[PATCH v2] ALSA: usb-mixer: us16x08: validate meter packet indices

[Shipei Qu <qu@darknavy.com>]

Hi,

resending with a properly formatted diff (the previous email had a malformed
patch header). The change itself is the same: while fuzzing a USB gadget that
emulates a Tascam US-16x08 we found that get_meter_levels_from_urb() in
mixer_us16x08.c uses a channel index taken directly from the 64-byte meter
packet to index meter_level[], comp_level[] and master_level[] without any
bounds checking. A malformed packet can therefore cause out-of-bounds writes in
the snd_us16x08_meter_store.

A malicious USB audio device (or USB gadget implementation) that pretends to be
a US-16x08-compatible interface can trigger this by sending crafted meter
packets. We have a small USB gadget-based PoC for this behaviour and can share
it if that would be helpful.

This driver is used by common distributions (e.g. Ubuntu) when a US-16x08 or
compatible USB audio device is present. The same pattern is present in current
mainline kernels.

This issue was first reported via security@kernel.org. The kernel security team
explained that, in the upstream threat model, USB endpoints are expected to be
trusted (i.e. only trusted devices should be bound to drivers), so they
consider this a normal bug rather than a security vulnerability, and asked us
to send a fix to the development lists. The patch below adds simple range
checks before updating these arrays.

Reported-by: DARKNAVY (@DarkNavyOrg) <vr@darknavy.com>
Signed-off-by: Shipei Qu <qu@darknavy.com>
---
 sound/usb/mixer_us16x08.c | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/sound/usb/mixer_us16x08.c b/sound/usb/mixer_us16x08.c
index 1c5712c31..f9df40730 100644
--- a/sound/usb/mixer_us16x08.c
+++ b/sound/usb/mixer_us16x08.c
@@ -655,17 +655,25 @@ static void get_meter_levels_from_urb(int s,
 	u8 *meter_urb)
 {
 	int val = MUC2(meter_urb, s) + (MUC3(meter_urb, s) << 8);
+	int ch = MUB2(meter_urb, s) - 1;
+
+	if (ch < 0)
+		return;
 
 	if (MUA0(meter_urb, s) == 0x61 && MUA1(meter_urb, s) == 0x02 &&
 		MUA2(meter_urb, s) == 0x04 && MUB0(meter_urb, s) == 0x62) {
-		if (MUC0(meter_urb, s) == 0x72)
-			store->meter_level[MUB2(meter_urb, s) - 1] = val;
-		if (MUC0(meter_urb, s) == 0xb2)
-			store->comp_level[MUB2(meter_urb, s) - 1] = val;
+		if (ch < SND_US16X08_MAX_CHANNELS) {
+			if (MUC0(meter_urb, s) == 0x72)
+				store->meter_level[ch] = val;
+			if (MUC0(meter_urb, s) == 0xb2)
+				store->comp_level[ch] = val;
+		}
 	}
 	if (MUA0(meter_urb, s) == 0x61 && MUA1(meter_urb, s) == 0x02 &&
-		MUA2(meter_urb, s) == 0x02 && MUB0(meter_urb, s) == 0x62)
-		store->master_level[MUB2(meter_urb, s) - 1] = val;
+		MUA2(meter_urb, s) == 0x02 && MUB0(meter_urb, s) == 0x62) {
+		if (ch < ARRAY_SIZE(store->master_level))
+			store->master_level[ch] = val;
+	}
 }
 
 /* Function to retrieve current meter values from the device.
-- 
2.45.1