linux-kernel - Re: [syzbot] [usb?] [media?] possible deadlock in vb2_video_unregister

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Message-ID: <98b81fbf-4905-4b45-a8c0-33e7399ef84a@collabora.com>
Date: Mon, 19 Feb 2024 16:49:47 +0100
From: Benjamin Gaignard <benjamin.gaignard@...labora.com>
To: syzbot <syzbot+3b1d4b3d5f7a358bf9a9@...kaller.appspotmail.com>,
 hdanton@...a.com, hverkuil-cisco@...all.nl, hverkuil@...all.nl,
 laurent.pinchart@...asonboard.com, linux-kernel@...r.kernel.org,
 linux-media@...r.kernel.org, linux-usb@...r.kernel.org,
 m.szyprowski@...sung.com, mchehab@...nel.org,
 syzkaller-bugs@...glegroups.com, tfiga@...omium.org
Subject: Re: [syzbot] [usb?] [media?] possible deadlock in
 vb2_video_unregister_device


Le 19/02/2024 à 14:10, syzbot a écrit :
> syzbot has bisected this issue to:
>
> commit c838530d230bc638d79b78737fc4488ffc28c1ee
> Author: Benjamin Gaignard <benjamin.gaignard@...labora.com>
> Date:   Thu Nov 9 16:34:59 2023 +0000
>
>      media: media videobuf2: Be more flexible on the number of queue stored buffers
>
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=166dc872180000
> start commit:   b401b621758e Linux 6.8-rc5
> git tree:       upstream
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=156dc872180000
> console output: https://syzkaller.appspot.com/x/log.txt?x=116dc872180000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=eff9f3183d0a20dd
> dashboard link: https://syzkaller.appspot.com/bug?extid=3b1d4b3d5f7a358bf9a9
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=13ffaae8180000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13ef909c180000
>
> Reported-by: syzbot+3b1d4b3d5f7a358bf9a9@...kaller.appspotmail.com
> Fixes: c838530d230b ("media: media videobuf2: Be more flexible on the number of queue stored buffers")
>
> For information about bisection process see: https://goo.gl/tpsmEJ#bisection

Hans,
I think the issue occur because of this part of the commit:
@@ -1264,7 +1264,7 @@ void vb2_video_unregister_device(struct video_device *vdev)
          */
         get_device(&vdev->dev);
         video_unregister_device(vdev);
-       if (vdev->queue && vdev->queue->owner) {
+       if (vdev->queue) {
                 struct mutex *lock = vdev->queue->lock ?
                         vdev->queue->lock : vdev->lock;

but I wonder if the correction shouldn't be to remove usbtv->vb2q_lock mutex in usbtv_video_free().

Any opinion ?

Regards,
Benjamin

>