Introduce reference counted allocations for panels to avoid
use-after-free. The patch adds the macro devm_drm_bridge_alloc()
to allocate a new refcounted panel. Followed the documentation for
drmm_encoder_alloc() and devm_drm_dev_alloc and other similar
implementations for this purpose.

Also adding drm_panel_get() and drm_panel_put() to suitably
increment and decrement the refcount

Signed-off-by: Anusha Srivatsa <asrivats@redhat.com>
---
 drivers/gpu/drm/drm_panel.c | 50 ++++++++++++++++++++++++++++++++++++++
 include/drm/drm_panel.h     | 58 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 108 insertions(+)

diff --git a/drivers/gpu/drm/drm_panel.c b/drivers/gpu/drm/drm_panel.c
index c627e42a7ce70459f50eb5095fffc806ca45dabf..b55e380e4a2f7ffd940c207e841c197d85113907 100644
--- a/drivers/gpu/drm/drm_panel.c
+++ b/drivers/gpu/drm/drm_panel.c
@@ -79,6 +79,7 @@ EXPORT_SYMBOL(drm_panel_init);
  */
 void drm_panel_add(struct drm_panel *panel)
 {
+	drm_panel_get(panel);
 	mutex_lock(&panel_lock);
 	list_add_tail(&panel->list, &panel_list);
 	mutex_unlock(&panel_lock);
@@ -96,6 +97,7 @@ void drm_panel_remove(struct drm_panel *panel)
 	mutex_lock(&panel_lock);
 	list_del_init(&panel->list);
 	mutex_unlock(&panel_lock);
+	drm_panel_put(panel);
 }
 EXPORT_SYMBOL(drm_panel_remove);
 
@@ -355,6 +357,54 @@ struct drm_panel *of_drm_find_panel(const struct device_node *np)
 }
 EXPORT_SYMBOL(of_drm_find_panel);
 
+/* Internal function (for refcounted panels) */
+void __drm_panel_free(struct kref *kref)
+{
+	struct drm_panel *panel = container_of(kref, struct drm_panel, refcount);
+	void *container = ((void *)panel) - panel->container_offset;
+
+	kfree(container);
+}
+EXPORT_SYMBOL(__drm_panel_free);
+
+static void drm_panel_put_void(void *data)
+{
+	struct drm_panel *panel = (struct drm_panel *)data;
+
+	drm_panel_put(panel);
+}
+
+void *__devm_drm_panel_alloc(struct device *dev, size_t size, size_t offset,
+			     const struct drm_panel_funcs *funcs)
+{
+	void *container;
+	struct drm_panel *panel;
+	int err;
+
+	if (!funcs) {
+		dev_warn(dev, "Missing funcs pointer\n");
+		return ERR_PTR(-EINVAL);
+	}
+
+	container = kzalloc(size, GFP_KERNEL);
+	if (!container)
+		return ERR_PTR(-ENOMEM);
+
+	panel = container + offset;
+	panel->container_offset = offset;
+	panel->funcs = funcs;
+	kref_init(&panel->refcount);
+
+	err = devm_add_action_or_reset(dev, drm_panel_put_void, panel);
+	if (err)
+		return ERR_PTR(err);
+
+	drm_panel_init(panel, dev, funcs, panel->connector_type);
+
+	return container;
+}
+EXPORT_SYMBOL(__devm_drm_panel_alloc);
+
 /**
  * of_drm_get_panel_orientation - look up the orientation of the panel through
  * the "rotation" binding from a device tree node
diff --git a/include/drm/drm_panel.h b/include/drm/drm_panel.h
index a9c042c8dea1a82ef979c7a68204e0b55483fc28..f7cfda0039c066ea2c2b26da5062015e61880971 100644
--- a/include/drm/drm_panel.h
+++ b/include/drm/drm_panel.h
@@ -28,6 +28,7 @@
 #include <linux/errno.h>
 #include <linux/list.h>
 #include <linux/mutex.h>
+#include <linux/kref.h>
 
 struct backlight_device;
 struct dentry;
@@ -266,8 +267,65 @@ struct drm_panel {
 	 * If true then the panel has been enabled.
 	 */
 	bool enabled;
+
+	/**
+	 * @container_offset: Offset of this struct within the container
+	 * struct embedding it. Used for refcounted panels to free the
+	 * embeddeing struct when the refcount drops to zero.
+	 */
+	size_t container_offset;
+	/**
+	 * @refcount: reference count for panels with dynamic lifetime
+	 */
+	struct kref refcount;
 };
 
