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    SubjectRe: [PATCH RESEND v3 0/5] media: uvcvideo: Fix race conditions
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    Hi Laurent,

    On 9/17/20 5:47 AM, Laurent Pinchart wrote:
    > Hi Guenter,
    >
    > On Wed, Sep 16, 2020 at 07:25:42PM -0700, Guenter Roeck wrote:
    >> Something seems to have gone wrong with v3 of this patch series.
    >> I am sure I sent it out, but I don't find it anywhere.
    >> Resending. Sorry for any duplicates.
    >
    > I haven't checked the mailing list, but I've found it in my inbox :-)
    > I'm not forgetting about you, just been fairly busy recently. I still
    > plan to try and provide an alternative implementation in the V4L2 core
    > (in a form that I think should even be moved to the cdev core) that
    > would fix this for all drivers.
    >
    Thanks for letting me know. As it turns out, this problem is responsible
    for about 2% of all Chromebook crashes, so I'll probably not wait for
    the series to be accepted upstream but apply it as-is to the various
    ChromeOS kernel branches.

    > By the way, as you managed to get hold of non-UVC webcams, one thing you
    > could try in your tests to make the drivers misbehave is to block on a
    > DQBUF call, and unplug the device at that time. When blocking, DQBUF
    > releases the driver lock (through the vb2ops .wait_prepare() and
    > .wait_finis() operations for drivers based on vb2), so this may allow
    > unregistration to proceed without waiting for userspace calls to
    > complete.
    >

    Good idea. I'll give it a try.

    Thanks,
    Guenter

    >> The uvcvideo code has no lock protection against USB disconnects
    >> while video operations are ongoing. This has resulted in random
    >> error reports, typically pointing to a crash in usb_ifnum_to_if(),
    >> called from usb_hcd_alloc_bandwidth(). A typical traceback is as
    >> follows.
    >>
    >> usb 1-4: USB disconnect, device number 3
    >> BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
    >> PGD 0 P4D 0
    >> Oops: 0000 [#1] PREEMPT SMP PTI
    >> CPU: 0 PID: 5633 Comm: V4L2CaptureThre Not tainted 4.19.113-08536-g5d29ca36db06 #1
    >> Hardware name: GOOGLE Edgar, BIOS Google_Edgar.7287.167.156 03/25/2019
    >> RIP: 0010:usb_ifnum_to_if+0x29/0x40
    >> Code: <...>
    >> RSP: 0018:ffffa46f42a47a80 EFLAGS: 00010246
    >> RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff904a396c9000
    >> RDX: ffff904a39641320 RSI: 0000000000000001 RDI: 0000000000000000
    >> RBP: ffffa46f42a47a80 R08: 0000000000000002 R09: 0000000000000000
    >> R10: 0000000000009975 R11: 0000000000000009 R12: 0000000000000000
    >> R13: ffff904a396b3800 R14: ffff904a39e88000 R15: 0000000000000000
    >> FS: 00007f396448e700(0000) GS:ffff904a3ba00000(0000) knlGS:0000000000000000
    >> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
    >> CR2: 0000000000000000 CR3: 000000016cb46000 CR4: 00000000001006f0
    >> Call Trace:
    >> usb_hcd_alloc_bandwidth+0x1ee/0x30f
    >> usb_set_interface+0x1a3/0x2b7
    >> uvc_video_start_transfer+0x29b/0x4b8 [uvcvideo]
    >> uvc_video_start_streaming+0x91/0xdd [uvcvideo]
    >> uvc_start_streaming+0x28/0x5d [uvcvideo]
    >> vb2_start_streaming+0x61/0x143 [videobuf2_common]
    >> vb2_core_streamon+0xf7/0x10f [videobuf2_common]
    >> uvc_queue_streamon+0x2e/0x41 [uvcvideo]
    >> uvc_ioctl_streamon+0x42/0x5c [uvcvideo]
    >> __video_do_ioctl+0x33d/0x42a
    >> video_usercopy+0x34e/0x5ff
    >> ? video_ioctl2+0x16/0x16
    >> v4l2_ioctl+0x46/0x53
    >> do_vfs_ioctl+0x50a/0x76f
    >> ksys_ioctl+0x58/0x83
    >> __x64_sys_ioctl+0x1a/0x1e
    >> do_syscall_64+0x54/0xde
    >>
    >> While there are not many references to this problem on mailing lists, it is
    >> reported on a regular basis on various Chromebooks (roughly 300 reports
    >> per month). The problem is relatively easy to reproduce by adding msleep()
    >> calls into the code.
    >>
    >> I tried to reproduce the problem with non-uvcvideo webcams, but was
    >> unsuccessful. I was unable to get Philips (pwc) webcams to work. gspca
    >> based webcams don't experience the problem, or at least I was unable to
    >> reproduce it (The gspa driver does not trigger sending USB messages in the
    >> open function, and otherwise uses the locking mechanism provided by the
    >> v4l2/vb2 core).
    >>
    >> I don't presume to claim that I found every issue, but this patch series
    >> should fix at least the major problems.
    >>
    >> The patch series was tested exensively on a Chromebook running chromeos-4.19
    >> and on a Linux system running a v5.8.y based kernel.
    >>
    >> v3:
    >> - In patch 5/5, add missing calls to usb_autopm_put_interface() and kfree()
    >> to failure code path
    >>
    >> v2:
    >> - Added details about problem frequency and testing with non-uvc webcams
    >> to summary
    >> - In patch 4/5, return EPOLLERR instead of -ENODEV on poll errors
    >> - Fix description in patch 5/5
    >>
    >> ----------------------------------------------------------------
    >> Guenter Roeck (5):
    >> media: uvcvideo: Cancel async worker earlier
    >> media: uvcvideo: Lock video streams and queues while unregistering
    >> media: uvcvideo: Release stream queue when unregistering video device
    >> media: uvcvideo: Protect uvc queue file operations against disconnect
    >> media: uvcvideo: Abort uvc_v4l2_open if video device is unregistered
    >>
    >> drivers/media/usb/uvc/uvc_ctrl.c | 11 ++++++----
    >> drivers/media/usb/uvc/uvc_driver.c | 12 ++++++++++
    >> drivers/media/usb/uvc/uvc_queue.c | 32 +++++++++++++++++++++++++--
    >> drivers/media/usb/uvc/uvc_v4l2.c | 45 ++++++++++++++++++++++++++++++++++++--
    >> drivers/media/usb/uvc/uvcvideo.h | 1 +
    >> 5 files changed, 93 insertions(+), 8 deletions(-)
    >

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