[PATCH v10 13/69] drm/bridge: Fix unlocked list access in drm_bridge_attach()

[Cristian Ciocaltea]

drm_bridge_attach() inspects bridge->list to warn about bridges being
attached without a previous drm_bridge_add() call, but it does so
without holding bridge_lock.

The list head can be concurrently written by drm_bridge_add(),
drm_bridge_remove() and __drm_bridge_free() while they operate on
neighbouring entries of bridge_list or bridge_lingering_list, all of
them under bridge_lock.  Hence the unlocked read is a data race, even
though it is unlikely to change the outcome of list_empty() in practice.

Perform the check with bridge_lock held.

Fixes: 76f1a9711b83 ("drm/bridge: add warning for bridges attached without being added")
Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea at collabora.com>
---
 drivers/gpu/drm/drm_bridge.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index 8b8f71c3ccff..9a658decad2a 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -600,8 +600,10 @@ int drm_bridge_attach(struct drm_encoder *encoder, struct drm_bridge *bridge,
 	if (!bridge->container)
 		DRM_WARN("DRM bridge corrupted or not allocated by devm_drm_bridge_alloc()\n");
 
-	if (list_empty(&bridge->list))
-		DRM_WARN("Missing drm_bridge_add() before attach\n");
+	scoped_guard(mutex, &bridge_lock) {
+		if (list_empty(&bridge->list))
+			DRM_WARN("Missing drm_bridge_add() before attach\n");
+	}
 
 	drm_bridge_get(bridge);
 

-- 
2.55.0