[PATCH v2] wifi: ar5523: validate rxlen against descriptor size and usblen in ar5523_data_rx_cb()

[pip-izony]

From: Seungjin Bae <eeodqql09@xxxxxxxxx>

In ar5523_data_rx_cb(), the `rxlen` variable is derived from desc->len,
which is provided by the USB device.

The function checks for an upper bound (rxlen > ar->rxbufsz) and for a
zero value (!rxlen), but it fails to check for a proper lower bound
against the size of the descriptor.

If a malicious device provides an `rxlen` value that is positive
but smaller than sizeof(struct ar5523_rx_desc), the subtraction in
the call to skb_put() will result in an integer underflow.

This passes a very large unsigned value to skb_put(), which then
triggers a kernel panic upon detecting the potential buffer overflow.

Fix this by adding a check to ensure `rxlen` is at least
sizeof(struct ar5523_rx_desc) before performing the subtraction.

Additionally, `rxlen` is not cross-checked against usblen (the number
of bytes actually received). Since the frame layout is
[ar5523_chunk][802.11 payload][ar5523_rx_desc] and `usblen` covers the
whole buffer, a valid rxlen cannot exceed `usblen - sizeof(chunk)`. A
device reporting a larger `rxlen` would cause skb_put() to expose
descriptor trailer bytes or uninitialized skb tail data as 802.11
payload to ieee80211_rx_irqsafe().

Add a check to reject such frames.

Fixes: b7d572e1871df ("ar5523: Add new driver")
Suggested-by: Jeff Johnson <jeff.johnson@xxxxxxxxxxxxxxxx>
Signed-off-by: Seungjin Bae <eeodqql09@xxxxxxxxx>
---
 v1 -> v2: cross-check rxlen against usblen to prevent overreading past the received payload
 
 drivers/net/wireless/ath/ar5523/ar5523.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/net/wireless/ath/ar5523/ar5523.c b/drivers/net/wireless/ath/ar5523/ar5523.c
index 66ac396858a5..3ee4b020f416 100644
--- a/drivers/net/wireless/ath/ar5523/ar5523.c
+++ b/drivers/net/wireless/ath/ar5523/ar5523.c
@@ -589,6 +589,18 @@ static void ar5523_data_rx_cb(struct urb *urb)
 		goto skip;
 	}
 
+	if (rxlen < sizeof(struct ar5523_rx_desc)) {
+		ar5523_dbg(ar, "RX: Bad descriptor (len=%u is too small)\n",
+			   rxlen);
+		goto skip;
+	}
+
+	if (rxlen > usblen - sizeof(struct ar5523_chunk)) {
+		ar5523_dbg(ar, "RX: rxlen too large (rxlen=%u usblen=%d)\n",
+			   rxlen, usblen);
+		goto skip;
+	}
+
 	skb_reserve(data->skb, sizeof(*chunk));
 	skb_put(data->skb, rxlen - sizeof(struct ar5523_rx_desc));
 
-- 
2.43.0