+void __drm_panel_free(struct kref *kref);
+
+/**
+ * drm_panel_get - Acquire a panel reference
+ * @panel: DRM panel
+ *
+ * This function increments the panel's refcount.
+ *
+ */
+static inline void drm_panel_get(struct drm_panel *panel)
+{
+
+	kref_get(&panel->refcount);
+}
+
+/**
+ * drm_panel_put - Release a panel reference
+ * @panel: DRM panel
+ *
+ * This function decrements the panel's reference count and frees the
+ * object if the reference count drops to zero.
+ */
+static inline void drm_panel_put(struct drm_panel *panel)
+{
+	kref_put(&panel->refcount, __drm_panel_free);
+}
+
+void *__devm_drm_panel_alloc(struct device *dev, size_t size, size_t offset,
+			     const struct drm_panel_funcs *funcs);
+
+/**
+ * devm_drm_panel_alloc - Allocate and initialize an refcounted panel
+ * @dev: struct device of the panel device
+ * @type: the type of the struct which contains struct &drm_panel
+ * @member: the name of the &drm_panel within @type
+ * @funcs: callbacks for this panel
+ *
+ * The returned refcount is initialised to 1
+ *
+ * Returns:
+ * Pointer to new panel, or ERR_PTR on failure.
+ */
+#define devm_drm_panel_alloc(dev, type, member, funcs) \
+	((type *)__devm_drm_panel_alloc(dev, sizeof(type), \
+					 offsetof(type, member), funcs))
+
 void drm_panel_init(struct drm_panel *panel, struct device *dev,
 		    const struct drm_panel_funcs *funcs,
 		    int connector_type);

-- 
2.48.1

Hi Anusha,

In addition to the feedback Luca already provided, I have a few comments

On Wed, Mar 12, 2025 at 08:54:42PM -0400, Anusha Srivatsa wrote:
> Introduce reference counted allocations for panels to avoid
> use-after-free. The patch adds the macro devm_drm_bridge_alloc()
> to allocate a new refcounted panel. Followed the documentation for
> drmm_encoder_alloc() and devm_drm_dev_alloc and other similar
> implementations for this purpose.
> 
> Also adding drm_panel_get() and drm_panel_put() to suitably
> increment and decrement the refcount
> 
> Signed-off-by: Anusha Srivatsa <asrivats@redhat.com>
> ---
>  drivers/gpu/drm/drm_panel.c | 50 ++++++++++++++++++++++++++++++++++++++
>  include/drm/drm_panel.h     | 58 +++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 108 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_panel.c b/drivers/gpu/drm/drm_panel.c
> index c627e42a7ce70459f50eb5095fffc806ca45dabf..b55e380e4a2f7ffd940c207e841c197d85113907 100644
> --- a/drivers/gpu/drm/drm_panel.c
> +++ b/drivers/gpu/drm/drm_panel.c
> @@ -79,6 +79,7 @@ EXPORT_SYMBOL(drm_panel_init);
>   */
>  void drm_panel_add(struct drm_panel *panel)
>  {
> +	drm_panel_get(panel);
>  	mutex_lock(&panel_lock);
>  	list_add_tail(&panel->list, &panel_list);
>  	mutex_unlock(&panel_lock);
> @@ -96,6 +97,7 @@ void drm_panel_remove(struct drm_panel *panel)
>  	mutex_lock(&panel_lock);
>  	list_del_init(&panel->list);
>  	mutex_unlock(&panel_lock);
> +	drm_panel_put(panel);
>  }
>  EXPORT_SYMBOL(drm_panel_remove);

I think these two should be added as a separate patch, with some
additional comment on why it's needed (because we store a pointer in the
panel list).

