In this guest blog from researcher Marcin Wiązowski, he details CVE-2023-21822 – a Use-After-Free (UAF) in win32kfull that could lead to a privilege escalation. The bug was reported through the ZDI program and later patched by Microsoft. Marcin has graciously provided this detailed write-up of the vulnerability, examines how it could be exploited, and a look at the patch Microsoft released to address the bug.
In the Windows kernel, there are three APIs intended for general use by device drivers for the purpose of creating bitmaps:
EngCreateDeviceSurface. Each of these APIs return a bitmap handle. If the caller wants to perform some drawing operations on the bitmap, the caller must first lock the bitmap by passing its handle to
EngLockSurface increases the bitmap’s reference counter and returns a pointer to a corresponding
SURFOBJ is a structure located in kernel memory containing all the information regarding the bitmap, such as the bitmap’s dimensions, pixel format, a pointer to the pixel buffer, and so forth. We’ll take a closer look at the
SURFOBJ structure later. After calling
EngLockSurface, the obtained
SURFOBJ pointer can be passed to various drawing APIs such as
EngBitBlt. See winddi.h for the complete list of these drawing APIs. After the caller is finished with drawing operations, they should call
EngUnlockSurface. At this point, the bitmap’s reference counter decreases to zero again, and the caller is no longer allowed to use the
SURFOBJ pointer. Finally, the caller can delete the bitmap by calling
EngDeleteSurface on its handle. Typical usage of these APIs is shown below:
All APIs discussed above are exported from
win32k.sys kernel-mode module. Note, though, that the functions in
win32k.sys are only wrappers, and the implementations are in
Many years ago, both display drivers and printer drivers worked in kernel mode, but since Windows Vista, printer drivers work only in user mode (hence User-Mode Printer Drivers, or UMPD). Two important facts emerge from this change:
— During printing operations, the kernel must now perform some callbacks to user mode to call the appropriate user-mode printer driver.
— To allow printer driver code to run in user mode, some kernel APIs have now been made available from user mode.
As a result, all the kernel APIs described above now have user-mode counterparts, exported from the
gdi32.dll user-mode module. Let’s try to execute the same code shown above, but, this time, from user mode:
Note the reference counter values shown in the comments. The value is still zero after locking the bitmap. Why is this?
Kernel-mode code is always trusted, while user-mode code is always untrusted. So, now that printer drivers execute in user mode, they are considered untrusted and potentially malicious.
Suppose that the user-mode
EngLockSurface call would increase the bitmap’s reference counter in the same way that the kernel-mode version does. An attacker, acting as a user-mode printer driver, could call
EngLockSurface many times in a loop on a bitmap in order to overflow the bitmap’s reference counter, causing it to wrap around to zero. Then the bitmap could be deleted, leading to a use-after-free on the bitmap.
For this reason, the Windows kernel has implemented a different approach. The
EngLockSurface API is expected to return a pointer to the bitmap’s
SURFOBJ record – and it does, but, in user mode, this is a user-mode copy of the “true”, kernel-mode
SURFOBJ record. We can reconstruct this user-mode data structure as follows:
EngLockSurface implementation returns a pointer to the
UMSO.so field, which is a copy of the true, kernel-mode
SURFOBJ record, so that everything will work as expected. Internally, the user-mode
EngLockSurface call jumps to its kernel-mode implementation
win32kfull.sys!NtGdiEngLockSurface, where the user-mode
UMSO record is allocated and filled in. In kernel mode, the “true”, kernel-mode
EngLockSurface call is made on the bitmap, which is needed to access the bitmap’s
SURFOBJ record so its data can be copied into the
UMSO.so field. Afterwards, though,
NtGdiEngLockSurface calls the kernel-mode
EngUnlockSurface, which decreases the bitmap’s reference counter to zero again. This explains the observed reference counter values.
