This is a note to let you know that I've just added the patch titled
ipc/msg: fix race around refcount
to the 3.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
ipc-msg-fix-race-around-refcount.patch
and it can be found in the queue-3.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <[email protected]> know about it.
>From [email protected] Fri Mar 28 10:23:19 2014
From: Konstantin Khlebnikov <[email protected]>
Date: Wed, 26 Mar 2014 14:12:19 +0400
Subject: ipc/msg: fix race around refcount
To: [email protected]
Cc: Andrew Morton <[email protected]>, [email protected]
Message-ID: <20140326101218.11221.74072.stgit@buzz>
From: Konstantin Khlebnikov <[email protected]>
[fixed differently in 6062a8dc0517bce23e3c2f7d2fea5e22411269a3 upstream.]
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 <[email protected]>
---
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);
}
/*
Patches currently in stable-queue which might be from [email protected]
are
queue-3.4/ipc-msg-fix-race-around-refcount.patch
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