[PATCH net v3] netrom: linearize and validate lengths in nr_rx_frame()

[Stanislav Fort <stanislav.fort@aisle.com>]

Linearize skb and add targeted length checks in nr_rx_frame() to avoid out-of-bounds reads and potential use-after-free when processing malformed NET/ROM frames.

- Linearize skb and require at least NR_NETWORK_LEN + NR_TRANSPORT_LEN (20 bytes) before reading network/transport fields.
- For existing sockets path, ensure NR_CONNACK includes the window byte (>= 21 bytes).
- For CONNREQ handling, ensure window (byte 20) and user address (bytes 21-27) are present (>= 28 bytes).
- Maintain existing BPQ extension handling:
  - NR_CONNACK len == 22 implies 1 extra byte (TTL)
  - NR_CONNREQ len == 37 implies 2 extra bytes (timeout)

Suggested-by: Eric Dumazet <edumazet@google.com>
Reported-by: Stanislav Fort <disclosure@aisle.com>
Signed-off-by: Stanislav Fort <disclosure@aisle.com>
---
 net/netrom/af_netrom.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/net/netrom/af_netrom.c b/net/netrom/af_netrom.c
index 3331669d8e33..f0660dd6d3b0 100644
--- a/net/netrom/af_netrom.c
+++ b/net/netrom/af_netrom.c
@@ -885,6 +885,11 @@ int nr_rx_frame(struct sk_buff *skb, struct net_device *dev)
 	 *	skb->data points to the netrom frame start
 	 */
 
+	if (skb_linearize(skb))
+		return 0;
+	if (skb->len < NR_NETWORK_LEN + NR_TRANSPORT_LEN)
+		return 0;
+
 	src  = (ax25_address *)(skb->data + 0);
 	dest = (ax25_address *)(skb->data + 7);
 
@@ -927,6 +932,11 @@ int nr_rx_frame(struct sk_buff *skb, struct net_device *dev)
 	}
 
 	if (sk != NULL) {
+		if (frametype == NR_CONNACK &&
+		    skb->len < NR_NETWORK_LEN + NR_TRANSPORT_LEN + 1) {
+			sock_put(sk);
+			return 0;
+		}
 		bh_lock_sock(sk);
 		skb_reset_transport_header(skb);
 
@@ -961,10 +971,14 @@ int nr_rx_frame(struct sk_buff *skb, struct net_device *dev)
 		return 0;
 	}
 
-	sk = nr_find_listener(dest);
+	/* Need window (byte 20) and user address (bytes 21-27) */
+	if (skb->len < NR_NETWORK_LEN + NR_TRANSPORT_LEN + 1 + AX25_ADDR_LEN)
+		return 0;
 
 	user = (ax25_address *)(skb->data + 21);
 
+	sk = nr_find_listener(dest);
+
 	if (sk == NULL || sk_acceptq_is_full(sk) ||
 	    (make = nr_make_new(sk)) == NULL) {
 		nr_transmit_refusal(skb, 0);
-- 
2.39.3 (Apple Git-146)