CVE-2026-74638: drm/v3d: Serialize the scheduler timeout handlers

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

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

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

drm/v3d: Serialize the scheduler timeout handlers

V3D exposes several independent hardware queues (BIN, RENDER, TFU and
CSD) but has only a single, global reset. A timeout on any one queue
therefore has to stop, reset and restart the schedulers of every other
queue as well. That makes concurrent timeout handlers unsafe.

`reset_lock` was never able to make them safe, as a driver-side lock can
only cover the driver's &drm_sched_backend_ops.timedout_job callback.
The scheduler handles the timed out job and its pending list around that
callback, outside of the driver's control, so a global reset triggered
by one queue can still interfere with another queue that is in the
middle of handling a timeout of its own.

Consequently, if a reset happens in the CSD queue while a CL-intensive
application is running, the global reset stops and restarts the CL
queue's scheduler while that queue is handling a timeout of its own. As
drm_sched_stop() and drm_sched_start() subtract and add the credits of
every job sitting on the pending list of the scheduler they are called
on, and as the CL queue's handler concurrently takes its job off that
same list and puts it back, the stop and the start no longer see the
same set of jobs. The CL queue is left with more credits in flight than
its limit:

[  327.302739] ------------[ cut here ]------------
[  327.302744] WARNING: CPU: 2 PID: 43 at drivers/gpu/drm/scheduler/sched_main.c:102 drm_sched_run_job_work+0x238/0x4d0 [gpu_sched]
[  327.302884] CPU: 2 UID: 0 PID: 43 Comm: kworker/u16:1 Not tainted 6.18.39-v8-16k+ #3 PREEMPT
[  327.302889] Hardware name: Raspberry Pi 5 Model B Rev 1.0 (DT)
[  327.302893] Workqueue: v3d_bin drm_sched_run_job_work [gpu_sched]
[  327.302984] Call trace:
[  327.302987]  drm_sched_run_job_work+0x238/0x4d0 [gpu_sched] (P)
[  327.302997]  process_scheduled_works+0x180/0x3d0
[  327.303010]  worker_thread+0x268/0x3e8
[  327.303016]  kthread+0x140/0x250
[  327.303022]  ret_from_fork+0x10/0x20
[  327.303031] ---[ end trace 0000000000000000 ]---

>From that point on, the credit count of the CL queue is broken, causing
a complete GPU hang and UI freeze.

The DRM scheduler already provides a mechanism to serialize the timeout
handlers of different schedulers: an ordered workqueue passed as
drm_sched_init()'s @timeout_wq parameter. By default, each scheduler
queues its timeout work on the system workqueue, which runs the handlers
concurrently. Give all of the queues a shared ordered workqueue instead,
as recommended by the DRM scheduler documentation for hardware that has
distinct queues but resets globally.

The Linux kernel CVE team has assigned CVE-2026-74638 to this issue.


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

	Fixed in 6.18.45 with commit 5884851a096d8afcdf91f0e542bac193035183a6
	Fixed in 7.1.9 with commit c22a45817b9c92aa0391db60e2ed467c7e6027d7
	Fixed in 7.2 with commit 4da94744707b27a3ae1197bdd7127da4505dc5b1

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-2026-74638
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/v3d/v3d_drv.h
	drivers/gpu/drm/v3d/v3d_sched.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/5884851a096d8afcdf91f0e542bac193035183a6
	https://git.kernel.org/stable/c/c22a45817b9c92aa0391db60e2ed467c7e6027d7
	https://git.kernel.org/stable/c/4da94744707b27a3ae1197bdd7127da4505dc5b1