[PATCH net] route: set the deleted fnhe fnhe_daddr to 0 in ip_del_fnhe to fix a race

[Xin Long]

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 thinks 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: deed49df7390 ("route: check and remove route cache when we get route")
Signed-off-by: Jon Maxwell <jmaxwel...@gmail.com>
Signed-off-by: Xin Long <lucien....@gmail.com>
---
 net/ipv4/route.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 5163b64..b0eb4fa 100644
--- 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;
-- 
2.1.0