[PATCH v1 net 1/3] ipv6: Flush exception route while replacing nexthop.

[Kuniyuki Iwashima <kuniyu@google.com>]

The following creates a cyclic reference between IPv6 route
and nexthop:

  1. create a nexthop A
  2. create a nexthop group w/ A
  3. create an IPv6 route with the nexthop group
  4. create an exception route w/ A
  5. replace A with a new nexthop B in the nexthop group
  6. remove the IPv6 route
  7. replace B with A in the nexthop group
  8. remove the nexthop group

Then, removing a device tied to the route gets stuck and
triggers the splat below. [0]

At 5., fib6_nh_release_dsts() is called, but it does not
flush exception routes, so the nexthop A continues to grab
the IPv6 route reference.

  nh_A->rt6i_exception_bucket[] -> rt6_ex->rt6i->from

At 6., fib6_purge_rt() calls rt6_flush_exceptions(), but it
only purges exceptions tied to nexthop B.  Also, the route is
unlinked from the nexthop group by list_del_init(&rt->nh_list).

  -> f6i->nh -> nhg->nh_entries[]

At 8., __remove_nexthop_fib() cannot find the IPv6 route, and
remove_nexthop_group() calls list_del_init(&nhge->nh_list) to
unlink the group from the nexthop A's nhe->grp_list.

  nhg->nh_entries[] -> nh_A -> ... -> f6i

Here, the nexthop A is reachable from net->nexthop.devhash[],
but remove_nexthop_from_groups() cannot find the nexthop group,
leaking the whole chain.

Let's call fib6_nh_flush_exceptions() in fib6_nh_release_dsts().

[0]:
unregister_netdevice: waiting for gre6 to become free. Usage count = 4
ref_tracker: netdev@000000009652f190 has 1/3 users at
     dst_alloc+0x14b/0x490
     ip6_rt_cache_alloc+0x27b/0x670
     __ip6_rt_update_pmtu+0x572/0x850
     ip6_tnl_xmit+0x8c5/0x1b70
     __gre6_xmit+0x7af/0xaa0
     ip6gre_tunnel_xmit+0x685/0xdc0
     dev_hard_start_xmit+0x204/0x5a0
     __dev_queue_xmit+0xe8a/0x2550
     ip6_finish_output+0x232/0x570
     ip6_output+0x226/0x3b0
     ip6_send_skb+0x133/0x250
     rawv6_push_pending_frames+0x503/0x6f0
     rawv6_sendmsg+0xdf0/0x1310
     sock_sendmsg_nosec+0xde/0x130
     __sys_sendto+0x27f/0x320
     __x64_sys_sendto+0xd9/0xf0

Fixes: 7bf4796dd099 ("nexthops: add support for replace")
Reported-by: Jason Zhang <jdz@google.com>
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 net/ipv6/route.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 616b86f89f63..fae698c5522b 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -3701,9 +3701,10 @@ void fib6_nh_release(struct fib6_nh *fib6_nh)
 {
 	struct rt6_exception_bucket *bucket;
 
+	fib6_nh_release_dsts(fib6_nh);
+
 	rcu_read_lock();
 
-	fib6_nh_flush_exceptions(fib6_nh, NULL);
 	bucket = fib6_nh_get_excptn_bucket(fib6_nh, NULL);
 	if (bucket) {
 		rcu_assign_pointer(fib6_nh->rt6i_exception_bucket, NULL);
@@ -3712,7 +3713,6 @@ void fib6_nh_release(struct fib6_nh *fib6_nh)
 
 	rcu_read_unlock();
 
-	fib6_nh_release_dsts(fib6_nh);
 	free_percpu(fib6_nh->rt6i_pcpu);
 
 	fib_nh_common_release(&fib6_nh->nh_common);
@@ -3722,6 +3722,8 @@ void fib6_nh_release_dsts(struct fib6_nh *fib6_nh)
 {
 	int cpu;
 
+	fib6_nh_flush_exceptions(fib6_nh, NULL);
+
 	if (!fib6_nh->rt6i_pcpu)
 		return;
 
-- 
2.56.0.317.g454f8302e1-goog