ipv6: Protect nh->f6i_list with spinlock and flag.

We will get rid of RTNL from RTM_NEWROUTE and SIOCADDRT.

Then, we may be going to add a route tied to a dying nexthop.

The nexthop itself is not freed during the RCU grace period, but
if we link a route after __remove_nexthop_fib() is called for the
nexthop, the route will be leaked.

To avoid the race between IPv6 route addition under RCU vs nexthop
deletion under RTNL, let's add a dead flag and protect it and
nh->f6i_list with a spinlock.

__remove_nexthop_fib() acquires the nexthop's spinlock and sets false
to nh->dead, then calls ip6_del_rt() for the linked route one by one
without the spinlock because fib6_purge_rt() acquires it later.

While adding an IPv6 route, fib6_add() acquires the nexthop lock and
checks the dead flag just before inserting the route.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250418000443.43734-15-kuniyu@amazon.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/net/nexthop.h b/include/net/nexthop.h
index d9fb44e8b321..572e69cda476 100644
--- a/include/net/nexthop.h
+++ b/include/net/nexthop.h
@@ -152,6 +152,8 @@ struct nexthop {
 	u8			protocol;   /* app managing this nh */
 	u8			nh_flags;
 	bool			is_group;
+	bool			dead;
+	spinlock_t		lock;       /* protect dead and f6i_list */
 
 	refcount_t		refcnt;
 	struct rcu_head		rcu;