Subject: CVE-2021-47533: drm/vc4: kms: Clear the HVS FIFO commit pointer once done

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

Description
===========

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

drm/vc4: kms: Clear the HVS FIFO commit pointer once done

Commit 9ec03d7f1ed3 ("drm/vc4: kms: Wait on previous FIFO users before a
commit") introduced a wait on the previous commit done on a given HVS
FIFO.

However, we never cleared that pointer once done. Since
drm_crtc_commit_put can free the drm_crtc_commit structure directly if
we were the last user, this means that it can lead to a use-after free
if we were to duplicate the state, and that stale pointer would even be
copied to the new state.

Set the pointer to NULL once we're done with the wait so that we don't
carry over a pointer to a free'd structure.

The Linux kernel CVE team has assigned CVE-2021-47533 to this issue.

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

	Issue introduced in 5.12 with commit 9ec03d7f1ed3 and fixed in 5.15.7 with commit 2931db9a5ed2
	Issue introduced in 5.12 with commit 9ec03d7f1ed3 and fixed in 5.16 with commit d134c5ff71c7

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-2021-47533
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/vc4/vc4_kms.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/2931db9a5ed219546cf2ae0546698faf78281b89
	https://git.kernel.org/stable/c/d134c5ff71c7f2320fc7997f2fbbdedf0c76889a