Once we call the user-mode
EngLockSurface, we are allowed to pass its result (which is a pointer to the copied
SURFOBJ data) to various drawing functions, such as
EngBitBlt. When corresponding calls are made from kernel mode, it works in a straightforward manner, but when calling from user mode, an additional layer is needed to translate the user-mode
SURFOBJ pointers into true, kernel-mode pointers. So, for example, if the user-mode code calls
gdi32.dll!EngLineTo, this will jump to the kernel-mode
win32kfull.sys!NtGdiEngLineTo wrapper. The wrapper will obtain the bitmap’s true kernel-mode
SURFOBJ record, so the kernel-mode
win32kfull.sys!EngLineTo drawing handler ultimately can be executed.
How does the kernel obtain the needed kernel-mode
SURFOBJ record? A
SURFOBJ record contains sensitive data such as the bitmap’s pixel buffer pointer, so the kernel never relies on the contents of
SURFOBJ records coming from user mode. Otherwise, there would be a security risk from malicious user-mode code that tampers with the contents of
UMSO.so structures. Instead, inside the wrapper function (such as
win32kfull.sys!NtGdiEngLineTo in the example above), the kernel verifies the
UMSO.magic value, and then uses the
UMSO.hsurf bitmap handle value to lock the bitmap by calling
EngLockSurface. In this way, the kernel safely obtains the requested bitmap’s kernel-mode
SURFOBJ record, which it can then pass to the appropriate kernel-mode
win32kfull.sys!EngXXX drawing function.
EngLockSurface function performs some validation on the supplied bitmap handle, meaning that not every kind of bitmap can be passed successfully to this call (we will discuss this in more detail later). But malicious user-mode code can now bypass this in any of these ways:
1) After making the
EngLockSurface call, we can delete the already-validated bitmap and create some other bitmap with the same handle value. We could choose to create a bitmap of a kind that couldn’t be successfully passed to
2) After making the
EngLockSurface call, we receive a pointer to a user-mode
SURFOBJ record, which, as we already know, is a part of a
UMSO record. So, we can overwrite the
UMSO.hsurf field, setting it to the handle of any bitmap that we want. We can choose to set it to the handle of a bitmap that couldn’t be successfully passed to
3) Simplest of all, we could prepare a
UMSO record from scratch, without making any
EngLockSurface call first. All we need to do is allocate some user-mode memory, set
0x554D534F, and set
UMSO.hsurf to the handle of a bitmap of our choice. The remaining part of this record (the
UMSO.so field, containing the
SURFOBJ record under normal circumstances) can be zeroed, as it will be disregarded by the kernel in any event.
Each of the three possibilities above will allow us to bypass the bitmap validation performed by the user-mode version of the
Now that we have seen that the validation can be bypassed, we must ask what is the purpose of that validation, and what ramifications does it have for security? To answer this question, we must look at the
SURFOBJ record definition. Some fields are publicly documented, while others have been reconstructed, as shown below:
flags field is undocumented, but it is known to contain some documented
HOOK_XXX flags found in the winddi.h header file. These flags tell the win32k subsystem which drawing operations should be handled by win32k itself, and which should instead be directed to a specialized device driver. The device driver is indicated by the bitmap’s
For example, suppose we want to draw a line on some bitmap. We’ll call
EngLineTo, passing a pointer to the bitmap’s
SURFOBJ record. Internally, the kernel will convert the requested line into a more general drawing construct known as a “path” (which can be a sequence of lines and curves). It will then check if the bitmap’s
SURFOBJ.flags field has the
HOOK_STROKEPATH flag set. If this flag is not present, it will use the generic code for drawing (“stroking”) paths provided by
HOOK_STROKEPATH is present, though, the kernel will direct the drawing request to the device driver specified by the
SURFOBJ.hdev field. The latter case, where possible, offers improved performance, as it allows individual device drivers to take advantage of accelerations offered by the specific hardware. For example, a graphics adapter may offer hardware-accelerated path stroking. Similarly, printer devices have specialized acceleration for outputting text.
