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

Vulnerability Description

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 object's 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."

References

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