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

[Takashi Iwai <tiwai@suse.de>]

On Thu, 06 Nov 2025 12:31:21 +0100,
Lizhi Xu wrote:
> 
> On Thu, 06 Nov 2025 11:02:08 +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@syzkaller.appspotmail.com
> > > Closes: https://syzkaller.appspot.com/bug?extid=bfd77469c8966de076f7
> > > Tested-by: syzbot+bfd77469c8966de076f7@syzkaller.appspotmail.com
> > > Signed-off-by: Lizhi Xu <lizhi.xu@windriver.com>
> > 
> > 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?

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?


thanks,

Takashi