: 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 AAAAAAAAAAAAAAAA
000000d0: 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 AAAAAAAAAAAAAAAA
000000e0: 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 AAAAAAAAAAAAAAAA
000000f0: 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 AAAAAAAAAAAAAAAA
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If the stack spraying part of the PoC code is disabled, we can immediately observe various kernel-mode addresses in the dumped memory area.
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.
