[PATCH v3] drm/v3d: Reject empty multisync extension to prevent infinite loop

[Ashutosh Desai <ashutoshdesai993@gmail.com>]

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.

Fixes: 9032d5f633ed ("drm/v3d: Detach job submissions IOCTLs to a new specific file")
Cc: stable@vger.kernel.org
Signed-off-by: Ashutosh Desai <ashutoshdesai993@gmail.com>
---
V2 -> V3: drop depth counter; instead reject empty multisync
          (in_sync_count == 0 && out_sync_count == 0) in
          v3d_get_multisync_submit_deps()
V1 -> V2: change cap from 16 to V3D_MAX_EXTENSIONS (7), add #define

v2: https://lore.kernel.org/dri-devel/20260413055230.3349114-1-ashutoshdesai993@gmail.com/
v1: https://lore.kernel.org/dri-devel/20260410013907.2404175-1-ashutoshdesai993@gmail.com/

 drivers/gpu/drm/v3d/v3d_submit.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/gpu/drm/v3d/v3d_submit.c b/drivers/gpu/drm/v3d/v3d_submit.c
index 18f2bf1fe89f..fc74351efad5 100644
--- a/drivers/gpu/drm/v3d/v3d_submit.c
+++ b/drivers/gpu/drm/v3d/v3d_submit.c
@@ -393,6 +393,11 @@ v3d_get_multisync_submit_deps(struct drm_file *file_priv,
 if (multisync.pad)
  -EINVAL;
 
+if (!multisync.in_sync_count && !multisync.out_sync_count) {
+drm_dbg(&v3d->drm, "Empty multisync extension
");
+return -EINVAL;
+}
+
 ret = v3d_get_multisync_post_deps(file_priv, se, multisync.out_sync_count,
 multisync.out_syncs);
 if (ret)
-- 
2.34.1