CVE-2021-47200

In the Linux kernel, the following vulnerability has been resolved:

drm/prime: Fix use after free in mmap with drm_gem_ttm_mmap

drm_gem_ttm_mmap() drops a reference to the gem object on success. If the gem objects refcount == 1 on entry to drm_gem_prime_mmap(), that drop will free the gem object, and the subsequent drm_gem_object_get() will be a UAF. Fix by grabbing a reference before calling the mmap helper.

This issue was forseen when the reference dropping was adding in commit 9786b65bc61ac (drm/ttm: fix mmap refcounting): For that to work properly the drm_gem_object_get() call in drm_gem_ttm_mmap() must be moved so it happens before calling obj->funcs->mmap(), otherwise the gem refcount would go down to zero.

Weakness

Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code.

Affected Software

Extended Description

The use of previously-freed memory can have any number of adverse consequences, ranging from the corruption of valid data to the execution of arbitrary code, depending on the instantiation and timing of the flaw. The simplest way data corruption may occur involves the system’s reuse of the freed memory. Use-after-free errors have two common and sometimes overlapping causes:

In this scenario, the memory in question is allocated to another pointer validly at some point after it has been freed. The original pointer to the freed memory is used again and points to somewhere within the new allocation. As the data is changed, it corrupts the validly used memory; this induces undefined behavior in the process. If the newly allocated data happens to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.

Potential Mitigations

References

• https://git.kernel.org/stable/c/4f8e469a2384dfa4047145b0093126462cbb6dc0
• https://git.kernel.org/stable/c/8244a3bc27b3efd057da154b8d7e414670d5044f
• https://git.kernel.org/stable/c/4f8e469a2384dfa4047145b0093126462cbb6dc0
• https://git.kernel.org/stable/c/8244a3bc27b3efd057da154b8d7e414670d5044f