Re: [PATCH v2] usb: gadget: uvc: Fix crash when encoding data for usb request

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Dan,

On Thu, Mar 24, 2022 at 03:23:29PM -0500, Dan Vacura wrote:
> On Thu, Mar 24, 2022 at 04:43:51PM +0200, Laurent Pinchart wrote:
> > On Fri, Mar 18, 2022 at 11:47:06AM -0500, Dan Vacura wrote:
> > > During the uvcg_video_pump() process, if an error occurs and
> > > uvcg_queue_cancel() is called, the buffer queue will be cleared out, but
> > > the current marker (queue->buf_used) of the active buffer (no longer
> > > active) is not reset. On the next iteration of uvcg_video_pump() the
> > > stale buf_used count will be used and the logic of min((unsigned
> > > int)len, buf->bytesused - queue->buf_used) may incorrectly calculate a
> > > nbytes size, causing an invalid memory access.
> > 
> > When uvcg_queue_cancel() is called, it will empty the queue->irqqueue.
> > The next uvcg_video_pump() iteration should thus get a NULL buffer when
> > calling uvcg_queue_head(), and shouldn't proceed to calling
> > video->encode(). Is the issue that the application queues further
> > buffers after cancellation, which puts a new buffer in the irqqueue ?
> 
> Yes, that's exactly what's happening. The application has one thread
> that is receiving camera frames and queuing them to the gadget driver,
> the other thread is waiting for empty buffers to dequeue, generated via
> uvcg_queue_next_buffer(), to send back for camera to fill. Is there a
> requirement to serialize this logic?

Not in userspace, any required serialization should be handled in the
kernel. Unless I'm mistaken we already have a lock to handle concurrent
QBUF and DQBUF calls, so it should be fine.

> In addition, I can check for the
> state of the buffers that have just been dequeued, but not sure what to
> do if there's a failure.

That's a good question, discussed below.

> > I wonder if we need to expand the discussion here to what should be done
> > if an error occurs in uvcg_video_pump(). We currently cancel the queue
> > and drop all queued buffers, but don't prevent more buffers to be
> > queued. Should we force the application to stop streaming in case of
> > error, clean up and restart ? Or are usb_ep_queue() errors expected to
> > happen from time to time, with graceful error recovery a required
> > feature of the gadget driver ?
> 
> Good question, this is out of my expertise, but I can comment about what
> I see in our current setup, qcom snapdragon chipsets with the dwc3
> drivers on 5.10-android. Depending on the host I connect to, the -18
> errors in uvc_video_complete() can occur during normal use, sometimes
> several times in a span of a few seconds. I wasn't seeing usb_eq_queue()
> errors. When the error occurs the device application doesn't do anything
> special and continues queuing subsequent buffers. Sometimes there is
> visible corruption in the received data, but the streaming gracefully
> recovers, mpeg or yuv.

So it looks like we need to handle this condition gracefully. This means
that we shouldn't cancel the queue, but continue with the next transfer,
doesn't it ?

> > > [80802.185460][  T315] configfs-gadget gadget: uvc: VS request completed
> > > with status -18.
> > > [80802.185519][  T315] configfs-gadget gadget: uvc: VS request completed
> > > with status -18.
> > > ...
> > > uvcg_queue_cancel() is called and the queue is cleared out, but the
> > > marker queue->buf_used is not reset.
> > > ...
> > > [80802.262328][ T8682] Unable to handle kernel paging request at virtual
> > > address ffffffc03af9f000
> > > ...
> > > ...
> > > [80802.263138][ T8682] Call trace:
> > > [80802.263146][ T8682]  __memcpy+0x12c/0x180
> > > [80802.263155][ T8682]  uvcg_video_pump+0xcc/0x1e0
> > > [80802.263165][ T8682]  process_one_work+0x2cc/0x568
> > > [80802.263173][ T8682]  worker_thread+0x28c/0x518
> > > [80802.263181][ T8682]  kthread+0x160/0x170
> > > [80802.263188][ T8682]  ret_from_fork+0x10/0x18
> > > [80802.263198][ T8682] Code: a8c12829 a88130cb a8c130
> > > 
> > > Fixes: d692522577c0 ("usb: gadget/uvc: Port UVC webcam gadget to use videobuf2 framework")
> > > Signed-off-by: Dan Vacura <w36195@motorola.com>
> > > 
> > > ---
> > > Changes in v2:
> > > - Add Fixes tag
> > > 
> > >  drivers/usb/gadget/function/uvc_queue.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > > 
> > > diff --git a/drivers/usb/gadget/function/uvc_queue.c b/drivers/usb/gadget/function/uvc_queue.c
> > > index d852ac9e47e7..2cda982f3765 100644
> > > --- a/drivers/usb/gadget/function/uvc_queue.c
> > > +++ b/drivers/usb/gadget/function/uvc_queue.c
> > > @@ -264,6 +264,8 @@ void uvcg_queue_cancel(struct uvc_video_queue *queue, int disconnect)
> > >  		buf->state = UVC_BUF_STATE_ERROR;
> > >  		vb2_buffer_done(&buf->buf.vb2_buf, VB2_BUF_STATE_ERROR);
> > >  	}
> > > +	queue->buf_used = 0;
> > > +
> > >  	/* This must be protected by the irqlock spinlock to avoid race
> > >  	 * conditions between uvc_queue_buffer and the disconnection event that
> > >  	 * could result in an interruptible wait in uvc_dequeue_buffer. Do not

-- 
Regards,

Laurent Pinchart