[PATCH v3 2/2] ALSA: usx2y: Guard the playback copy

[Sinan Turunc <enscrpt@pm.me>]

usx2y_urb_play_prepare() sums the packet lengths of a whole URB, copies
that many frames with a single wrap, and sizes each playback packet from
the capture packet's actual_length. Two assumptions are unchecked: that
the URB fits in the ring buffer, and that a packet fits in the playback
endpoint's maxpacksize, from which tmpbuf is allocated.

Return -EPIPE, as the existing packet-size check does, when the URB is
larger than the buffer or a packet is larger than maxpacksize. The first
case is prevented by the buffer bound in the previous patch and this is
the guard for it; the second depends only on the device.

The first case is reachable on the unpatched driver with ordinary
packets: on 7.3-rc7, a USB/IP device presenting the US-122 interface
layout and sending 48-frame packets, with a 100-frame playback buffer
(two 50-frame periods, inside the stock constraints), KASAN reports:

  BUG: KASAN: out-of-bounds in i_usx2y_urb_complete+0x79b/0x1ac0
  Read of size 18446744073709551280 at addr ffff88800e6b02e0 by task vhci_rx/80

With both patches, the same device and a 200-frame playback buffer is
refused at hw_params with EINVAL, and a 400-frame buffer plays for 30 s
with the position advancing inside the buffer, no KASAN report and
neither guard message. The maxpacksize check is not exercised: the
emulated device reports the same wMaxPacketSize on both endpoints.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Sinan Turunc <enscrpt@pm.me>
---
v3: new patch, split out of the v3 draft; the review of v2 found both
unchecked assumptions. The buffer case is confirmed on the unpatched
driver and the guards tested on 7.3-rc7, see above.

 sound/usb/usx2y/usbusx2yaudio.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/sound/usb/usx2y/usbusx2yaudio.c b/sound/usb/usx2y/usbusx2yaudio.c
index ee224b4dfb0..5f241d45401 100644
--- a/sound/usb/usx2y/usbusx2yaudio.c
+++ b/sound/usb/usx2y/usbusx2yaudio.c
@@ -112,7 +112,8 @@ static int usx2y_urb_play_prepare(struct snd_usx2y_substream *subs,
 		/* calculate the size of a packet */
 		counts = cap_urb->iso_frame_desc[pack].actual_length / usx2y->stride;
 		count += counts;
-		if (counts < 43 || counts > 50) {
+		if (counts < 43 || counts > 50 ||
+		    counts * usx2y->stride > subs->maxpacksize) {
 			dev_err(&usx2y->dev->dev, "%s: should not be here with counts=%i\n",
 				__func__, counts);
 			return -EPIPE;
@@ -125,6 +126,16 @@ static int usx2y_urb_play_prepare(struct snd_usx2y_substream *subs,
 		urb->iso_frame_desc[pack].length = cap_urb->iso_frame_desc[pack].actual_length;
 	}
 	if (atomic_read(&subs->state) >= STATE_PRERUNNING) {
+		/*
+		 * The copy below wraps once, which needs count <= buffer_size.
+		 * snd_usx2y_pcm_open() guarantees it; guard anyway.
+		 */
+		if (unlikely(count > runtime->buffer_size)) {
+			dev_err_ratelimited(&usx2y->dev->dev,
+					    "%s: URB of %i frames exceeds %lu-frame buffer\n",
+					    __func__, count, runtime->buffer_size);
+			return -EPIPE;
+		}
 		if (subs->hwptr + count > runtime->buffer_size) {
 			/* err, the transferred area goes over buffer boundary.
 			 * copy the data to the temp buffer.
-- 
2.56.0