Re: [PATCH] ALSA: usb-audio: Clamp implicit feedback packet count to URB capacity

[Takashi Iwai]

On Sat, 12 Sep 2026 22:05:30 +0200,
Xiang Mei wrote:
> 
> data_ep_set_params() allocates each data URB for exactly u->packets
> isochronous frames, so urb->iso_frame_desc[] has u->packets slots and
> ctx->packets is the driver's only record of that limit. For an implicit
> feedback sink, snd_usb_queue_pending_output_urbs() overwrites it with the
> sync source's packet count, which is calculated independently from the
> capture endpoint's parameters. When that count is larger,
> prepare_playback_urb() and prepare_silent_urb() can write
> iso_frame_desc[] past the allocation; their existing bounds limit payload
> bytes, not the descriptor index.
> 
> The reproducer uses a high-speed UAC2 device declaring bInterval 1 for
> implicit feedback capture (8 packets) and bInterval 4 for playback
> (1 packet). On the first capture completion after the stream starts, it
> accesses seven descriptors spanning 112 bytes beyond the one-packet URB:
> 
>   BUG: KASAN: slab-out-of-bounds in prepare_playback_urb (sound/usb/pcm.c:1560)
>   Write of size 4 at addr ffff88801e696ad0 by task vhci_rx/178
>    prepare_playback_urb (sound/usb/pcm.c:1560)
>    prepare_outbound_urb (sound/usb/endpoint.c:340)
>    snd_usb_queue_pending_output_urbs (sound/usb/endpoint.c:501)
>    snd_complete_urb (sound/usb/endpoint.c:1834)
>    __usb_hcd_giveback_urb (drivers/usb/core/hcd.c:1657)
>    usb_hcd_giveback_urb (drivers/usb/core/hcd.c:1741)
>    vhci_rx_loop (drivers/usb/usbip/vhci_rx.c:107)
>    kthread (kernel/kthread.c:436)
>   The buggy address belongs to the object at ffff88801e696a00
>    which belongs to the cache kmalloc-256 of size 256
>   The buggy address is located 0 bytes to the right of
>    allocated 208-byte region [ffff88801e696a00, ffff88801e696ad0)
> 
> Record the allocated packet count per endpoint and clamp both the adopted
> count and the packet-size copy to it. Fold the Format Type II delimiter
> into urb_packs before the allocation loop so the recorded limit matches
> every URB.
> 
> Fixes: cf044e441902 ("ALSA: usb-audio: Update the number of packets properly at receiving")
> Reported-by: co+8eacd4fa193b1b28@xxxxxxx
> Closes: https://lore.kernel.org/all/22xPn8drvIUtYgVeQnBiNqXuevOTpBAjepLz%40bugs.sh/
> Cc: stable@xxxxxxxxxxxxxxx
> Assisted-by: Claude:claude-opus-5
> Signed-off-by: Xiang Mei <xmei5@xxxxxxx>

Applied now.  Thanks.


Takashi