drm/bridge: add next_bridge pointer to struct drm_bridge

Many bridge drivers store a next_bridge pointer in their private data and
use it for attach and sometimes other purposes. This is going to be risky
when bridge hot-unplug is used.

Considering this example scenario:

  1. pipeline: encoder --> bridge A --> bridge B --> bridge C
  2. encoder takes a reference to bridge B
  3. bridge B takes a next_bridge reference to bridge C
  4. encoder calls (bridge B)->b_foo(), which in turns references
     next_bridge, e.g.:

       b_foo() {
           bar(b->next_bridge);
       }

If bridges B and C are removed, bridge C can be freed but B is still
allocated because the encoder holds a reference to B. So when step 4
happens, 'b->next-bridge' would be a use-after-free.

Calling drm_bridge_put() in the B bridge .remove function does not solve
the problem as it leaves a (potentially long) risk window between B removal
and the final deallocation of B. A safe moment to put the B reference is in
__drm_bridge_free(), when the last reference has been put. This can be done
by drivers in the .destroy func. However to avoid the need for so many
drivers to implement a .destroy func, just offer a next_bridge pointer to
all bridges that is automatically put it in __drm_bridge_free(), exactly
when the .destroy func is called.

Suggested-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/all/20251201-thick-jasmine-oarfish-1eceb0@houat/
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Link: https://patch.msgid.link/20251216-drm-bridge-alloc-getput-drm_of_find_bridge-v3-6-b5165fab8058@bootlin.com
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
index 42ec33116d18..3e6cbfa9dc44 100644
--- a/include/drm/drm_bridge.h
+++ b/include/drm/drm_bridge.h
@@ -1279,6 +1279,17 @@ struct drm_bridge {
 	 * @hpd_cb.
 	 */
 	void *hpd_data;
+
+	/**
+	 * @next_bridge: Pointer to the following bridge, automatically put
+	 * when this bridge is freed (i.e. at destroy time). This is for
+	 * drivers needing to store a pointer to the next bridge in the
+	 * chain, and ensures any code still holding a reference to this
+	 * bridge after its removal cannot use-after-free the next
+	 * bridge. Any other bridge pointers stored by the driver must be
+	 * put in the .destroy callback by driver code.
+	 */
+	struct drm_bridge *next_bridge;
 };
 
 static inline struct drm_bridge *