In the October 2011 Patch Tuesday, Microsoft released update MS11-077 to fix a null pointer de-reference vulnerability (CVE-2011-1985). In this paper, we will reverse engineer the patch for MS11-077 (CVE-2011-1985) to get a better understanding of the vulnerability fixed by this patch.
Unpatched File: win32k.sys (version: 5.1.2600.6119)
Patched File: win32k.sys (version: 5.1.2600.6149)
- Using binary diff, we can see the changes that were made to the vulnerable file win32k.sys. Figure 1 below shows the TurboDiff results.
Figure 1: TurboDiff Results
As you can see in Figure 1 above, while most of the functions are identical, there are a couple of functions that look ‘suspicious’ and some others that are ‘changed’. The large number of changes is not a surprise because Microsoft has fixed four different vulnerabilities with this patch.
- Taking a closer look at all the functions that were changed, you will see that the changes made to functions ‘NtUserfnINLBOXSTRING’, ‘NtUserfnSENTDDEMSG’ and ‘NtUserfnINCBOXSTRING’ are all the same. Figure 2, below shows the changes made.
Figure 2: Binary Diff for function NtUserfnINLBOXSTRING(x,x,x,x,x).
- Looking at the binary difference, it is clear that the patch is checking that the arg_0 (first argument passed to the function) is 0xFFFFFFFF and if it is 0xFFFFFFFF, call _UserSetLastError() with 0x578 and return from the function.
- This gives us two pointers to exploit the vulnerability. The first is that the arg_0 has to be 0xFFFFFFFF. The second pointer is that the patched function bails out setting system error code to 0x578. This is the system error code for ERROR_INVALID_WINDOW_HANDLE, thus hinting us that the argument is of type HWND.
Everything until now is pretty simple and it looks easy to exploit this vulnerability. However, the really challenge here is finding a user mode function that will call the vulnerable function. It turns out this isn’t very straightforward, and we will need to understand the Windows GUI subsystem.
Win32 GDI Subsystem:
The GDI (Graphics Device Interface) APIs are implemented in the GDI32.DLL and include all the low-level graphics services such as drawing lines, displaying BMPs etc. The GDI APIs make system calls into the WIN32k.sys to implement most APIs. The User APIs are implemented in USER32.DLL module and include all higher-level GUI-related services such as window management, menus, dialog boxes, user controls etc. USER heavily relies on GDI to do its work.
One of the most important means of communication in Windows is Messages. Windows-based applications are event-driven and act upon messages sent to them. The way you program in Windows is by responding to events. These events are called Messages. Messages can signal many events, caused by the user, the operating system, or another program. Each window, owned by a thread, has a window procedure (function) for processing input messages and dispatching them to the operating system. If a thread accesses any of the user interface or GDI system calls (handled by win32k.sys), the kernel creates a THREADINFO structure which holds three message queues used to process input. These are the input queue, the post queue, and the send queue. The input queue is primarily used for mouse and keyboard messages, while the send and post queues are used for synchronous (send) and asynchronous (post) window messages respectively.
Asynchronous messages are used in one-way communication between window threads and are typically used to notify a window to perform a specific task. Asynchronous messages are handled by the PostMessage APIs and are sent to the post queue of the receiving thread. The sender does not wait for the processing to complete in the receiving thread and thus returns immediately.
Synchronous messages differ from asynchronous messages as the sender typically waits for a response to be provided or a timeout to occur before continuing execution. Thus, they require mechanisms to ensure that the threads are properly synchronized and in the expected state. Synchronous messages use the SendMessage APIs which in turn directs execution to the NtUserMessageCall system call in win32k.sys.
This information is enough for us to take our analysis further.
Hitting the vulnerable function:
As described above, the message mechanism plays an integral role in the user interface component of the Windows operating system. There are many different types of message codes and those less than 0x400 are reserved by operating system. Depending upon the type of message code, NtUserMessageCall() calls a particular function to handle the message. Let’s take a closer look at how NtUserMessageCall, calls the appropriate functions to handle different message types.
As seen in the above figure, the function first checks if the Msg code is less than 0x400(EAX has the Msg code) to check if it’s a system message code. Each Message code denotes an index in the win32k!MessageTable byte array. This byte value is than logically AND to 0x3F, since the last 6bits of the byte obtained from win32k!MessageTable determines the function that will handle the Message code. _gapfnMessageCall is a function table that stores address of all the functions that can handle different messages. See Figures below to see how _gapfnMessageCall table looks.
Thus if we can get the index of our vulnerable function in _gapfnMessageCall, we can easily compute how we can call the vulnerable function. The index of our vulnerable functions are 29(0x1D), 27(0x27) and 43(0x2B) for NtUserfnINLBOXSTRING(),NtUserfnINCBOXSTRING() and NtUserfnSENTDDEMSG() respectively.
Following is the pseudo code to compute Msg codes for hitting the vulnerable function:
for i in range[0x00 to 0x400]
if MessageTable[i] & 0x3F == 0x1D //NtUserfnINLBOXSTRING() Hit!
if MessageTable[i] & 0x3F == 0x1B //NtUserfnINCBOXSTRING() Hit!
if MessageTable[i] & 0x3F == 0x2B //NtUserfnSENTDDEMSG() Hit!
Proof of Concept:
Other Possible Msg codes for hitting vulnerable functions are:
0x143, 0x14A, 0x14C, 0x14D, 0x158, 0x180, 0x181, 0x18C, 0x18F, 0x1A2, 0x1AA, 0x1AB, 0x1AC, 0x1AD, 0x3E2, 0x3E3, 0x3E5, 0x3E6, 0x3E7, 0x3E8
As we've seen above, it is pretty easy to trigger this vulnerability. We would recommend our customers to scan their environment for QID 90746 and apply this security update as soon as possible.