Re: [PATCH < v3.10] ipc/msg: fix race around refcount

["Luís Henriques" <luis.henriques@canonical.com>]

On Wed, Mar 26, 2014 at 02:12:19PM +0400, Konstantin Khlebnikov wrote:
> 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>

Thank you, I'm queuing this for the 3.5 kernel.

Cheers,
--
Lu�s

> ---
>  ipc/msg.c |    2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/ipc/msg.c b/ipc/msg.c
> index 7385de2..25f1a61 100644
> --- a/ipc/msg.c
> +++ b/ipc/msg.c
> @@ -296,7 +296,9 @@ static void freeque(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp)
>  	}
>  	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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