CVE-2026-72094: dma-buf: dma-fence: Fix potential NULL pointer dereference

CVE-2026-72094: dma-buf: dma-fence: Fix potential NULL pointer dereference

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

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

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

dma-buf: dma-fence: Fix potential NULL pointer dereference

The commit mentioned in the fixes tag below introduced a mechanism
through which fence producers can fully decouple from fence consumers.
This, desirable, mechanism is based on the fence's signaled-bit as the
"decoupling point".

A sophisticated interaction between RCU and atomic instructions attempts
to ensure that fence consumers can still interact with fence producers
through the dma_fence_ops (callback pointers into the producer).

This is the desired behavior: to check for decoupling, the signaled-bit
is first checked. If it's not yet signaled, RCU ensures that the ops
pointer cannot yet be NULL.

Hereby, dma_fence_signal_timestamp_locked() first sets the signaled-bit,
and then sets the ops pointer to NULL. Readers first load the ops
pointer, and then check through the signaled-bit whether the pointer can
legally be accessed.

These set and load operations could occur out of order on weakly ordered
platforms. This problem can be solved very elegantly by using the ops
pointer itself as the synchronization point. The pointer is either NULL,
or cannot become NULL while it is being used thanks to RCU.

Replace the signaled-bit check in dma_fence_timeline_name() and
dma_fence_driver_name().

The Linux kernel CVE team has assigned CVE-2026-72094 to this issue.


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

	Issue introduced in 7.1 with commit f4cc3ab824d6772a48ca9d9c74ac623b3309985d and fixed in 7.1.5 with commit 15ecfdf0ef6f6d874d0a26690d300857b39ebfd0
	Issue introduced in 7.1 with commit f4cc3ab824d6772a48ca9d9c74ac623b3309985d and fixed in 7.2-rc2 with commit 035219a760edb35ae9a9e96beba7f122e26a997b

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-2026-72094
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/dma-buf/dma-fence.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/15ecfdf0ef6f6d874d0a26690d300857b39ebfd0
	https://git.kernel.org/stable/c/035219a760edb35ae9a9e96beba7f122e26a997b