net: Do not access possibly freed net_pkt

As the net_pkt might have already been sent by net_if_try_send_data()
function, the pkt might already contain garbage data. So do not try
to access if after that send call but remember the used iface and family
and use them to update the statistics.

The issue was seen with qemu_x86_64 and qemu_cortex_a53 when CONFIG_SMP
was enabled.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>

Author: jukkar

subsys/net/ip/net_core.c

@@ -366,6 +366,8 @@ static inline bool process_multicast(struct net_pkt *pkt)
 
 int net_try_send_data(struct net_pkt *pkt, k_timeout_t timeout)
 {
+	struct net_if *iface;
+	int family;
 	int status;
 	int ret;
 
@@ -439,18 +441,27 @@ int net_try_send_data(struct net_pkt *pkt, k_timeout_t timeout)
 	}
 #endif
 
+	/* The pkt might contain garbage already after the call to
+	 * net_if_try_send_data(), so do not use pkt after that call.
+	 * Remember the iface and family for statistics update.
+	 */
+	if (IS_ENABLED(CONFIG_NET_STATISTICS)) {
+		iface = net_pkt_iface(pkt);
+		family = net_pkt_family(pkt);
+	}
+
 	if (net_if_try_send_data(net_pkt_iface(pkt), pkt, timeout) == NET_DROP) {
 		ret = -EIO;
 		goto err;
 	}
 
 	if (IS_ENABLED(CONFIG_NET_STATISTICS)) {
-		switch (net_pkt_family(pkt)) {
+		switch (family) {
 		case AF_INET:
-			net_stats_update_ipv4_sent(net_pkt_iface(pkt));
+			net_stats_update_ipv4_sent(iface);
 			break;
 		case AF_INET6:
-			net_stats_update_ipv6_sent(net_pkt_iface(pkt));
+			net_stats_update_ipv6_sent(iface);
 			break;
 		}
 	}