So, if we prepare a bitmap that has a screen-related
SURFOBJ.hdev value, and also has the appropriate
HOOK_XXX flag set, and we pass it to one of the
EngXXX drawing APIs, there is the possibility of reaching an entry point of a specialized display driver, working in kernel mode. This could be
cdd.dll!DrvXXX in the single-monitor case or
win32kfull.sys!MulXXX in the multi-monitor case (though, there is not always a simple relationship between the requested functionality and the driver entry point ultimately called, as noted in the example above). The pointer to the bitmap’s
SURFOBJ record will be passed as a parameter to the driver’s entry point.
Further note that some
EngXXX APIs take not only one bitmap as a parameter, but rather two: a source bitmap and a destination bitmap. (Some optionally also take a mask bitmap, but that is not interesting for us). An example of such an API is
EngBitBlt, which copies a rectangle of pixels from a source bitmap to a destination bitmap. APIs that work on two bitmaps use the
SURFOBJ.hdev values of the destination bitmap when determining the ultimate device driver to receive the call. Nonetheless, when the final driver’s entry point is called, both the source and destination bitmaps are passed to it.
Hence, a properly prepared, screen-related bitmap, when passed to some
EngXXX API as the destination bitmap, allows us to reach a kernel-mode display driver, while also allowing an arbitrary bitmap of our choice to be passed as the source bitmap.
There is still no obvious security problem here, but let’s look at the
SURFOBJ record definition once again. It contains a
dhsurf field (not to be confused with the
hsurf field discussed above). The win32k subsystem treats
SURFOBJ.dhsurf as an opaque value. It is reserved for individual device drivers to use for their internal purposes. Setting this field on a new bitmap is easy: the
EngCreateDeviceSurface bitmap creation APIs just take the
dhsurf value as a parameter. Both the Canonical Display Driver (
cdd.dll, used for single-monitor graphics output) and the multi-display driver (
win32kfull.sys!MulXXX) expect to work only with their own bitmaps – bitmaps with
SURFOBJ.dhsurf values set by that specific driver – rather than on arbitrary bitmaps created from user mode (or by other drivers). Internally, each of these drivers use the
SURFOBJ.dhsurf value as a pointer to a block of kernel-mode memory, containing private data owned by that driver.
But we can reach a kernel-mode display driver by passing a properly prepared, destination bitmap to the
EngXXX call, and we can also pass some arbitrary bitmap of our choice as the source bitmap to the same
EngXXX call. This source bitmap can be an arbitrary bitmap we created, and its
SURFOBJ.dhsurf value may point to arbitrary controllable memory. The kernel-mode display driver, such as the Canonical Display Driver, will work on this block of memory as if it were its own block of kernel-mode memory. This means “game over”.
For these reasons, the user-mode
EngLockSurface implementation has validation to reject screen-related bitmaps that could be used to reach a kernel-mode display driver. But, thanks to the vulnerability described above, we can bypass this
EngLockSurface validation easily. In fact, we can get away with not calling
EngLockSurface at all, and just preparing the needed
UMSO record from scratch instead, as we have explained.
We must first notice that user-mode
EngXXX calls are intended to be used by user-mode printer drivers only, so most of these APIs will fail unless they are called during a callback from kernel to user-mode for a printing operation. But this doesn’t complicate things too much: the user-mode part of the callback is implemented as a
gdi32.dll!GdiPrinterThunk function, which is a public export from
gdi32.dll. It’s enough to hook or patch this function and perform our main exploitation there. This function receives four parameters (the input buffer, the input buffer size, the output buffer, and the output buffer size), but we don’t need the parameters during our exploitation at all. (However, if you are interested in more details, see Selecting Bitmaps into Mismatched Device Contexts. In particular, see sections titled “User-Mode Printer Drivers (UMPD)” and “Hooking the UMPD implementation”.)
We first need to get a callback from the kernel to our hooked
gdi32.dll!GdiPrinterThunk function. To achieve this, we need to initiate some printing operation. First we must locate an installed printer. There is at least one virtual printer installed by default on every Windows machine. We can locate installed printers using a call to the user-mode
winspool.drv!EnumPrintersA/W API. Then we must create a printer-related device context:
This call will go down to kernel mode, which will then perform several callbacks to user mode again – so our hooked
gdi32.dll!GdiPrinterThunk function will be invoked, exactly as we need. Our main exploitation phase starts here.
