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    Subject[ 16/38] neighbour: Fixed race condition at tbl->nht
    3.0-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Michel Machado <michel@digirati.com.br>

    [ Upstream commit 84338a6c9dbb6ff3de4749864020f8f25d86fc81 ]

    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>
    CC: "David S. Miller" <davem@davemloft.net>
    CC: Eric Dumazet <eric.dumazet@gmail.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    net/core/neighbour.c | 2 ++
    1 file changed, 2 insertions(+)
    --- a/net/core/neighbour.c
    +++ b/net/core/neighbour.c
    @@ -823,6 +823,8 @@ 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



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