CVE-2026-74682

In the Linux kernel, the following vulnerability has been resolved:

ALSA: usb-audio: fix OOB write on Type II inbound URBs

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 <baul.lee@xbow.com>