FAILED: patch "[PATCH] drm/v3d: Reject empty multisync extension to prevent infinite" failed to apply to 6.6-stable tree

The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x fb44d589bf3148e13452185a6e772a7efbf2d684
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026051531-plexiglas-mumbling-3aa6@gregkh' --subject-prefix 'PATCH 6.6.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From fb44d589bf3148e13452185a6e772a7efbf2d684 Mon Sep 17 00:00:00 2001
From: Ashutosh Desai <[email protected]>
Date: Wed, 15 Apr 2026 05:00:00 +0000
Subject: [PATCH] drm/v3d: Reject empty multisync extension to prevent infinite
loop
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

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.

index 18f2bf1fe89f..fc74351efad5 100644
@@ -393,6 +393,11 @@ v3d_get_multisync_submit_deps(struct drm_file *file_priv,
 	if (multisync.pad)
 		return -EINVAL;
 
+	if (!multisync.in_sync_count && !multisync.out_sync_count) {
+		drm_dbg(&v3d->drm, "Empty multisync extension\n");
+		return -EINVAL;
+	}
+
 	ret = v3d_get_multisync_post_deps(file_priv, se, multisync.out_sync_count,
 					  multisync.out_syncs);
 	if (ret)

From: Ashutosh Desai <[email protected]>

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.

---
 drivers/gpu/drm/v3d/v3d_gem.c | 5 +++++
 1 file changed, 5 insertions(+)

index ef991a9b1c6c..057f9525a8a4 100644
@@ -597,6 +597,11 @@ v3d_get_multisync_submit_deps(struct drm_file *file_priv,
 	if (multisync.pad)
 		return -EINVAL;
 
+	if (!multisync.in_sync_count && !multisync.out_sync_count) {
+		DRM_DEBUG("Empty multisync extension\n");
+		return -EINVAL;
+	}
+
 	ret = v3d_get_multisync_post_deps(file_priv, data, multisync.out_sync_count,
 					  multisync.out_syncs);
 	if (ret)

2.54.0