>  
> @@ -355,6 +357,54 @@ struct drm_panel *of_drm_find_panel(const struct device_node *np)
>  }
>  EXPORT_SYMBOL(of_drm_find_panel);
>  
> +/* Internal function (for refcounted panels) */
> +void __drm_panel_free(struct kref *kref)
> +{
> +	struct drm_panel *panel = container_of(kref, struct drm_panel, refcount);
> +	void *container = ((void *)panel) - panel->container_offset;
> +
> +	kfree(container);
> +}
> +EXPORT_SYMBOL(__drm_panel_free);
> +
> +static void drm_panel_put_void(void *data)
> +{
> +	struct drm_panel *panel = (struct drm_panel *)data;
> +
> +	drm_panel_put(panel);
> +}
> +
> +void *__devm_drm_panel_alloc(struct device *dev, size_t size, size_t offset,
> +			     const struct drm_panel_funcs *funcs)
> +{
> +	void *container;
> +	struct drm_panel *panel;
> +	int err;
> +
> +	if (!funcs) {
> +		dev_warn(dev, "Missing funcs pointer\n");
> +		return ERR_PTR(-EINVAL);
> +	}
> +
> +	container = kzalloc(size, GFP_KERNEL);
> +	if (!container)
> +		return ERR_PTR(-ENOMEM);
> +
> +	panel = container + offset;
> +	panel->container_offset = offset;
> +	panel->funcs = funcs;
> +	kref_init(&panel->refcount);
> +
> +	err = devm_add_action_or_reset(dev, drm_panel_put_void, panel);
> +	if (err)
> +		return ERR_PTR(err);
> +
> +	drm_panel_init(panel, dev, funcs, panel->connector_type);
> +
> +	return container;
> +}
> +EXPORT_SYMBOL(__devm_drm_panel_alloc);

Similarly, here, I think we'd need to split that some more. Ideally, we
should have a series of patches doing

1: Adding that allocation function you have right now, but using
   devm_kzalloc

2: Adding the reference counting to drm_panel, with drm_panel_get /
   drm_panel_put and the devm_action to put the reference in
   __devm_drm_panel_alloc()

3: Adding X patches to add calls to drm_bridge_get/drm_bridge_put
   everywhere it's needed, starting indeed by
   drm_panel_add/drm_panel_put. We don't have to do all of them in that
   series though. of_drm_find_panel though will probably merit a series
   of its own, given we'd have to fix all its callers too.

4: Convert some panels to the new allocation function. You already did
   that with panel_simple so there's nothing to change yet, but once we
   agree on the API we should mass convert all the panels.

Maxime

Hello Anusha,

On Wed, 12 Mar 2025 20:54:42 -0400
Anusha Srivatsa <asrivats@redhat.com> wrote:

> Introduce reference counted allocations for panels to avoid
> use-after-free. The patch adds the macro devm_drm_bridge_alloc()
> to allocate a new refcounted panel. Followed the documentation for
> drmm_encoder_alloc() and devm_drm_dev_alloc and other similar
> implementations for this purpose.
> 
> Also adding drm_panel_get() and drm_panel_put() to suitably
> increment and decrement the refcount
> 
> Signed-off-by: Anusha Srivatsa <asrivats@redhat.com>

I'm very happy to see the very first step of the panel rework mentioned
by Maxime see the light! :-)

This patch looks mostly good to me, and the similarity with my bridge
refcounting work is by itself reassuring.

I have a few notes, one is relevant and the others are minor details,
see below.

In the Subject line: s/allocatons/allocations/

[...]

> +void *__devm_drm_panel_alloc(struct device *dev, size_t size, size_t offset,
> +			     const struct drm_panel_funcs *funcs)
> +{
> +	void *container;
> +	struct drm_panel *panel;
> +	int err;
> +
> +	if (!funcs) {
> +		dev_warn(dev, "Missing funcs pointer\n");
> +		return ERR_PTR(-EINVAL);
> +	}
> +
> +	container = kzalloc(size, GFP_KERNEL);
> +	if (!container)
> +		return ERR_PTR(-ENOMEM);
> +
> +	panel = container + offset;
> +	panel->container_offset = offset;
> +	panel->funcs = funcs;
> +	kref_init(&panel->refcount);
> +
> +	err = devm_add_action_or_reset(dev, drm_panel_put_void, panel);
> +	if (err)
> +		return ERR_PTR(err);
> +
> +	drm_panel_init(panel, dev, funcs, panel->connector_type);

panel->connector_type here is uninitialized. You are passing
panel->connector_type to drm_panel_init(), which will then copy it into
panel->connector_type itself.

