Subject: CVE-2024-26909: soc: qcom: pmic_glink_altmode: fix drm bridge use-after-free

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

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

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

soc: qcom: pmic_glink_altmode: fix drm bridge use-after-free

A recent DRM series purporting to simplify support for "transparent
bridges" and handling of probe deferrals ironically exposed a
use-after-free issue on pmic_glink_altmode probe deferral.

This has manifested itself as the display subsystem occasionally failing
to initialise and NULL-pointer dereferences during boot of machines like
the Lenovo ThinkPad X13s.

Specifically, the dp-hpd bridge is currently registered before all
resources have been acquired which means that it can also be
deregistered on probe deferrals.

In the meantime there is a race window where the new aux bridge driver
(or PHY driver previously) may have looked up the dp-hpd bridge and
stored a (non-reference-counted) pointer to the bridge which is about to
be deallocated.

When the display controller is later initialised, this triggers a
use-after-free when attaching the bridges:

	dp -> aux -> dp-hpd (freed)

which may, for example, result in the freed bridge failing to attach:

	[drm:drm_bridge_attach [drm]] *ERROR* failed to attach bridge /soc@0/phy@88eb000 to encoder TMDS-31: -16

or a NULL-pointer dereference:

	Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
	...
	Call trace:
	  drm_bridge_attach+0x70/0x1a8 [drm]
	  drm_aux_bridge_attach+0x24/0x38 [aux_bridge]
	  drm_bridge_attach+0x80/0x1a8 [drm]
	  dp_bridge_init+0xa8/0x15c [msm]
	  msm_dp_modeset_init+0x28/0xc4 [msm]

The DRM bridge implementation is clearly fragile and implicitly built on
the assumption that bridges may never go away. In this case, the fix is
to move the bridge registration in the pmic_glink_altmode driver to
after all resources have been looked up.

Incidentally, with the new dp-hpd bridge implementation, which registers
child devices, this is also a requirement due to a long-standing issue
in driver core that can otherwise lead to a probe deferral loop (see
commit fbc35b45f9f6 ("Add documentation on meaning of -EPROBE_DEFER")).

[DB: slightly fixed commit message by adding the word 'commit']

The Linux kernel CVE team has assigned CVE-2024-26909 to this issue.

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

	Issue introduced in 6.3 with commit 080b4e24852b and fixed in 6.6.23 with commit 2bbd65c6ca56
	Issue introduced in 6.3 with commit 080b4e24852b and fixed in 6.7.11 with commit ef45aa2841e1
	Issue introduced in 6.3 with commit 080b4e24852b and fixed in 6.8 with commit b979f2d50a09

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-2024-26909
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/soc/qcom/pmic_glink_altmode.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/2bbd65c6ca567ed8dbbfc4fb945f57ce64bef342
	https://git.kernel.org/stable/c/ef45aa2841e15b649e5417fe3d4de395fe462781
	https://git.kernel.org/stable/c/b979f2d50a099f3402418d7ff5f26c3952fb08bb