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

[Hans Verkuil]

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@xxxxxxxxxxxxxxxx>
> ---
>  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
> 

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/