3.2.57-rc1 review patch. If anyone has any objections, please let me know.
------------------
From: Konstantin Khlebnikov <k.khlebnikov@xxxxxxxxxxx>
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@xxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
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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