net: ipv4: fix infinite loop on secondary addr promotion

secondary address promotion causes infinite loop -- it arranges
for ifa->ifa_next to point back to itself.

Problem is that 'prev_prom' and 'last_prim' might point at the same entry,
so 'last_sec' pointer must be obtained after prev_prom->next update.

Fixes: 2638eb8b ("net: ipv4: provide __rcu annotation for ifa_list")
Reported-by: Ran Rozenstein <ranro@mellanox.com>
Reported-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/devinet.c

@@ -428,8 +428,9 @@ static void __inet_del_ifa(struct in_device *in_dev,
 		if (prev_prom) {
 			struct in_ifaddr *last_sec;
 
-			last_sec = rtnl_dereference(last_prim->ifa_next);
 			rcu_assign_pointer(prev_prom->ifa_next, next_sec);
+
+			last_sec = rtnl_dereference(last_prim->ifa_next);
 			rcu_assign_pointer(promote->ifa_next, last_sec);
 			rcu_assign_pointer(last_prim->ifa_next, promote);
 		}