Re: [PATCH v7] media: vb2: Take queue or device lock in mmap-related vb2 ioctl handlers

[Sasha Levin <sasha.levin@oracle.com>]

Ping?

On 05/23/2014 09:54 AM, Hans Verkuil wrote:
> Hi Laurent,
> 
> This patch caused a circular locking dependency as reported by Sasha Levin:
> 
> https://lkml.org/lkml/2014/5/5/366
> 
> The reason is that copy_to/from_user is called in video_usercopy() with the
> core lock held. The copy functions can fault which takes the mmap_sem. If it
> was just video_usercopy() then it would be fairly easy to solve this, but
> the copy_to_/from_user functions are also called from read and write and they
> can be used in other unexpected places.
> 
> I'm not sure if vb2_fop_get_unmapped_area() is a problem. I suspect (but I'm
> not sure) that when that one is called the mmap_sem isn't taken, in which
> case taking the lock is fine.
> 
> But taking the lock in vb2_fop_mmap() does cause lockdep problems.
> 
> Ideally I would like to drop taking that lock in vb2_fop_mmap and resolve the
> race condition that it intended to fix in a different way.
> 
> Regards,
> 
> 	Hans
> 
> On 08/06/2013 10:10 PM, Laurent Pinchart wrote:
>> The vb2_fop_mmap() and vb2_fop_get_unmapped_area() functions are plug-in
>> implementation of the mmap() and get_unmapped_area() file operations
>> that calls vb2_mmap() and vb2_get_unmapped_area() on the queue
>> associated with the video device. Neither the
>> vb2_fop_mmap/vb2_fop_get_unmapped_area nor the
>> v4l2_mmap/vb2_get_unmapped_area functions in the V4L2 core take any
>> lock, leading to race conditions between mmap/get_unmapped_area and
>> other buffer-related ioctls such as VIDIOC_REQBUFS.
>>
>> Fix it by taking the queue or device lock around the vb2_mmap() and
>> vb2_get_unmapped_area() calls.
>>
>> Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
>> ---
>>  drivers/media/v4l2-core/videobuf2-core.c | 18 ++++++++++++++++--
>>  1 file changed, 16 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/media/v4l2-core/videobuf2-core.c b/drivers/media/v4l2-core/videobuf2-core.c
>> index 9fc4bab..c9b50c7 100644
>> --- a/drivers/media/v4l2-core/videobuf2-core.c
>> +++ b/drivers/media/v4l2-core/videobuf2-core.c
>> @@ -2578,8 +2578,15 @@ EXPORT_SYMBOL_GPL(vb2_ioctl_expbuf);
>>  int vb2_fop_mmap(struct file *file, struct vm_area_struct *vma)
>>  {
>>  	struct video_device *vdev = video_devdata(file);
>> +	struct mutex *lock = vdev->queue->lock ? vdev->queue->lock : vdev->lock;
>> +	int err;
>>  
>> -	return vb2_mmap(vdev->queue, vma);
>> +	if (lock && mutex_lock_interruptible(lock))
>> +		return -ERESTARTSYS;
>> +	err = vb2_mmap(vdev->queue, vma);
>> +	if (lock)
>> +		mutex_unlock(lock);
>> +	return err;
>>  }
>>  EXPORT_SYMBOL_GPL(vb2_fop_mmap);
>>  
>> @@ -2685,8 +2692,15 @@ unsigned long vb2_fop_get_unmapped_area(struct file *file, unsigned long addr,
>>  		unsigned long len, unsigned long pgoff, unsigned long flags)
>>  {
>>  	struct video_device *vdev = video_devdata(file);
>> +	struct mutex *lock = vdev->queue->lock ? vdev->queue->lock : vdev->lock;
>> +	int ret;
>>  
>> -	return vb2_get_unmapped_area(vdev->queue, addr, len, pgoff, flags);
>> +	if (lock && mutex_lock_interruptible(lock))
>> +		return -ERESTARTSYS;
>> +	ret = vb2_get_unmapped_area(vdev->queue, addr, len, pgoff, flags);
>> +	if (lock)
>> +		mutex_unlock(lock);
>> +	return ret;
>>  }
>>  EXPORT_SYMBOL_GPL(vb2_fop_get_unmapped_area);
>>  #endif
>>
>