Hi Guenter, On Sun, Aug 30, 2020 at 01:48:24PM -0700, Guenter Roeck wrote: > On 8/30/20 8:58 AM, Laurent Pinchart wrote: > > On Sun, Aug 30, 2020 at 08:04:38AM -0700, Guenter Roeck wrote: > >> 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 this is problem rarely observed in the field, it is relatively easy > >> to reproduce by adding msleep() calls into the code. > >> > >> 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. > > > > I'll review each patch individually, but I think 2/5, 4/5 and 5/5 should > > be handled in the V4L2 core, not the uvcvideo driver. Otherwise we would > > have to replicate that logic in all drivers, while I think it can easily > > be implemented in a generic fashion as previously discussed. > > > The problem is that the v4l2 core already does support locking. There is > a global lock, in struct video_device, a queue lock in struct v4l2_m2m_ctx, > and another queue lock in struct vb2_queue. However, all of those have > to be initialized from the driver. The uvcvideo driver uses its own locks and > does not set the lock pointers in the various generic structures. I was able > to figure out how to use the uvcvideo specific locks in the uvcvideo > driver, but all my attempts to initialize and use the generic locks failed. > > It may well be that the generic code isn't entirely clean - for example > I am not sure if the lock protection in v4l2_open() is complete since > it doesn't handle disconnects after checking if the video device is still > registered (and I don't really see the point of the second video_is_registered() > call in v4l2_open). However, that may just be a lack of understanding on my > side on how the code is supposed to work. Maybe the actual device open function > is expected to have its own protection against underlying hardware removal > and video device unregistration while opening the device. > > [ Regarding the second call to video_is_registered() in v4l2_open(): > Add msleep(5000) between it and the call to the driver open function, > disconnect the device during the sleep, and it will happily call the device > open function on a non-registered video device. That is what patch 5/5 tries > to fix or the uvcvideo driver. > The same problem applies to other file operations in v4l2-dev.c: They all > check if the video device is registered before calling the device > specific code, but I don't really see the point of doing that because > there is no protection against unregistration after the check was made > and before/while the device specific code is running. > Patch 4/5 tries to fix this for the uvcvideo driver. > If that is a bug in the v4l2 code, I'll be happy to work on a fix, > but the only generic fix I could think of would be to utilize the lock in > struct video_device ... but that lock isn't initialized by the uvcvideo > driver. > ] > > Either case, I don't think my understanding of the interaction between > v4l2 and uvcvideo is good enough to make more invasive changes. I _think_ > any generic improvement should start with refactoring the uvcvideo code to > use the v4l2 locking mechanism. However, from the exchange here, my > understanding is that this locking mechanism is not used on purpose. That > means we'll have a uvcvideo specific locking mechanism, period, and I don't > think it is even possible to solve the problem without utilizing this locking > mechanism. > > Of course, it may as well be that I am completely off track and clueless. > After all, the first time I looked into this code was about two weeks ago. > So please bear with me if I talk nonsense. It would be rather impolite to claim you're clueless, given that you managed to write this patch series only two weeks after first looking into the problem :-) I'll try to prototype what I envision would be a good solution in the V4L2 core. If stars align, I may even try to push it one level up, to the chardev layer. Would you then be able to test it ? > >> ---------------------------------------------------------------- > >> 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: In uvc_v4l2_open, check if video device is registered > >> > >> 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(-) -- Regards, Laurent Pinchart
