[PATCH 4/4] ipc,msg: prevent race with rmid in msgsnd,msgrcv

[Davidlohr Bueso <davidlohr@hp.com>]

This fixes a race in both msgrcv() and msgsnd() between finding the msg and
actually dealing with the queue, as another thread can delete shmid
underneath us if we are preempted before acquiring the kern_ipc_perm.lock.

Manfred illustrates this nicely:

Assume a preemptible kernel that is preempted just after
> msq = msq_obtain_object_check(ns, msqid)
in do_msgrcv().
The only lock that is held is rcu_read_lock().

Now the other thread processes IPC_RMID.
When the first task is resumed, then it will happily wait for messages on a
deleted queue.

Fix this by checking for if the queue has been deleted after taking the lock.

Reported-by: Manfred Spraul <manfred@colorfullife.com>
Cc: stable@vger.kernel.org # for 3.11
Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
---
 ipc/msg.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/ipc/msg.c b/ipc/msg.c
index 06e8aae..4e7d95a 100644
--- a/ipc/msg.c
+++ b/ipc/msg.c
@@ -690,6 +690,12 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext,
 		if (ipcperms(ns, &msq->q_perm, S_IWUGO))
 			goto out_unlock0;
 
+		/* raced with RMID? */
+		if (msq->q_perm.deleted) {
+			err = -EIDRM;
+			goto out_unlock0;
+		}
+
 		err = security_msg_queue_msgsnd(msq, msg, msgflg);
 		if (err)
 			goto out_unlock0;
@@ -896,6 +902,13 @@ long do_msgrcv(int msqid, void __user *buf, size_t bufsz, long msgtyp, int msgfl
 			goto out_unlock1;
 
 		ipc_lock_object(&msq->q_perm);
+
+		/* raced with RMID? */
+		if (msq->q_perm.deleted) {
+			msg = ERR_PTR(-EIDRM);
+			goto out_unlock0;
+		}
+
 		msg = find_msg(msq, &msgtyp, mode);
 		if (!IS_ERR(msg)) {
 			/*
-- 
1.7.11.7