diff mbox series

[v2,2/5] drm/bridge/panel: hold a reference to the wrapped panel

Message ID 20260713-drm_refcount_wiring-v2-2-d3bb61f4bd4d@redhat.com (mailing list archive)
State New
Headers
Series drm/panel: refcounting panel lookups and references |

Commit Message

Albert Esteve July 13, 2026, 11:53 a.m. UTC 
drm_panel_bridge_add_typed() stores a pointer to the drm_panel it
wraps, but never acquires a reference to it. If the panel device
goes away while a panel_bridge still exists, the dangling pointer can
be dereferenced through panel_bridge->panel.

Acquire a reference in drm_panel_bridge_add_typed() with drm_panel_get()
and release it in each teardown path.

Reviewed-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Albert Esteve <aesteve@redhat.com>
---
 drivers/gpu/drm/bridge/panel.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

Comments

sashiko-bot@kernel.org July 13, 2026, 12:06 p.m. UTC | #1 
Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:

New issues:
- [High] Dereference of `panel_bridge` after it may have been freed inside `devm_drm_panel_bridge_release()`.

Pre-existing issues:
- [High] `drm_panel_bridge_remove()` accesses `panel->dev` for devres removal, but the underlying `struct device` may have already been freed.
--

--- Patch [2]: [PATCH v2 2/5] drm/bridge/panel: hold a reference to the wrapped panel ---
commit 94f4ef45af3a89d595cd95811619b47c422771c3
Author: Albert Esteve <aesteve@redhat.com>

drm/bridge/panel: hold a reference to the wrapped panel

This patch acquires a reference to the wrapped drm_panel in
drm_panel_bridge_add_typed() using drm_panel_get() and releases it in
the teardown paths to prevent dereferencing dangling pointers.

> diff --git a/drivers/gpu/drm/bridge/panel.c b/drivers/gpu/drm/bridge/panel.c
> index 4978ec98a0828..6b98ad19508df 100644
> --- a/drivers/gpu/drm/bridge/panel.c
> +++ b/drivers/gpu/drm/bridge/panel.c
[ ... ]
> @@ -316,6 +316,7 @@ EXPORT_SYMBOL(drm_panel_bridge_add_typed);
>  void drm_panel_bridge_remove(struct drm_bridge *bridge)
>  {
>  	struct panel_bridge *panel_bridge;
> +	struct drm_panel *panel;
>  
>  	if (!bridge)
>  		return;
> @@ -326,10 +327,12 @@ void drm_panel_bridge_remove(struct drm_bridge *bridge)
>  	}
>  
>  	panel_bridge = drm_bridge_to_panel_bridge(bridge);
> +	panel = panel_bridge->panel;
>  
>  	drm_bridge_remove(bridge);
>  	/* TODO remove this after reworking panel_bridge lifetime */
> -	devm_drm_put_bridge(panel_bridge->panel->dev, bridge);
> +	devm_drm_put_bridge(panel->dev, bridge);

[Severity: High]
This is a pre-existing issue, but can accessing panel->dev here lead to
a use-after-free of the struct device?

While this patch successfully keeps the drm_panel structure alive via
drm_panel_get(), it does not appear to take a get_device() reference to
the underlying struct device (panel->dev). 

If the panel device was completely unregistered, could the struct device
memory be freed while the drm_panel object remains alive, causing
devm_drm_put_bridge() to operate on a dangling pointer?

> +	drm_panel_put(panel);
>  }
>  EXPORT_SYMBOL(drm_panel_bridge_remove);
>  
> @@ -357,11 +360,14 @@ EXPORT_SYMBOL(drm_panel_bridge_set_orientation);
>  static void devm_drm_panel_bridge_release(struct device *dev, void *res)
>  {
>  	struct drm_bridge *bridge = *(struct drm_bridge **)res;
> +	struct panel_bridge *panel_bridge;
>  
>  	if (!bridge)
>  		return;
>  
> +	panel_bridge = drm_bridge_to_panel_bridge(bridge);
>  	drm_bridge_remove(bridge);
> +	drm_panel_put(panel_bridge->panel);

[Severity: High]
Could this result in a use-after-free of the panel_bridge structure?

If the consumer device unbinds, devm_drm_panel_bridge_release() calls
drm_bridge_remove(bridge). If this drops the final reference to the
bridge, it triggers __drm_bridge_free() which frees the panel_bridge
structure.

The very next line calls drm_panel_put(panel_bridge->panel). Does this
dereference the potentially freed panel_bridge pointer?

Perhaps panel_bridge->panel should be cached in a local variable before
calling drm_bridge_remove()?

>  }
>  
>  /**
diff mbox series

Patch

diff --git a/drivers/gpu/drm/bridge/panel.c b/drivers/gpu/drm/bridge/panel.c
index 4978ec98a0828..6b98ad19508df 100644
--- a/drivers/gpu/drm/bridge/panel.c
+++ b/drivers/gpu/drm/bridge/panel.c
@@ -294,7 +294,7 @@  struct drm_bridge *drm_panel_bridge_add_typed(struct drm_panel *panel,
 		return (void *)panel_bridge;
 
 	panel_bridge->connector_type = connector_type;
-	panel_bridge->panel = panel;
+	panel_bridge->panel = drm_panel_get(panel);
 
 	panel_bridge->bridge.of_node = panel->dev->of_node;
 	panel_bridge->bridge.ops = DRM_BRIDGE_OP_MODES;
@@ -316,6 +316,7 @@  EXPORT_SYMBOL(drm_panel_bridge_add_typed);
 void drm_panel_bridge_remove(struct drm_bridge *bridge)
 {
 	struct panel_bridge *panel_bridge;
+	struct drm_panel *panel;
 
 	if (!bridge)
 		return;
@@ -326,10 +327,12 @@  void drm_panel_bridge_remove(struct drm_bridge *bridge)
 	}
 
 	panel_bridge = drm_bridge_to_panel_bridge(bridge);
+	panel = panel_bridge->panel;
 
 	drm_bridge_remove(bridge);
 	/* TODO remove this after reworking panel_bridge lifetime */
-	devm_drm_put_bridge(panel_bridge->panel->dev, bridge);
+	devm_drm_put_bridge(panel->dev, bridge);
+	drm_panel_put(panel);
 }
 EXPORT_SYMBOL(drm_panel_bridge_remove);
 
@@ -357,11 +360,14 @@  EXPORT_SYMBOL(drm_panel_bridge_set_orientation);
 static void devm_drm_panel_bridge_release(struct device *dev, void *res)
 {
 	struct drm_bridge *bridge = *(struct drm_bridge **)res;
+	struct panel_bridge *panel_bridge;
 
 	if (!bridge)
 		return;
 
+	panel_bridge = drm_bridge_to_panel_bridge(bridge);
 	drm_bridge_remove(bridge);
+	drm_panel_put(panel_bridge->panel);
 }
 
 /**