From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: stable@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	patches@lists.linux.dev,
	Huihui Huang <hhhuang@smu.edu.sg>,
	Johannes Berg <johannes.berg@intel.com>
Subject: [PATCH 6.1 073/609] wifi: at76c50x-usb: avoid length underflow in at76_guess_freq()
Date: Mon, 17 Aug 2026 15:26:09 +0200
Message-ID: <20260817132545.955207902@linuxfoundation.org>
In-Reply-To: <20260817132543.039278408@linuxfoundation.org>
References: <20260817132543.039278408@linuxfoundation.org>
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6.1-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Huihui Huang <hhhuang@smu.edu.sg>

commit 61a799ffd1e5a4fd3702d547828b7ff3d161468e upstream.

at76_guess_freq() checks only that the received frame is at least a bare
802.11 header (24 bytes) before subtracting the fixed management-body
offset:

	len -= el_off;

For both beacon and probe response frames, el_off is 36. If the frame is
shorter than el_off, subtracting it causes the calculated IE length to
wrap. The length is eventually passed to cfg80211_find_elem_match() as a
very large unsigned value, so the element walk runs beyond the RX skb.

This path is reached from at76_rx_tasklet() while scanning. If the device
delivers a truncated beacon or probe response, the oversized IE length
causes an out-of-bounds read during scanning.

Skip the IE lookup if the frame does not reach the variable elements,
before subtracting el_off.

Fixes: 1264b951463a ("at76c50x-usb: add driver")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Huihui Huang <hhhuang@smu.edu.sg>
Link: https://patch.msgid.link/20260715140815.1242033-1-hhhuang@smu.edu.sg
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/net/wireless/atmel/at76c50x-usb.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

--- a/drivers/net/wireless/atmel/at76c50x-usb.c
+++ b/drivers/net/wireless/atmel/at76c50x-usb.c
@@ -1527,13 +1527,16 @@ static inline int at76_guess_freq(struct
 
 	if (ieee80211_is_probe_resp(hdr->frame_control)) {
 		el_off = offsetof(struct ieee80211_mgmt, u.probe_resp.variable);
-		el = ((struct ieee80211_mgmt *)hdr)->u.probe_resp.variable;
 	} else if (ieee80211_is_beacon(hdr->frame_control)) {
 		el_off = offsetof(struct ieee80211_mgmt, u.beacon.variable);
-		el = ((struct ieee80211_mgmt *)hdr)->u.beacon.variable;
 	} else {
 		goto exit;
 	}
+
+	if (len < el_off)
+		goto exit;
+
+	el = priv->rx_skb->data + el_off;
 	len -= el_off;
 
 	el = cfg80211_find_ie(WLAN_EID_DS_PARAMS, el, len);