Re: [PATCH 6/7] v4l2-device: v4l2_device_release_subdev_node can't reference sd

From: Hans Verkuil <hverkuil-cisco@xs4all.nl>
To: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: linux-media@vger.kernel.org, Helen Koike <helen.koike@collabora.com>
Subject: Re: [PATCH 6/7] v4l2-device: v4l2_device_release_subdev_node can't reference sd
Date: Fri, 1 Mar 2019 12:46:52 +0100	[thread overview]
Message-ID: <51113ff6-1213-ee6b-93ab-381a55b5e2e5@xs4all.nl> (raw)
In-Reply-To: <20190222113257.GO3522@pendragon.ideasonboard.com>

On 2/22/19 12:32 PM, Laurent Pinchart wrote:
> Hi Hans,
> 
> Thank you for the patch.
> 
> On Thu, Feb 21, 2019 at 03:21:47PM +0100, Hans Verkuil wrote:
>> When the v4l-subdev device node is released it calls the
>> v4l2_device_release_subdev_node() function which sets sd->devnode
>> to NULL.
>>
>> However, the v4l2_subdev struct may already be released causing this
>> to write in freed memory.
>>
>> Instead just use the regular video_device_release release function
>> (just calls kfree) and set sd->devnode to NULL right after the
>> video_unregister_device() call.
> 
> This seems a bit of a workaround. The devnode can access the subdev in
> multiple ways, it should really keep a reference to the subdev to ensure
> it doesn't get freed early.

It's not the link from the devnode to the subdev (that's done through
video_get_drvdata()), it's the link from the subdev to the devnode.

As soon as the video device is unregistered sd->devnode should be set
to NULL. It is in fact how sd->devnode is used: as a check if the devnode
was registered.

The only other place where it is used is in v4l2_subdev_notify_event to
send an event to the devnode, and after unregistering the video device
you no longer want to do that, so setting sd->devnode to NULL after it
is unregistered is the right thing to do.

FYI, I'll post a v2 of this series soon.

Regards,

	Hans

> 
>> Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
>> ---
>>  drivers/media/v4l2-core/v4l2-device.c | 10 ++--------
>>  1 file changed, 2 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/media/v4l2-core/v4l2-device.c b/drivers/media/v4l2-core/v4l2-device.c
>> index e0ddb9a52bd1..57a7b220fa4d 100644
>> --- a/drivers/media/v4l2-core/v4l2-device.c
>> +++ b/drivers/media/v4l2-core/v4l2-device.c
>> @@ -216,13 +216,6 @@ int v4l2_device_register_subdev(struct v4l2_device *v4l2_dev,
>>  }
>>  EXPORT_SYMBOL_GPL(v4l2_device_register_subdev);
>>  
>> -static void v4l2_device_release_subdev_node(struct video_device *vdev)
>> -{
>> -	struct v4l2_subdev *sd = video_get_drvdata(vdev);
>> -	sd->devnode = NULL;
>> -	kfree(vdev);
>> -}
>> -
>>  int v4l2_device_register_subdev_nodes(struct v4l2_device *v4l2_dev)
>>  {
>>  	struct video_device *vdev;
>> @@ -250,7 +243,7 @@ int v4l2_device_register_subdev_nodes(struct v4l2_device *v4l2_dev)
>>  		vdev->dev_parent = sd->dev;
>>  		vdev->v4l2_dev = v4l2_dev;
>>  		vdev->fops = &v4l2_subdev_fops;
>> -		vdev->release = v4l2_device_release_subdev_node;
>> +		vdev->release = video_device_release;
>>  		vdev->ctrl_handler = sd->ctrl_handler;
>>  		err = __video_register_device(vdev, VFL_TYPE_SUBDEV, -1, 1,
>>  					      sd->owner);
>> @@ -319,6 +312,7 @@ void v4l2_device_unregister_subdev(struct v4l2_subdev *sd)
>>  	}
>>  #endif
>>  	video_unregister_device(sd->devnode);
>> +	sd->devnode = NULL;
>>  	if (!sd->owner_v4l2_dev)
>>  		module_put(sd->owner);
>>  }
> 