[PATCH 4.4 132/230] route: set the deleted fnhe fnhe_daddr to 0 in ip_del_fnhe to fix a race

From: Greg Kroah-Hartman
Date: Fri Mar 22 2019 - 07:35:24 EST

4.4-stable review patch. If anyone has any objections, please let me know.


From: Xin Long <lucien.xin@xxxxxxxxx>

[ Upstream commit ee60ad219f5c7c4fb2f047f88037770063ef785f ]

The race occurs in __mkroute_output() when 2 threads lookup a dst:

find_exception() [fnhe expires]
ip_del_fnhe() [fnhe is deleted]

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: deed49df7390 ("route: check and remove route cache when we get route")
Signed-off-by: Jon Maxwell <jmaxwell37@xxxxxxxxx>
Signed-off-by: Xin Long <lucien.xin@xxxxxxxxx>
Reviewed-by: David Ahern <dsahern@xxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
net/ipv4/route.c | 4 ++++
1 file changed, 4 insertions(+)

--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1604,6 +1604,10 @@ static void ip_del_fnhe(struct fib_nh *n
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;
kfree_rcu(fnhe, rcu);