Date:   Mon, 17 Aug 2020 13:00:49 +0200
From:   Hans Verkuil <hverkuil-cisco@...all.nl>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Guenter Roeck <linux@...ck-us.net>
Cc:     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 17/08/2020 01:51, Laurent Pinchart wrote:
> Hi Guenter,
> 
> On Sun, Aug 16, 2020 at 08:54:18AM -0700, Guenter Roeck wrote:
>> 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@...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.

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.

> 
> 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.

Regards,

	Hans

> 
>> 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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