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

[Baul Lee <baul.lee@xbow.com>]

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>

Fixes: 8fdff6a319e7 ("ALSA: snd-usb: implement new endpoint streaming model")
Reported-by: Federico Kirschbaum <federico.kirschbaum@xbow.com>
Reported-by: Baul Lee <baul.lee@xbow.com>
Cc: stable@vger.kernel.org
Signed-off-by: Baul Lee <baul.lee@xbow.com>
---
 sound/usb/endpoint.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/sound/usb/endpoint.c b/sound/usb/endpoint.c
index dca06ba1c67e..a1d449f2a342 100644
--- a/sound/usb/endpoint.c
+++ b/sound/usb/endpoint.c
@@ -385,13 +385,15 @@ static int prepare_inbound_urb(struct snd_usb_endpoint *ep,
 	case SND_USB_ENDPOINT_TYPE_DATA:
 		offs = 0;
 		for (i = 0; i < urb_ctx->packets; i++) {
+			if (offs + ep->curpacksize > urb_ctx->buffer_size)
+				break;
 			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;
+		urb->number_of_packets = i;
 		break;
 
 	case SND_USB_ENDPOINT_TYPE_SYNC:
@@ -1243,10 +1245,10 @@ static int data_ep_set_params(struct snd_usb_endpoint *ep)
 		u->index = i;
 		u->ep = ep;
 		u->packets = urb_packs;
-		u->buffer_size = maxsize * u->packets;
 
 		if (fmt->fmt_type == UAC_FORMAT_TYPE_II)
 			u->packets++; /* for transfer delimiter */
+		u->buffer_size = maxsize * u->packets;
 		u->urb = usb_alloc_urb(u->packets, GFP_KERNEL);
 		if (!u->urb)
 			goto out_of_memory;
-- 
2.50.1 (Apple Git-155)