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 <[email protected]>
Link: https://lore.kernel.org/all/20251201-thick-jasmine-oarfish-1eceb0@houat/
Signed-off-by: Luca Ceresoli <[email protected]>
---
drivers/gpu/drm/drm_bridge.c | 2 ++
include/drm/drm_bridge.h | 11 +++++++++++
2 files changed, 13 insertions(+)
diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index 64aa69dcf46f..6dcf8f6d3ecf 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -275,6 +275,8 @@ static void __drm_bridge_free(struct kref *kref)
if (bridge->funcs->destroy)
bridge->funcs->destroy(bridge);
+ drm_bridge_put(bridge->next_bridge);
+
kfree(bridge->container);
}
diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
index 8f10d2fd6016..63660256c4f5 100644
--- a/include/drm/drm_bridge.h
+++ b/include/drm/drm_bridge.h
@@ -1278,6 +1278,17 @@ 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 *
--
2.52.0
