Re: [PATCH] ALSA: line6: reject oversized playback packets

[Takashi Iwai]

On Sat, 19 Sep 2026 01:40:14 +0200,
Xiang Mei wrote:
> 
> Each playback URB is handed a slice of out.buffer that is exactly
> LINE6_ISO_PACKETS * max_packet_size_out bytes, sized from the OUT
> endpoint, but the number of bytes written into that slice is never
> compared against it.
> 
> submit_audio_out_urb() takes the frame count from prev_fsize, which
> audio_in_callback() derived from the *IN* endpoint's received packet
> length, and rescales it with the playback frame size; when prev_fsize is
> still zero it synthesizes a length from the sample rate instead.  On a
> device declaring a large IN wMaxPacketSize and a small OUT
> wMaxPacketSize, the resulting memcpy(), or the memset() when the
> playback stream is idle, runs past its slice and, as the reproducer
> below shows, beyond the allocation.
> 
> usb_submit_urb() is not a backstop.  max_packet_size_out comes from
> usb_maxpacket(), which returns only the low 11 bits of wMaxPacketSize,
> while USB core validates a high-speed isochronous length against that
> base scaled by usb_endpoint_maxp_mult().  A length of up to three times
> the allocated slice is therefore accepted, and even a rejected URB is
> only rejected after the write.
> 
> Reject a packet that does not fit the slice the driver allocated for it.
> Clamping it instead would silently shorten the capture-derived rate
> feedback and desynchronize the two streams.
> 
>   BUG: KASAN: slab-out-of-bounds in submit_audio_out_urb (sound/usb/line6/playback.c:229)
>   Write of size 1024 at addr ffff888100bbd400 by task kworker/1:2/5002
>   Workqueue: events line6_startup_work
>   Call Trace:
>    __asan_memcpy (mm/kasan/shadow.c:106)
>    submit_audio_out_urb (sound/usb/line6/playback.c:229)
>    line6_submit_audio_out_all_urbs (sound/usb/line6/playback.c:291)
>    line6_stream_start (sound/usb/line6/pcm.c:194)
>    line6_pcm_acquire (sound/usb/line6/pcm.c:337)
>    line6_startup_work (sound/usb/line6/driver.c:728)
>    process_one_work (kernel/workqueue.c:3396)
>    worker_thread (kernel/workqueue.c:3479 kernel/workqueue.c:3560)
>    kthread (kernel/kthread.c:436)
>    ret_from_fork (arch/x86/kernel/process.c:158)
>    ret_from_fork_asm (arch/x86/entry/entry_64.S:245)
> 
>   The buggy address belongs to the object at ffff888100bbd400
>    which belongs to the cache kmalloc-256 of size 256
>   The buggy address is located 0 bytes inside of
>    allocated 256-byte region [ffff888100bbd400, ffff888100bbd500)
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 7a0f55aeeb8f ("ALSA: line6: Support assymetrical in/out configurations")
> Reported-by: <co+be8fa7ea6f77fdce@xxxxxxx>
> Assisted-by: LLM
> Signed-off-by: Xiang Mei <xmei5@xxxxxxx>

Applied now.  Thanks.


Takashi