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

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

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