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

From: Sasha Levin
Date: Wed Jun 25 2014 - 12:58:58 EST


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@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
>>
>

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