Subtle information disclosure in WIN32K.SYS syscall return values

While performing some random research related to the WIN32K.SYS driver syscalls a few months ago, I stumbled on an interesting finding – when examining the full 32-bit (or in the case of the original research – 64) return values, some of the services seemed to return unusual numbers, for example 0xfffffa8000ea0000. After investigating the subject for a while, it turned out that a part of the WIN32K syscall interface indeed does not always return a value of the native CPU size (as opposed to the core NT kernel). Instead, some of the graphical services seem to be explicitly declared as:

VOID NtUserRandomService( ... )

 or

USHORT NtUserRandomService( ... )

Funny enough to find such types of quirks in the Windows kernel, eh? A list of the flawed syscall names is as follows:

• ETHREAD (full disclosure)
  • NtUserAlterWindowStyle
  • NtUserSetThreadState
  • NtUserNotifyWinEvent
  • NtUserModifyUserStartupInfoFlags
  • NtUserSetThreadLayoutHandles
  • NtUserNotifyIMEStatus
  • NtUserSetThreadLayoutHandles
  • NtUserSetRipFlags (Windows XP only)
  • NtUserSetDbgTag (Windows XP only)
• ETHREAD (partial disclosure)
  • NtUserRegisterClassExWOW
  • NtUserGetKeyState
  • NtUserGetAsyncKeyState
  • NtUserVkKeyScanEx
  • NtUserSetWindowWord
  • NtUserSetClassWord
• W32THREAD (full disclosure)
  • NtGdiEngUnlockSurface
  • NtGdiPATHOBJ_vEnumStartClipLines
  • NtGdiEngDeletePath
  • NtGdiEngDeleteClip
  • NtGdiFONTOBJ_vGetInfo
• Other (unknown?)
  • NtGdiFlush
  • NtGdiEngUnlockSurface

The above list was created based on the Windows 7 64-bit graphical system call table, although I do not guarantee it is by any means complete (system calls might have been added, removed, or altered since then). As can be seen, a user-mode application is primarily able to read the addresses of two, internal kernel-mode structures (assigned to the currently executed thread, thus no serious information disclosure is going on, here). These are:

• ETHREAD – the very standard NT kernel structure, assigned to every single thread running in the system,
• W32THREAD – an internal, and pretty much unexplored structure, allocated on demand (i.e. when the thread calls a win32k service for the first time). For more details, you’re advised to take a look at the Mysteries of win32k & GDI post on the Woodmann RCE Forums.

In order to confirm the story a little, let’s take a look at the example NtUserGetKeyState system call epilogue:

mov     [ecx+48h], edx
mov     eax, [eax+70h]
mov     [ecx+4Ch], eax
call    _LeaveCrit@0    ; LeaveCrit()
mov     ax, di
pop     edi
pop     esi
pop     ebp
retn    4

Clearly, the code doesn’t care about the upper 16 bits of the return value (leading to a partial disclosure) – that’s also the case for the rest of the aforementioned routines. A complete log from a Windows 7 64-bit instance of a Proof of Concept code follows:

--- 64bit Kernel-address disclosure:
[+] [ETHREAD] NtUserAlterWindowStyle          : 0xfffffa8000ea80b0
[+] [ETHREAD] NtUserSetThreadState            : 0xfffffa8000ea80b0
[+] [ETHREAD] NtUserNotifyWinEvent            : 0xfffffa8000ea80b0
[+] [ETHREAD] NtUserModifyUserStartupInfoFlags: 0xfffffa8000ea80b0
[+] [ETHREAD] NtUserSetThreadLayoutHandles    : 0xfffffa8000ea80b0
[+] [ETHREAD] NtUserNotifyIMEStatus           : 0xfffffa8000ea80b0
[+] [ETHREAD] NtUserSetThreadLayoutHandles    : 0xfffffa8000ea80b0
[+] [W32THREAD] NtGdiEngUnlockSurface         : 0xfffff900c216f010
[+] [W32THREAD] NtGdiPATHOBJ_vEnumStartClipLines: 0xfffff900c216f010
[+] [W32THREAD] NtGdiEngDeletePath            : 0xfffff900c216f010
[+] [W32THREAD] NtGdiEngDeleteClip            : 0xfffff900c216f010
[+] [W32THREAD] NtGdiFONTOBJ_vGetInfo         : 0xfffff900c216f010

--- 48bit Kernel-address disclosure (the AX value is uncontrolled):
[+] [ETHREAD] NtUserRegisterClassExWOW        : 0xfffffa8000ea0000
[+] [ETHREAD] NtUserGetKeyState               : 0xfffffa8000ea0000
[+] [ETHREAD] NtUserGetAsyncKeyState          : 0xfffffa8000ea0000
[+] [ETHREAD] NtUserVkKeyScanEx               : 0xfffffa8000ea0332
[+] [ETHREAD] NtUserSetWindowWord             : 0xfffffa8000ea0000
[+] [ETHREAD] NtUserSetClassWord              : 0xfffffa8000ea0000

Since the disclosed addresses do not pose a direct threat to the system security, the MSRC team did not consider the issue worth fixing. Well, although I do agree that it’s a minor issue, it is still amusing for me to find such peculiarities in the main Windows graphical kernel module ;) No source code this time, although I do encourage you to fire your IDA and verify the findings by yourself (Windows WIN32K.SYS System Call Table might come in handy).

Take care!

PS. It seems that I wasn’t the only one coming across this matter – Tavis Ormandy has apparently tweeted about it a year ago, as well ;) Nice one.

PS2. I received the first, valid submission for Pimp CrackMe – a reversing challenge by Gynvael and me (see: link). Well done!