Subject: CVE-2021-47200: drm/prime: Fix use after free in mmap with drm_gem_ttm_mmap

From: Greg Kroah-Hartman <[email-masked]>

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."

The Linux kernel CVE team has assigned CVE-2021-47200 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 5.5 with commit 9786b65bc61a and fixed in 5.15.5 with commit 4f8e469a2384
	Issue introduced in 5.5 with commit 9786b65bc61a and fixed in 5.16 with commit 8244a3bc27b3

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2021-47200
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	drivers/gpu/drm/drm_prime.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/4f8e469a2384dfa4047145b0093126462cbb6dc0
	https://git.kernel.org/stable/c/8244a3bc27b3efd057da154b8d7e414670d5044f