[PATCH v2 net-next 9/9] neighbour: Check n->parms->tbl under tbl->lock in ___neigh_create().

Sashiko reported that if ___neigh_create() is called outside of
RCU, it could create a new entry for dev in a different netns
while __dev_change_net_namespace() is running concurrently:

  creator
    ipv6_neigh_create(dev, ...)
      tbl = nd_table(dev_net(dev))   <- old netns
      <preempted>

  __dev_change_net_namespace()
    NETDEV_UNREGISTER
    rcu_barrier()
    dev_net_set(dev, new_net)
    NETDEV_REGISTER                  <- new in6_dev

  creator resumes
    neigh_create(old_tbl, ...)
      ndisc_constructor()
        in6_dev_get(dev)             <- new in6_dev, live parms

Let's check n->parms->tbl under tbl->lock.

Fixes: cda2962b6e2b ("neighbour: Namespacify neigh_tables.")
Reported-by: netdev-bot+sashiko@kernel.org
Closes: https://lore.kernel.org/netdev/179090663305.434549.17214258093385242325@kernel.org/ (local)
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 net/core/neighbour.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 0d883c043956..90335895b415 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c

@@ -690,7 +690,7 @@ ___neigh_create(struct neigh_table *tbl, const void *pkey,
 
 	hash_val = tbl->hash(n->primary_key, dev, nht->hash_rnd) >> (32 - nht->hash_shift);
 
-	if (n->parms->dead) {
+	if (n->parms->dead || n->parms->tbl != tbl) {
 		rc = ERR_PTR(-EINVAL);
 		goto out_tbl_unlock;
 	}

-- 
2.56.0.rc1.315.gc6ed9934b7-goog