From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx106.postini.com [74.125.245.106]) by kanga.kvack.org (Postfix) with SMTP id F10D46B0032 for ; Tue, 3 Sep 2013 10:00:29 -0400 (EDT) Received: by mail-bk0-f53.google.com with SMTP id d7so2187564bkh.12 for ; Tue, 03 Sep 2013 07:00:28 -0700 (PDT) From: Manfred Spraul Subject: [PATCH] ipc/msg.c: Fix lost wakeup in msgsnd(). Date: Tue, 3 Sep 2013 16:00:08 +0200 Message-Id: <1378216808-2564-1-git-send-email-manfred@colorfullife.com> Sender: owner-linux-mm@kvack.org List-ID: To: Linus Torvalds , Andrew Morton Cc: Davidlohr Bueso , Sedat Dilek , Davidlohr Bueso , linux-next , LKML , Stephen Rothwell , linux-mm , Andi Kleen , Rik van Riel , Jonathan Gonzalez , Vineet Gupta , Manfred Spraul 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() calls first reads all messages from the queue and then sleep, 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-by: Vineet Gupta Signed-off-by: Manfred Spraul --- ipc/msg.c | 12 +++++------- 1 file changed, 5 insertions(+), 7 deletions(-) diff --git a/ipc/msg.c b/ipc/msg.c index 9f29d9e..b65fdf1 100644 --- a/ipc/msg.c +++ b/ipc/msg.c @@ -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(); -- 1.8.3.1 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org