Re: [PATCH] ALSA: usb-audio: fix OOB write on Type II inbound URBs

On Wed, 05 Aug 2026 03:34:41 +0200,
Baul Lee wrote:
> 
> data_ep_set_params() sizes each URB transfer buffer before it adds the
> Format Type II transfer delimiter:
> 
> 	u->packets = urb_packs;
> 	u->buffer_size = maxsize * u->packets;
> 
> 	if (fmt->fmt_type == UAC_FORMAT_TYPE_II)
> 		u->packets++; /* for transfer delimiter */
> 	u->urb = usb_alloc_urb(u->packets, GFP_KERNEL);
> 
> buffer_size is computed from the pre-increment packet count and never
> recomputed, so for a Type II endpoint the buffer is one packet short of
> the packet count the URB is built with.
> 
> prepare_inbound_urb() then lays out one iso frame per packet and never
> consults buffer_size:
> 
> 	offs = 0;
> 	for (i = 0; i < urb_ctx->packets; i++) {
> 		urb->iso_frame_desc[i].offset = offs;
> 		urb->iso_frame_desc[i].length = ep->curpacksize;
> 		offs += ep->curpacksize;
> 	}
> 
> 	urb->transfer_buffer_length = offs;
> 	urb->number_of_packets = urb_ctx->packets;
> 
> The last descriptor therefore points one packet past the end of the
> transfer buffer, where the host controller writes device data on every
> inbound transfer.  prepare_silent_urb() and prepare_playback_urb() bound
> their fill loops by ctx->buffer_size, so only capture is affected.
> 
> fmt_type comes from the device's audio streaming descriptors, so any
> device advertising a Type II capture format hits this once userspace sets
> hw_params on the stream.
> 
> KASAN on 7.2.0-rc5 (arm64) with a dummy_hcd/raw-gadget device, one report
> per inbound transfer:
> 
>   BUG: KASAN: slab-out-of-bounds in dummy_timer
>   Write of size 64 at addr ffff0000186171c0 by task cons02/166
>    __asan_memcpy
>    dummy_timer
>    hrtimer_run_softirq
>   Allocated by task 166:
>    usb_alloc_coherent
>    snd_usb_endpoint_set_params
>   The buggy address is located 0 bytes to the right of
>    allocated 64-byte region [ffff000018617180, ffff0000186171c0)
> 
> Compute buffer_size after the delimiter packet has been accounted for,
> and bound the fill loop by buffer_size, as prepare_silent_urb() already
> does on the outbound side.  This grows every Type II URB allocation by
> one maxsize packet.
> 
> Discovered by XBOW, triaged by Baul Lee <[email protected]>
> 
> Fixes: 8fdff6a319e7 ("ALSA: snd-usb: implement new endpoint streaming model")
> Reported-by: Federico Kirschbaum <[email protected]>
> Reported-by: Baul Lee <[email protected]>
> Cc: [email protected]
> Signed-off-by: Baul Lee <[email protected]>

Applied now.  Thanks.


Takashi