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

[Xiang Mei]

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>
---
 sound/usb/card.h     |  1 +
 sound/usb/endpoint.c | 12 +++++++-----
 2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/sound/usb/card.h b/sound/usb/card.h
index e34d92d576a2..8299ac241c60 100644
--- a/sound/usb/card.h
+++ b/sound/usb/card.h
@@ -116,6 +116,7 @@ struct snd_usb_endpoint {
 	unsigned int phase;		/* phase accumulator */
 	unsigned int maxpacksize;	/* max packet size in bytes */
 	unsigned int maxframesize;      /* max packet size in frames */
+	unsigned int max_urb_packs;	/* packets allocated per data URB */
 	unsigned int max_urb_frames;	/* max URB size in frames */
 	unsigned int curpacksize;	/* current packet size in bytes (for capture) */
 	unsigned int curframesize;      /* current packet size in frames (for capture) */
diff --git a/sound/usb/endpoint.c b/sound/usb/endpoint.c
index 0835943d7b0e..b72ce1e9bfb1 100644
--- a/sound/usb/endpoint.c
+++ b/sound/usb/endpoint.c
@@ -492,9 +492,10 @@ int snd_usb_queue_pending_output_urbs(struct snd_usb_endpoint *ep,
 
 		/* copy over the length information */
 		if (implicit_fb) {
-			ctx->packets = packet->packets;
+			ctx->packets = min_t(int, packet->packets,
+					     ep->max_urb_packs);
 			memcpy(ctx->packet_size, packet->packet_size,
-			       packet->packets * sizeof(packet->packet_size[0]));
+			       ctx->packets * sizeof(packet->packet_size[0]));
 		}
 
 		/* call the data handler to fill in playback data */
@@ -1242,15 +1243,16 @@ static int data_ep_set_params(struct snd_usb_endpoint *ep)
 			ep->nurbs = min(max_urbs, urbs_per_period * ep->cur_buffer_periods);
 	}
 
+	if (fmt->fmt_type == UAC_FORMAT_TYPE_II)
+		urb_packs++; /* for transfer delimiter */
+	ep->max_urb_packs = urb_packs;
+
 	/* allocate and initialize data urbs */
 	for (i = 0; i < ep->nurbs; i++) {
 		struct snd_urb_ctx *u = &ep->urb[i];
 		u->index = i;
 		u->ep = ep;
 		u->packets = urb_packs;
-
-		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)
-- 
2.43.0