On 8/16/20 5:18 AM, Laurent Pinchart wrote: > Hi Guenter, > > CC'ing Hans Verkuil and Sakari Ailus for the discussion about handling > file operations and disconnect in V4L2. > > On Sat, Aug 15, 2020 at 05:33:15PM -0700, Guenter Roeck wrote: >> + linux-uvc-devel@xxxxxxxxxxxxxxxxxxxxx >> + linux-media@xxxxxxxxxxxxxxx >> + laurent.pinchart@xxxxxxxxxxxxxxxx >> >> and changed subject >> >> On Fri, Aug 14, 2020 at 10:07:39PM -0400, Alan Stern wrote: >>> On Fri, Aug 14, 2020 at 04:07:03PM -0700, Guenter Roeck wrote: >>>> Hi all, >>>> >>>> over time, there have been a number of reports of crashes in usb_ifnum_to_if(), >>>> called from usb_hcd_alloc_bandwidth, which is in turn called from usb_set_interface(). >>>> Examples are [1] [2] [3]. A typical backtrace is: >>>> >>>> <3>[ 3489.445468] intel_sst_acpi 808622A8:00: sst: Busy wait failed, cant send this msg >>>> <6>[ 3490.507273] usb 1-4: USB disconnect, device number 3 >>>> <1>[ 3490.516670] BUG: unable to handle kernel NULL pointer dereference at 0000000000000000 >>>> <6>[ 3490.516680] PGD 0 P4D 0 >>>> <4>[ 3490.516687] Oops: 0000 [#1] PREEMPT SMP PTI >>>> <4>[ 3490.516693] CPU: 0 PID: 5633 Comm: V4L2CaptureThre Not tainted 4.19.113-08536-g5d29ca36db06 #1 >>>> <4>[ 3490.516696] Hardware name: GOOGLE Edgar, BIOS Google_Edgar.7287.167.156 03/25/2019 >>>> <4>[ 3490.516706] RIP: 0010:usb_ifnum_to_if+0x29/0x40 >>>> <4>[ 3490.516710] Code: ee 0f 1f 44 00 00 55 48 89 e5 48 8b 8f f8 03 00 00 48 85 c9 74 27 44 0f b6 41 04 4d 85 c0 74 1d 31 ff 48 8b 84 f9 98 00 00 00 <48> 8b 10 0f b6 52 02 39 f2 74 0a 48 ff c7 4c 39 c7 72 e5 31 c0 5d >>>> <4>[ 3490.516714] RSP: 0018:ffffa46f42a47a80 EFLAGS: 00010246 >>>> <4>[ 3490.516718] RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff904a396c9000 >>>> <4>[ 3490.516721] RDX: ffff904a39641320 RSI: 0000000000000001 RDI: 0000000000000000 >>>> <4>[ 3490.516724] RBP: ffffa46f42a47a80 R08: 0000000000000002 R09: 0000000000000000 >>>> <4>[ 3490.516727] R10: 0000000000009975 R11: 0000000000000009 R12: 0000000000000000 >>>> <4>[ 3490.516731] R13: ffff904a396b3800 R14: ffff904a39e88000 R15: 0000000000000000 >>>> <4>[ 3490.516735] FS: 00007f396448e700(0000) GS:ffff904a3ba00000(0000) knlGS:0000000000000000 >>>> <4>[ 3490.516738] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 >>>> <4>[ 3490.516742] CR2: 0000000000000000 CR3: 000000016cb46000 CR4: 00000000001006f0 >>>> <4>[ 3490.516745] Call Trace: >>>> <4>[ 3490.516756] usb_hcd_alloc_bandwidth+0x1ee/0x30f >>>> <4>[ 3490.516762] usb_set_interface+0x1a3/0x2b7 >>>> <4>[ 3490.516773] uvc_video_start_transfer+0x29b/0x4b8 [uvcvideo] >>>> <4>[ 3490.516781] uvc_video_start_streaming+0x91/0xdd [uvcvideo] >>>> <4>[ 3490.516787] uvc_start_streaming+0x28/0x5d [uvcvideo] >>>> <4>[ 3490.516795] vb2_start_streaming+0x61/0x143 [videobuf2_common] >>>> <4>[ 3490.516801] vb2_core_streamon+0xf7/0x10f [videobuf2_common] >>>> <4>[ 3490.516807] uvc_queue_streamon+0x2e/0x41 [uvcvideo] >>>> <4>[ 3490.516814] uvc_ioctl_streamon+0x42/0x5c [uvcvideo] >>>> <4>[ 3490.516820] __video_do_ioctl+0x33d/0x42a >>>> <4>[ 3490.516826] video_usercopy+0x34e/0x5ff >>>> <4>[ 3490.516831] ? video_ioctl2+0x16/0x16 >>>> <4>[ 3490.516837] v4l2_ioctl+0x46/0x53 >>>> <4>[ 3490.516843] do_vfs_ioctl+0x50a/0x76f >>>> <4>[ 3490.516848] ksys_ioctl+0x58/0x83 >>>> <4>[ 3490.516853] __x64_sys_ioctl+0x1a/0x1e >>>> <4>[ 3490.516858] do_syscall_64+0x54/0xde >>>> >>>> I have been able to reproduce the problem on a Chromebook by strategically placing >>>> msleep() calls into usb_set_interface() and usb_disable_device(). Ultimately, the >>>> problem boils down to lack of protection against device removal in usb_set_interface() >>>> [and/or possibly other callers of usb_ifnum_to_if()]. >>>> >>>> Sequence of events is roughly as follows: >>>> >>>> - usb_set_interface() is called and proceeds to some point, possibly to >>>> mutex_lock(hcd->bandwidth_mutex); >>>> - Device removal event is detected, and usb_disable_device() is called >>> >>> At this point all interface drivers get unbound (their disconnect >>> routines are called). >>> >>>> - usb_disable_device() starts removing actconfig data. It has removed >>>> and cleared dev->actconfig->interface[i], but not dev->actconfig >>>> - usb_set_interface() calls usb_hcd_alloc_bandwidth(), which calls >>>> usb_ifnum_to_if() >>>> - In usb_ifnum_to_if(), dev->actconfig