> +
> +	/**
> +	 * @container_offset: Offset of this struct within the container
> +	 * struct embedding it. Used for refcounted panels to free the
> +	 * embeddeing struct when the refcount drops to zero.
> +	 */
> +	size_t container_offset;

While storing the offset obviously works, and that's what I had
implemented in my latest bridge refcounting series, after some
discussion with Maxime we agreed storing a container pointer instead of
the offset is cleaner. I think it would be good here as well.

See: https://lore.kernel.org/lkml/20250227-macho-convivial-tody-cea7dc@houat/

> +/**
> + * drm_panel_get - Acquire a panel reference
> + * @panel: DRM panel
> + *
> + * This function increments the panel's refcount.
> + *
> + */
> +static inline void drm_panel_get(struct drm_panel *panel)
> +{
> +

Remove empty line.

> +void *__devm_drm_panel_alloc(struct device *dev, size_t size, size_t offset,
> +			     const struct drm_panel_funcs *funcs);
> +
> +/**
> + * devm_drm_panel_alloc - Allocate and initialize an refcounted panel

s/an/a/ -- same typo as in my bridge series so I'm fixing it in my
series as well :)

> + * @dev: struct device of the panel device
> + * @type: the type of the struct which contains struct &drm_panel
> + * @member: the name of the &drm_panel within @type
> + * @funcs: callbacks for this panel
> + *
> + * The returned refcount is initialised to 1

In my opinion it is important to clarify that the caller does not have
to explicitly call drm_panel_put() on the returned pointer, because
devm will do it. Without clarifying, a user might think they need to,
and that would result in an extra put, which would be a bug.

Adapting from [0], that would be:

 * The returned refcount is initialized to 1. This reference will be
 * automatically dropped via devm (by calling drm_panel_put()) when @dev
 * is removed.

[0] https://lore.kernel.org/lkml/20250206-hotplug-drm-bridge-v6-14-9d6f2c9c3058@bootlin.com/

Luca

-- 
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

On Thu, Mar 13, 2025 at 11:09:44AM +0100, Luca Ceresoli wrote:
> Hello Anusha,
> 
> On Wed, 12 Mar 2025 20:54:42 -0400
> Anusha Srivatsa <asrivats@redhat.com> wrote:
> 
> > Introduce reference counted allocations for panels to avoid
> > use-after-free. The patch adds the macro devm_drm_bridge_alloc()
> > to allocate a new refcounted panel. Followed the documentation for
> > drmm_encoder_alloc() and devm_drm_dev_alloc and other similar
> > implementations for this purpose.
> > 
> > Also adding drm_panel_get() and drm_panel_put() to suitably
> > increment and decrement the refcount
> > 
> > Signed-off-by: Anusha Srivatsa <asrivats@redhat.com>
> 
> I'm very happy to see the very first step of the panel rework mentioned
> by Maxime see the light! :-)
> 
> This patch looks mostly good to me, and the similarity with my bridge
> refcounting work is by itself reassuring.
> 
> I have a few notes, one is relevant and the others are minor details,
> see below.
> 
> In the Subject line: s/allocatons/allocations/
> 
> [...]
> 
> > +void *__devm_drm_panel_alloc(struct device *dev, size_t size, size_t offset,
> > +			     const struct drm_panel_funcs *funcs)
> > +{
> > +	void *container;
> > +	struct drm_panel *panel;
> > +	int err;
> > +
> > +	if (!funcs) {
> > +		dev_warn(dev, "Missing funcs pointer\n");
> > +		return ERR_PTR(-EINVAL);
> > +	}
> > +
> > +	container = kzalloc(size, GFP_KERNEL);
> > +	if (!container)
> > +		return ERR_PTR(-ENOMEM);
> > +
> > +	panel = container + offset;
> > +	panel->container_offset = offset;
> > +	panel->funcs = funcs;
> > +	kref_init(&panel->refcount);
> > +
> > +	err = devm_add_action_or_reset(dev, drm_panel_put_void, panel);
> > +	if (err)
> > +		return ERR_PTR(err);
> > +
> > +	drm_panel_init(panel, dev, funcs, panel->connector_type);
> 
> panel->connector_type here is uninitialized. You are passing
> panel->connector_type to drm_panel_init(), which will then copy it into
> panel->connector_type itself.

Yeah, we need to have the connector type as a parameter.

Maxime