First, we need to obtain a bitmap with a screen-related
SURFOBJ.hdev value and a useful
HOOK_XXX flag set in its
SURFOBJ.flags field. To obtain such a bitmap, we can create a window with proper parameters, obtain the window’s device context, and grab the underlying bitmap. The obtained bitmap will act as our destination bitmap:
We also need a source bitmap, with its
SURFOBJ.dhsurf field pointing to controlled user-mode memory (our
Now we can prepare two
UMSO records, one for the destination bitmap and one for the source bitmap:
At this point, we have everything that we need to make a malicious
EngXXX call with our bitmaps. Our screen-related, destination bitmap will have all the defined
HOOK_XXX flags set, so we are free to choose any of the
EngXXX APIs that accept two bitmaps:
Through reverse engineering the Canonical Display Driver or multi-display driver internals, we can learn how to prepare the user-mode
FakeDhsurfBlock so that the call to the display driver yields exploitable memory primitives.
As discussed earlier, each of the user-mode
EngXXX drawing APIs (such as
EngBitBlt) calls its corresponding kernel-mode
win32kfull.sys!NtGdiEngXXX wrapper, where, amongst other things, user-mode
SURFOBJ pointers are converted to kernel-mode
SURFOBJ pointers. Afterwards, a kernel-mode
win32kfull.sys!EngXXX driver endpoint is called to perform the requested drawing operation.
Although it’s not related to our vulnerability, it’s worth mentioning that, for the duration of the
gdi32.dll!GdiPrinterThunk user-mode callback, the kernel maintains a mapping of known user-mode
SURFOBJ records to kernel-mode
SURFOBJ records. When a user-mode printer driver passes a user-mode
SURFOBJ pointer to some user-mode
EngXXX call, the kernel will try to use the mapping to find the corresponding kernel-mode
SURFOBJ pointer so it can be passed to the corresponding kernel-mode
The mapping is prepared before the user-mode
GdiPrinterThunk callback begins. This is because some bitmaps may be passed to the callback as parameters (though, during our exploitation, we made no use of the
GdiPrinterThunk input data). However, this means that bitmaps “locked” later, that is, by calls to
EngLockSurface made from inside the callback, are not present in the mapping.
win32kfull.sys!NtGdiEngXXX receives a user-mode
SURFOBJ pointer as a parameter and is not able to find it in the mapping, it assumes that the received
SURFOBJ record is contained in an
UMSO record (as its
Before the patch, such cases were directed to the internal
win32kfull.sys!UMPDSURFOBJ::GetLockedSURFOBJ function, where the
UMSO.magic value would be verified against the
0x554D534F value, and then the kernel-mode
EngLockSurface call would be made on the
UMSO.hsurf handle value, yielding the needed pointer to the “true”, kernel-mode
SURFOBJ record, as discussed earlier.
As you may have noticed, the name
GetLockedSURFOBJ is misleading, as it suggests that the bitmap is already locked. In reality, when coming from user mode, a bitmap’s reference counter is still zero. And as we saw above, a malicious user-mode printer driver may not have called
EngLockSurface at all, but instead just prepared the needed
UMSO record from scratch.
After the patch, the function name was changed to
GetLockableSURFOBJ. A user-mode printer driver can still perform all the manipulations described above, but now
GetLockableSURFOBJ considers the received bitmap handle (
UMSO.hsurf) as untrusted. After using the
UMSO.hsurf value to lock the bitmap in kernel mode,
GetLockableSURFOBJ now once again performs the same bitmap validation that is performed when calling the user-mode
EngLockSurface API. This validation is performed by calling
win32kfull.sys!IsSurfaceLockable. In this way, screen-related bitmaps that could be used to reach the kernel-mode display driver from within the user-mode printer driver are now rejected by
Thanks again to Marcin for providing this thorough write-up. He has contributed multiple bugs to the ZDI program over the last few years, and we certainly hope to see more submissions from them in the future. Until then, follow the team on Twitter, Mastodon, LinkedIn, or Instagram for the latest in exploit techniques and security patches.
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