linux-kernel - [ 002/117] ipc/msg.c: Fix lost wakeup in msgsnd().

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Date:	Tue, 24 Sep 2013 17:17:48 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org, Rik van Riel <riel@...hat.com>,
	Manfred Spraul <manfred@...orfullife.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Sedat Dilek <sedat.dilek@...il.com>
Subject: [ 002/117] ipc/msg.c: Fix lost wakeup in msgsnd().

3.11-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Manfred Spraul <manfred@...orfullife.com>

commit bebcb928c820d0ee83aca4b192adc195e43e66a2 upstream.

The check if the queue is full and adding current to the wait queue of
pending msgsnd() operations (ss_add()) must be atomic.

Otherwise:
 - the thread that performs msgsnd() finds a full queue and decides to
   sleep.
 - the thread that performs msgrcv() first reads all messages from the
   queue and then sleeps, because the queue is empty.
 - the msgrcv() calls do not perform any wakeups, because the msgsnd()
   task has not yet called ss_add().
 - then the msgsnd()-thread first calls ss_add() and then sleeps.

Net result: msgsnd() and msgrcv() both sleep forever.

Observed with msgctl08 from ltp with a preemptible kernel.

Fix: Call ipc_lock_object() before performing the check.

The patch also moves security_msg_queue_msgsnd() under ipc_lock_object:
 - msgctl(IPC_SET) explicitely mentions that it tries to expunge any
   pending operations that are not allowed anymore with the new
   permissions.  If security_msg_queue_msgsnd() is called without locks,
   then there might be races.
 - it makes the patch much simpler.

Reported-and-tested-by: Vineet Gupta <Vineet.Gupta1@...opsys.com>
Acked-by: Rik van Riel <riel@...hat.com>
Signed-off-by: Manfred Spraul <manfred@...orfullife.com>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Sedat Dilek <sedat.dilek@...il.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 ipc/msg.c |   12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

--- a/ipc/msg.c
+++ b/ipc/msg.c
@@ -680,16 +680,18 @@ long do_msgsnd(int msqid, long mtype, vo
 		goto out_unlock1;
 	}
 
+	ipc_lock_object(&msq->q_perm);
+
 	for (;;) {
 		struct msg_sender s;
 
 		err = -EACCES;
 		if (ipcperms(ns, &msq->q_perm, S_IWUGO))
-			goto out_unlock1;
+			goto out_unlock0;
 
 		err = security_msg_queue_msgsnd(msq, msg, msgflg);
 		if (err)
-			goto out_unlock1;
+			goto out_unlock0;
 
 		if (msgsz + msq->q_cbytes <= msq->q_qbytes &&
 				1 + msq->q_qnum <= msq->q_qbytes) {
@@ -699,10 +701,9 @@ long do_msgsnd(int msqid, long mtype, vo
 		/* queue full, wait: */
 		if (msgflg & IPC_NOWAIT) {
 			err = -EAGAIN;
-			goto out_unlock1;
+			goto out_unlock0;
 		}
 
-		ipc_lock_object(&msq->q_perm);
 		ss_add(msq, &s);
 
 		if (!ipc_rcu_getref(msq)) {
@@ -730,10 +731,7 @@ long do_msgsnd(int msqid, long mtype, vo
 			goto out_unlock0;
 		}
 
-		ipc_unlock_object(&msq->q_perm);
 	}
-
-	ipc_lock_object(&msq->q_perm);
 	msq->q_lspid = task_tgid_vnr(current);
 	msq->q_stime = get_seconds();
 


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