route: set the deleted fnhe fnhe_daddr to 0 in ip_del_fnhe to fix a race

lxin · davem330 · commit ee60ad219f5c · 2019-03-08T10:50:34.000-08:00
The race occurs in __mkroute_output() when 2 threads lookup a dst: CPU A CPU B find_exception() find_exception() [fnhe expires] ip_del_fnhe() [fnhe is deleted] rt_bind_exception() In rt_bind_exception() it will bind a deleted fnhe with the new dst, and this dst will get no chance to be freed. It causes a dev defcnt leak and consecutive dmesg warnings: unregister_netdevice: waiting for ethX to become free. Usage count = 1 Especially thanks Jon to identify the issue. This patch fixes it by setting fnhe_daddr to 0 in ip_del_fnhe() to stop binding the deleted fnhe with a new dst when checking fnhe's fnhe_daddr and daddr in rt_bind_exception(). It works as both ip_del_fnhe() and rt_bind_exception() are protected by fnhe_lock and the fhne is freed by kfree_rcu(). Fixes: deed49d ("route: check and remove route cache when we get route") Signed-off-by: Jon Maxwell <jmaxwell37@gmail.com> Signed-off-by: Xin Long <lucien.xin@gmail.com> Reviewed-by: David Ahern <dsahern@gmail.com> Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/route.c b/net/ipv4/route.c
@@ -1303,6 +1303,10 @@ static void ip_del_fnhe(struct fib_nh *nh, __be32 daddr)
 		if (fnhe->fnhe_daddr == daddr) {
 			rcu_assign_pointer(*fnhe_p, rcu_dereference_protected(
 				fnhe->fnhe_next, lockdep_is_held(&fnhe_lock)));
+			/* set fnhe_daddr to 0 to ensure it won't bind with
+			 * new dsts in rt_bind_exception().
+			 */
+			fnhe->fnhe_daddr = 0;
 			fnhe_flush_routes(fnhe);
 			kfree_rcu(fnhe, rcu);
 			break;
