Subject: CVE-2023-52486: drm: Don't unref the same fb many times by mistake due to deadlock handling

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

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

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

drm: Don't unref the same fb many times by mistake due to deadlock handling

If we get a deadlock after the fb lookup in drm_mode_page_flip_ioctl()
we proceed to unref the fb and then retry the whole thing from the top.
But we forget to reset the fb pointer back to NULL, and so if we then
get another error during the retry, before the fb lookup, we proceed
the unref the same fb again without having gotten another reference.
The end result is that the fb will (eventually) end up being freed
while it's still in use.

Reset fb to NULL once we've unreffed it to avoid doing it again
until we've done another fb lookup.

This turned out to be pretty easy to hit on a DG2 when doing async
flips (and CONFIG_DEBUG_WW_MUTEX_SLOWPATH=y). The first symptom I
saw that drm_closefb() simply got stuck in a busy loop while walking
the framebuffer list. Fortunately I was able to convince it to oops
instead, and from there it was easier to track down the culprit.

The Linux kernel CVE team has assigned CVE-2023-52486 to this issue.

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

	Fixed in 4.19.307 with commit 376e21a9e4c2
	Fixed in 5.4.269 with commit 9dd334a82450
	Fixed in 5.10.210 with commit f55261469be8
	Fixed in 5.15.149 with commit b4af63da9d94
	Fixed in 6.1.76 with commit 62f2e79cf9f4
	Fixed in 6.6.15 with commit d7afdf360f4a
	Fixed in 6.7.3 with commit bfd0feb1b109
	Fixed in 6.8 with commit cb4daf271302

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-2023-52486
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_plane.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/376e21a9e4c2c63ee5d8d3aa74be5082c3882229
	https://git.kernel.org/stable/c/9dd334a8245011ace45e53298175c7b659edb3e7
	https://git.kernel.org/stable/c/f55261469be87c55df13db76dc945f6bcd825105
	https://git.kernel.org/stable/c/b4af63da9d94986c529d74499fdfe44289acd551
	https://git.kernel.org/stable/c/62f2e79cf9f4f47cc9dea9cebdf58d9f7b5695e0
	https://git.kernel.org/stable/c/d7afdf360f4ac142832b098b4de974e867cc063c
	https://git.kernel.org/stable/c/bfd0feb1b109cb63b87fdcd00122603787c75a1a
	https://git.kernel.org/stable/c/cb4daf271302d71a6b9a7c01bd0b6d76febd8f0c