media: cec: fix a deadlock situation

The cec_devnode struct has a lock meant to serialize access
to the fields of this struct. This lock is taken during
device node (un)registration and when opening or releasing a
filehandle to the device node. When the last open filehandle
is closed the cec adapter might be disabled by calling the
adap_enable driver callback with the devnode.lock held.

However, if during that callback a message or event arrives
then the driver will call one of the cec_queue_event()
variants in cec-adap.c, and those will take the same devnode.lock
to walk the open filehandle list.

This obviously causes a deadlock.

This is quite easy to reproduce with the cec-gpio driver since that
uses the cec-pin framework which generated lots of events and uses
a kernel thread for the processing, so when adap_enable is called
the thread is still running and can generate events.

But I suspect that it might also happen with other drivers if an
interrupt arrives signaling e.g. a received message before adap_enable
had a chance to disable the interrupts.

This patch adds a new mutex to serialize access to the fhs list.
When adap_enable() is called the devnode.lock mutex is held, but
not devnode.lock_fhs. The event functions in cec-adap.c will now
use devnode.lock_fhs instead of devnode.lock, ensuring that it is
safe to call those functions from the adap_enable callback.

This specific issue only happens if the last open filehandle is closed
and the physical address is invalid. This is not something that
happens during normal operation, but it does happen when monitoring
CEC traffic (e.g. cec-ctl --monitor) with an unconfigured CEC adapter.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Cc: <stable@vger.kernel.org>  # for v5.13 and up
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

1 files changed, 9 insertions, 2 deletions

diff --git a/include/media/cec.h b/include/media/cec.h
index 208c9613c07e..77346f757036 100644
--- a/include/media/cec.h
+++ b/include/media/cec.h
@@ -26,13 +26,17 @@
  * @dev:	cec device
  * @cdev:	cec character device
  * @minor:	device node minor number
+ * @lock:	lock to serialize open/release and registration
  * @registered:	the device was correctly registered
  * @unregistered: the device was unregistered
+ * @lock_fhs:	lock to control access to @fhs
  * @fhs:	the list of open filehandles (cec_fh)
- * @lock:	lock to control access to this structure
  *
  * This structure represents a cec-related device node.
  *
+ * To add or remove filehandles from @fhs the @lock must be taken first,
+ * followed by @lock_fhs. It is safe to access @fhs if either lock is held.
+ *
  * The @parent is a physical device. It must be set by core or device drivers
  * before registering the node.
  */
@@ -43,10 +47,13 @@ struct cec_devnode {
 
 	/* device info */
 	int minor;
+	/* serialize open/release and registration */
+	struct mutex lock;
 	bool registered;
 	bool unregistered;
+	/* protect access to fhs */
+	struct mutex lock_fhs;
 	struct list_head fhs;
-	struct mutex lock;
 };
 
 struct cec_adapter;