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authorKonstantin Khlebnikov <k.khlebnikov@samsung.com>2014-03-26 14:12:19 +0400
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2014-03-30 21:40:31 -0700
commit79dd68bfe15ac3da7e76d2db7fdbccc74026e97f (patch)
parentebaacf5c50f552d1bab829451d9bda5eb55fe337 (diff)
ipc/msg: fix race around refcount
[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 <k.khlebnikov@samsung.com>
1 files changed, 2 insertions, 0 deletions
diff --git a/ipc/msg.c b/ipc/msg.c
index 7385de25788a..25f1a6139584 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);
+ ipc_lock_by_ptr(&msq->q_perm);
+ ipc_unlock(&msq->q_perm);