Message-ID: <96cc3d39-ff23-8def-e199-9c4cb22cc02f@xs4all.nl>
Date:   Wed, 19 Aug 2020 09:27:05 +0200
From:   Hans Verkuil <hverkuil-cisco@...all.nl>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     Guenter Roeck <linux@...ck-us.net>,
        linux-usb <linux-usb@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Alan Stern <stern@...land.harvard.edu>,
        linux-media@...r.kernel.org, linux-uvc-devel@...ts.sourceforge.net,
        Sakari Ailus <sakari.ailus@....fi>
Subject: Re: Protecting uvcvideo againt USB device disconnect [Was: Re:
 Protecting usb_set_interface() against device removal]

On 19/08/2020 03:30, Laurent Pinchart wrote:
> Hi Hans,
> 
> On Mon, Aug 17, 2020 at 01:00:49PM +0200, Hans Verkuil wrote:
>> On 17/08/2020 01:51, Laurent Pinchart wrote:
>>> On Sun, Aug 16, 2020 at 08:54:18AM -0700, Guenter Roeck wrote:
>>>> On 8/16/20 5:18 AM, Laurent Pinchart wrote:
>>>>> 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@...ts.sourceforge.net
>>>>>> + linux-media@...r.kernel.org
>>>>>> + laurent.pinchart@...asonboard.com
>>>>>>
>>>>>> 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.
>>>
>>> No worries.
>>>
>>>>>> 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 was thinking about adding a new function to be called from the
>>> disconnect handler to implement the wait on end of userspace access, but
>>> video_device_unregister() seems an even better idea.
>>> v4l2_device_unregister() is probably not very useful as v4l2_device
>>> isn't exposed to userspace, only video_device is (and v4l2_subdev and
>>> media_device, but that's a different story, although probably still an
>>> issue for the latter in the UVC driver).
>>
>> Actually, all that is needed is to take the ioctl serialization lock in the disconnect
>> function.
> 
> It's not just ioctls though, the other file operations also need to be
> handled (read, write, mmap).

Correct. And AFAIK all vb2-based drivers do take that serialization lock in
the file ops.

> 
>> See last paragraph in 1.4.1 here:
>>
>> https://hverkuil.home.xs4all.nl/spec/driver-api/v4l2-dev.html
>>
>> Since uvc uses its own lock, you need to take that one.
> 
> Drivers that use their own lock do so to avoid serializing all ioctls.

Let's agree to disagree :-)

In my experience it is just too hard to keep track of locking and with
very little advantages. You are the only developer that I know of that
insists on doing your own locking. Luckily you are very good at your
job, but everyone else uses the v4l2/vb2 core locking.

> This means that different ioctls may be covered by different locks
> (possibly with part of some ioctls running without locking). I don't
> think we can just dismiss the issue saying those drivers need to
> implement the disconnection manually. It would be much better to
> integrate handling of userspace access with video_device_unregister()
> like proposed above, as that will work for all drivers in a transparent
> way. It would also be fairly simple to implement in the V4L2 core.

I'm not really sure what you want. Should video_unregister_device()
take the core lock (i.e. vdev->lock)?

Regards,

	Hans

> 
>>> We also have a v4l2_device_disconnect() function which is supposed to
>>> handle hot-pluggable device disconnection, but it's fairly useless (I'd
>>> even say harmful as it gives the illusion that hotplugging is correctly
>>> handled, while in reality the media subsystem is plagged by hot-unplug
>>> issues :-S).
>>
>> The v4l2_device_disconnect() is there to remove a v4l2_dev reference to
>> the device that is about to be removed when the disconnect() exists.
>> Otherwise v4l2_dev->dev would point to a missing device.
>>
>> However, I wonder if it is still needed: commit 236c5441d703 from 2011 added
>> code to take a reference to v4l2_dev->dev in v4l2_device_register(). This
>> should prevent the device from disappearing until v4l2_device_unregister() is
>> called. I suspect that v4l2_device_disconnect() can be removed completely, and
>> instead v4l2_device_unregister() just calls put_device(v4l2_dev->dev).
>>
>> I don't like v4l2_device_disconnect() either, so if this works, then that would
>> be a nice simplification.
>>
>>>> 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 ?
>>>
>>> I'm not aware of anyone working on this.
>>>
>>>>> 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().
>>>
>>> I'd rather avoid that, as the cleanup functions in the UVC driver are
>>> generally meant to free memory when the last user disappears. While no
>>> new userspace operation will be started after disconnection once the
>>> above fix will be in place, there's one operation we can't avoid: the
>>> file release. This will access some of the memory allocated by the
>>> driver, and while the current implementation probably doesn't access in
>>> .release() any memory freed by uvc_ctrl_cleanup_device(), I think it's a
>>> good practice to only shut down the userspace API in .disconnect(), and
>>> free memory when the last reference is released.
>>>
>>>> An alternative might
>>>> be to add a uvc_ctrl_stop_device() function which would just cancel the
>>>> worker.
>>>
>>> I think that would be best. Should stream->async_wq (in uvc_video.c) be
>>> similarly flushed ? The driver does so in stream->async_wq(), called
>>> from uvc_video_stop_transfer(), itself called from
>>> uvc_video_stop_streaming() (among other places, that are either error
>>> paths or system suspend handling). The call stack goes to
>>> uvc_stop_streaming(), and, through the videobuf2 helpers, to
>>> vb2_queue_release() called by uvc_queue_release() itself called by
>>> uvc_v4l2_release() (in the non-disconnect case,
>>> uvc_video_stop_streaming() will be called through videobuf2 by
>>> uvc_queue_streamoff(), in response to a VIDIOC_STREAMOFF ioctl). We thus
>>> flush the workqueue too late, and also access the device in
>>> uvc_video_stop_streaming() long after .disconnect() returns.
>>>
>>> I think uvc_video_stop_streaming() could be called in uvc_disconnect()
>>> after uvc_unregister_video().
> 

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