Re: [PATCH] ALSA: usb-audio: Prevent urb from writing out of bounds

[Lizhi Xu]

On Thu, 06 Nov 2025 12:49:51 +0100, Takashi Iwai wrote:
> > > > The calculation rule for the actual data length written to the URB's
> > > > transfer buffer differs from that used to allocate the URB's transfer
> > > > buffer, and in this problem, the value used during allocation is smaller.
> > > >
> > > > This ultimately leads to write out-of-bounds errors when writing data to
> > > > the transfer buffer.
> > > >
> > > > To prevent out-of-bounds writes to the transfer buffer, a check between
> > > > the size of the bytes to be written and the size of the allocated bytes
> > > > should be added before performing the write operation.
> > > >
> > > > When the written bytes are too large, -EPIPE is returned instead of
> > > > -EAGAIN, because returning -EAGAIN might result in push back to ready
> > > > list again.
> > > >
> > > > Based on the context of calculating the bytes to be written here, both
> > > > copy_to_urb() and copy_to_urb_quirk() require a check for the size of
> > > > the bytes to be written before execution.
> > > >
> > > > syzbot reported:
> > > > BUG: KASAN: slab-out-of-bounds in copy_to_urb+0x261/0x460 sound/usb/pcm.c:1487
> > > > Write of size 264 at addr ffff88801107b400 by task syz.0.17/5461
> > > >
> > > > Call Trace:
> > > >  copy_to_urb+0x261/0x460 sound/usb/pcm.c:1487
> > > >  prepare_playback_urb+0x953/0x13d0 sound/usb/pcm.c:1611
> > > >
> > > > Reported-by: syzbot+bfd77469c8966de076f7@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > > Closes: https://syzkaller.appspot.com/bug?extid=bfd77469c8966de076f7
> > > > Tested-by: syzbot+bfd77469c8966de076f7@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > > Signed-off-by: Lizhi Xu <lizhi.xu@xxxxxxxxxxxxx>
> > >
> > > I'm afraid that this doesn't address the root cause at all.
> > > The description above sounds plausible, but not pointing to "why".
> > >
> > > The bytes is frames * stride, so the question is why a too large
> > > frames is calculated.  I couldn't have time to check the details, but
> > > there should be rather some weird condition / parameters to trigger
> > > this, and we should check that at first.
> > During debugging, I discovered that the value of ep->packsize[0] is 22,
> > which causes the counts calculated by
> > counts = snd_usb_endpoint_next_packet_size(ep, ctx, i, avail);
> > to be 22, resulting in a frames value of 22 * 6 = 132;
> > Meanwhile, the stride value is 2, which ultimately results in
> > bytes = frames * stride = 132 * 2 = 264;
> > @@ -1241,6 +1252,10 @@ static int data_ep_set_params(struct snd_usb_endpoint *ep)
> > 	u->buffer_size = maxsize * u->packets;
> > 	...
> > 	u->urb->transfer_buffer =
> >                 usb_alloc_coherent(chip->dev, u->buffer_size,
> >                                    GFP_KERNEL, &u->urb->transfer_dma);
> >
> > Here, when calculating u->buffer_size = maxsize * u->packets;
> > maxsize = 9, packets = 6, which results in only 54 bytes allocated to
> > transfer_buffer;
> 
> Hm, so the problem is rather the calculation of the buffer size.
> The size sounds extremely small.  Which parameters (rates, formats,
> etc) are used for achieving this?
rates: 22050
format: 2
channels: 1
/////////////////////////////
stride: 2
packets: 6
data interval: 0
frame_bits: 16
> 
> The calculation of u->buffer_size is a bit complex, as maxsize is
> adjusted in many different ways.  Is it limited due to wMaxPacketSize
> setup?
Yes, it's because the value of ep->maxpacksize is 9 that the maxsize
value is 9.

BR,
Lizhi