neighbour: Fixed race condition at tbl->nht

When the fixed race condition happens:

1. While function neigh_periodic_work scans the neighbor hash table
pointed by field tbl->nht, it unlocks and locks tbl->lock between
buckets in order to call cond_resched.

2. Assume that function neigh_periodic_work calls cond_resched, that is,
the lock tbl->lock is available, and function neigh_hash_grow runs.

3. Once function neigh_hash_grow finishes, and RCU calls
neigh_hash_free_rcu, the original struct neigh_hash_table that function
neigh_periodic_work was using doesn't exist anymore.

4. Once back at neigh_periodic_work, whenever the old struct
neigh_hash_table is accessed, things can go badly.

Signed-off-by: Michel Machado <michel@digirati.com.br>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index e287346e09343f1f315866fc7e77fb3770d927b0..2a83914b027743fbf047aae18315ab0197bc6e0c 100644 (file)
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -826,6 +826,8 @@ next_elt:
                write_unlock_bh(&tbl->lock);
                cond_resched();
                write_lock_bh(&tbl->lock);
+               nht = rcu_dereference_protected(tbl->nht,
+                                               lockdep_is_held(&tbl->lock));
        }
        /* Cycle through all hash buckets every base_reachable_time/2 ticks.
         * ARP entry timeouts range from 1/2 base_reachable_time to 3/2