CVE-2021-47108: drm/mediatek: hdmi: Perform NULL pointer check for mtk_hdmi_conf

CVE-2021-47108: drm/mediatek: hdmi: Perform NULL pointer check for mtk_hdmi_conf

[Greg Kroah-Hartman]

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

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

drm/mediatek: hdmi: Perform NULL pointer check for mtk_hdmi_conf

In commit 41ca9caaae0b
("drm/mediatek: hdmi: Add check for CEA modes only") a check
for CEA modes was added to function mtk_hdmi_bridge_mode_valid()
in order to address possible issues on MT8167;
moreover, with commit c91026a938c2
("drm/mediatek: hdmi: Add optional limit on maximal HDMI mode clock")
another similar check was introduced.

Unfortunately though, at the time of writing, MT8173 does not provide
any mtk_hdmi_conf structure and this is crashing the kernel with NULL
pointer upon entering mtk_hdmi_bridge_mode_valid(), which happens as
soon as a HDMI cable gets plugged in.

To fix this regression, add a NULL pointer check for hdmi->conf in the
said function, restoring HDMI functionality and avoiding NULL pointer
kernel panics.

The Linux kernel CVE team has assigned CVE-2021-47108 to this issue.


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

	Issue introduced in 5.14 with commit c91026a938c2 and fixed in 5.15.12 with commit 71d07ebc5000
	Issue introduced in 5.14 with commit c91026a938c2 and fixed in 5.16 with commit 3b8e19a0aa39

Please see https://www.kernel.org or 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-2021-47108
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/mediatek/mtk_hdmi.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/71d07ebc5000b9c1d140e99e7493b0bafa954776
	https://git.kernel.org/stable/c/3b8e19a0aa3933a785be9f1541afd8d398c4ec69