batman-adv: fix skb deref after free

BugLink: https://bugs.launchpad.net/bugs/1868629

commit 63d443ef upstream.

batadv_send_skb_to_orig() calls dev_queue_xmit() so we can't use skb->len.

Fixes: 95332477 ("batman-adv: network coding - buffer unicast packets before forward")
Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <a@unstable.cc>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Kelsey Skunberg <kelsey.skunberg@canonical.com>

net/batman-adv/routing.c

@@ -585,6 +585,7 @@ static int batadv_route_unicast_packet(struct sk_buff *skb,
 	struct batadv_unicast_packet *unicast_packet;
 	struct ethhdr *ethhdr = eth_hdr(skb);
 	int res, hdr_len, ret = NET_RX_DROP;
+	unsigned int len;
 
 	unicast_packet = (struct batadv_unicast_packet *)skb->data;
 
@@ -625,6 +626,7 @@ static int batadv_route_unicast_packet(struct sk_buff *skb,
 	if (hdr_len > 0)
 		batadv_skb_set_priority(skb, hdr_len);
 
+	len = skb->len;
 	res = batadv_send_skb_to_orig(skb, orig_node, recv_if);
 
 	/* translate transmit result into receive result */
@@ -632,7 +634,7 @@ static int batadv_route_unicast_packet(struct sk_buff *skb,
 		/* skb was transmitted and consumed */
 		batadv_inc_counter(bat_priv, BATADV_CNT_FORWARD);
 		batadv_add_counter(bat_priv, BATADV_CNT_FORWARD_BYTES,
-				   skb->len + ETH_HLEN);
+				   len + ETH_HLEN);
 
 		ret = NET_RX_SUCCESS;
 	} else if (res == NET_XMIT_POLICED) {