Ack: [Lucid][CVE-2013-4483] ipc/msg: fix race around refcount
brad.figg at canonical.com
Fri May 9 15:13:42 UTC 2014
On 05/09/2014 09:35 AM, Luis Henriques wrote:
> From: Konstantin Khlebnikov <k.khlebnikov at 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);
> (caller locks spinlock)
> expunge_all(msq, -EIDRM);
> ss_wakeup(&msq->q_senders, 1);
> msg_rmid(ns, msq);
> 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
> ("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 at samsung.com>
> Signed-off-by: Ben Hutchings <ben at decadent.org.uk>
> (cherry picked from 91182754daa6ca26dd2e97ee0b0f6e9e37d33324 3.2.y)
> BugLink: http://bugs.launchpad.net/bugs/1248713
> Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
> ipc/msg.c | 2 ++
> 1 file changed, 2 insertions(+)
> diff --git a/ipc/msg.c b/ipc/msg.c
> index 779f762..75075fc 100644
> --- a/ipc/msg.c
> +++ b/ipc/msg.c
> @@ -297,7 +297,9 @@ static void freeque(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp)
> atomic_sub(msq->q_cbytes, &ns->msg_bytes);
> + ipc_lock_by_ptr(&msq->q_perm);
> + ipc_unlock(&msq->q_perm);
More information about the kernel-team