Subject: Re: ipc-msg broken again on 3.11-rc7?

From: Manfred Spraul <[email-masked]>
Date: Tue, 03 Sep 2013 12:16:54 +0200

Hi Vineet,

On 09/03/2013 11:51 AM, Vineet Gupta wrote:
> On 09/03/2013 02:53 PM, Manfred Spraul wrote:
>>
>> The access to msq->q_cbytes is not protected.
>>
>> Vineet, could you try to move the test for free space after ipc_lock?
>> I.e. the lock must not get dropped between testing for free space and
>> enqueueing the messages.
> Hmm, the code movement is not trivial. I broke even the simplest of cases (patch
> attached). This includes the additional change which Linus/Davidlohr had asked for.
The attached patch should work. Could you try it?

--
     Manfred
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();