linux-kernel - [PATCH v3 06/22] drm/bridge: add next_bridge pointer to struct drm

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Message-Id: <20251216-drm-bridge-alloc-getput-drm_of_find_bridge-v3-6-b5165fab8058@bootlin.com>
Date: Tue, 16 Dec 2025 18:58:39 +0100
From: Luca Ceresoli <luca.ceresoli@...tlin.com>
To: Andrzej Hajda <andrzej.hajda@...el.com>, 
 Neil Armstrong <neil.armstrong@...aro.org>, Robert Foss <rfoss@...nel.org>, 
 Laurent Pinchart <Laurent.pinchart@...asonboard.com>, 
 Jonas Karlman <jonas@...boo.se>, Jernej Skrabec <jernej.skrabec@...il.com>, 
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, 
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, 
 Jonathan Corbet <corbet@....net>, Alexey Brodkin <abrodkin@...opsys.com>, 
 Phong LE <ple@...libre.com>, Liu Ying <victor.liu@....com>, 
 Shawn Guo <shawnguo@...nel.org>, Sascha Hauer <s.hauer@...gutronix.de>, 
 Pengutronix Kernel Team <kernel@...gutronix.de>, 
 Fabio Estevam <festevam@...il.com>, Kevin Hilman <khilman@...libre.com>, 
 Jerome Brunet <jbrunet@...libre.com>, 
 Martin Blumenstingl <martin.blumenstingl@...glemail.com>
Cc: Hui Pu <Hui.Pu@...ealthcare.com>, 
 Thomas Petazzoni <thomas.petazzoni@...tlin.com>, 
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
 linux-doc@...r.kernel.org, imx@...ts.linux.dev, 
 linux-arm-kernel@...ts.infradead.org, linux-amlogic@...ts.infradead.org, 
 Luca Ceresoli <luca.ceresoli@...tlin.com>
Subject: [PATCH v3 06/22] 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@...nel.org>
Link: https://lore.kernel.org/all/20251201-thick-jasmine-oarfish-1eceb0@houat/
Signed-off-by: Luca Ceresoli <luca.ceresoli@...tlin.com>
---
 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