is not NULL, but >>>> dev->actconfig->interface[i] is NULL >>>> - crash >>>> >>>> Question is what we can do about this. Checking if dev->state != USB_STATE_NOTATTACHED >>>> in usb_ifnum_to_if() might be a possible approach, but strictly speaking it would >>>> still be racy since there is still no lock against device removal. I have not tried >>>> calling usb_lock_device() in usb_set_interface() - would that possibly be an option ? >>> >>> As far as I know, protecting against these races is the responsibility >>> of the USB interface drivers. They must make sure that their disconnect >>> routines block until all outstanding calls to usb_set_interface return >>> (in fact, until all outstanding device accesses have finished). >>> >>> For instance, in the log extract you showed, it's obvious that the >>> uvc_start_streaming routine was running after the disconnect routine had >>> returned, which looks like a bug in itself: Once the disconnect routine >>> returns, the driver is not supposed to try to access the device at all >>> because some other driver may now be bound to it. >>> >>> We can't just call usb_lock_device from within usb_set_interface, >>> because usb_set_interface is often called with that lock already held. >>> >> I had a closer look into the uvcvideo driver and compared it to other usb >> drivers, including drivers in drivers/media/usb/ which connect to the video >> subsystem. >> >> The usbvideo driver lacks protection against calls to uvc_disconnect() while > > Are you confusing usbvideo and uvcvideo ? Both exist, and uvcvideo would > have been called usbvideo if the former hadn't already been in use. > Yes, sorry :-(. I am not sure how s/uvc/usb/ happened. >> calls into file operations are ongoing. This is pretty widespread, and not >> even limited to file operations (for example, there is a worker which is only >> canceled in uvc_delete, not in ucv_disconnect). The existing protection only >> ensures that no file operations are started after the call to ucv_disconnect, >> but that is insufficient. >> >> Other drivers do have that protection and make sure that no usb operations >> can happen after the disconnect call. >> >> The only remedy I can see is to rework the usbvideo driver and add the >> necessary protections. At first glance, it looks like this may be a >> substantial amount of work. I'd sign up for that, but before I start, >> I would like to get input from the usbvideo community. Is such an effort >> already going on ? If yes, how can I help ? If not, is the problem >> understood and accepted ? Are there any ideas on how to solve it ? > > This is something that has been discussed before, and needs to be solved > in the V4L2 framework itself, not in individual drivers. Not only would > this avoid rolling out the same code manually everywhere (in different > incorrect ways, as races are difficult to solve and implementations are > more often wrong than right), but it will also avoid similar issues for > non-USB devices. > You mean code that ensures that no user-space v4l2 operation is in progress after video_device_unregister / v4l2_device_unregister return ? I agree, that would simplify the necessary changes on the uvc side. I actually came from the other side - I assumed that there is a reason that is not already the case, and that the problem therefore has to be resolved on the driver side. So I guess the next question is: Is this already being addressed on the v4l2 side ? > It shouldn't take more than two flags (to track user-space operations in > progress and disconnection), a spinlock and a wait queue entry. I'm not > sure if someone has already given it a try, and don't recall why this > hasn't been done yet, as it should be fairly straightforward. > > On the UVC side, the work queue probably has to be flushed in > uvc_disconnect(). I'd keep the destroy call in uvc_delete() though. > Please make sure to look for potential race conditions between the URB > completion handler and the .disconnect() handler (they shouldn't be any, > but I haven't checked lately myself). > My current solution for this problem is to call uvc_ctrl_cleanup_device() from uvc_disconnect(), after uvc_unregister_video(). An alternative might be to add a uvc_ctrl_stop_device() function which would just cancel the worker. Thanks, Guenter
