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 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."