Commit bebcb928 authored Sep 03, 2013 by Manfred Spraul Committed by Linus Torvalds Sep 03, 2013

ipc/msg.c: Fix lost wakeup in msgsnd().



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@synopsys.com>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: stable@vger.kernel.org  # for 3.11
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent ec1882a9

ipc/msg.c

+5 −7

Original line number	Diff line number	Diff line
		@@ -680,16 +680,18 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext,
		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, void __user *mtext,
		/* 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, void __user *mtext,
		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();