CVE-2026-46314: drm/v3d: Reject empty multisync extension to prevent infinite loop

CVE-2026-46314: drm/v3d: Reject empty multisync extension to prevent infinite loop

[Greg Kroah-Hartman]

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

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

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

drm/v3d: Reject empty multisync extension to prevent infinite loop

v3d_get_extensions() walks a userspace-provided singly-linked list of
ioctl extensions without any bound on the chain length. A local user
can craft a self-referential extension (ext->next == &ext) with zero
in_sync_count and out_sync_count, which bypasses the existing duplicate-
extension guard:

    if (se->in_sync_count || se->out_sync_count)
            return -EINVAL;

The guard never fires because v3d_get_multisync_post_deps() returns
immediately when count is zero, leaving both fields at zero on every
iteration. The result is an infinite loop in kernel context, blocking
the calling thread and pegging a CPU core indefinitely.

Fix this by rejecting a multisync extension where both in_sync_count
and out_sync_count are zero in v3d_get_multisync_submit_deps(). An
empty multisync carries no synchronization information and serves no
useful purpose, so returning -EINVAL for such an extension is the
correct defense against this attack vector.

The Linux kernel CVE team has assigned CVE-2026-46314 to this issue.


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

	Issue introduced in 5.16 with commit e4165ae8304e5ea822fbe5909dd3be5445c058b7 and fixed in 6.18.33 with commit 4fa42a249e8cd6ed17aea04e5695b6e9001f2433
	Issue introduced in 5.16 with commit e4165ae8304e5ea822fbe5909dd3be5445c058b7 and fixed in 7.0.9 with commit 9c5164781cb388d219d8f49fa0f0b04cf86ad544
	Issue introduced in 5.16 with commit e4165ae8304e5ea822fbe5909dd3be5445c058b7 and fixed in 7.1-rc1 with commit fb44d589bf3148e13452185a6e772a7efbf2d684

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-2026-46314
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_submit.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/4fa42a249e8cd6ed17aea04e5695b6e9001f2433
	https://git.kernel.org/stable/c/9c5164781cb388d219d8f49fa0f0b04cf86ad544
	https://git.kernel.org/stable/c/fb44d589bf3148e13452185a6e772a7efbf2d684