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    Subject[PATCH 3.2 06/18] ipc/msg: fix race around refcount
    3.2.57-rc1 review patch.  If anyone has any objections, please let me know.

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

    From: Konstantin Khlebnikov <k.khlebnikov@samsung.com>

    In older kernels (before v3.10) ipc_rcu_hdr->refcount was non-atomic int.
    There was possuble double-free bug: do_msgsnd() calls ipc_rcu_putref() under
    msq->q_perm->lock and RCU, while freequeue() calls it while it holds only
    'rw_mutex', so there is no sinchronization between them. Two function
    decrements '2' non-atomically, they both can get '0' as result.

    do_msgsnd() freequeue()

    msq = msg_lock_check(ns, msqid);
    ...
    ipc_rcu_getref(msq);
    msg_unlock(msq);
    schedule();
    (caller locks spinlock)
    expunge_all(msq, -EIDRM);
    ss_wakeup(&msq->q_senders, 1);
    msg_rmid(ns, msq);
    msg_unlock(msq);
    ipc_lock_by_ptr(&msq->q_perm);
    ipc_rcu_putref(msq); ipc_rcu_putref(msq);
    < both may get get --(...)->refcount == 0 >

    This patch locks ipc_lock and RCU around ipc_rcu_putref in freequeue.
    ( RCU protects memory for spin_unlock() )

    Similar bugs might be in other users of ipc_rcu_putref().

    In the mainline this has been fixed in v3.10 indirectly in commmit
    6062a8dc0517bce23e3c2f7d2fea5e22411269a3
    ("ipc,sem: fine grained locking for semtimedop") by Rik van Riel.
    That commit optimized locking and converted refcount into atomic.

    I'm not sure that anybody should care about this bug: it's very-very unlikely
    and no longer exists in actual mainline. I've found this just by looking into
    the code, probably this never happens in real life.

    Signed-off-by: Konstantin Khlebnikov <k.khlebnikov@samsung.com>
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    ipc/msg.c | 2 ++
    1 file changed, 2 insertions(+)

    --- a/ipc/msg.c
    +++ b/ipc/msg.c
    @@ -296,7 +296,9 @@ static void freeque(struct ipc_namespace
    }
    atomic_sub(msq->q_cbytes, &ns->msg_bytes);
    security_msg_queue_free(msq);
    + ipc_lock_by_ptr(&msq->q_perm);
    ipc_rcu_putref(msq);
    + ipc_unlock(&msq->q_perm);
    }

    /*


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