linux-kernel - [PATCH 4/5] ipc/msg: Lockless security checks for msgsnd

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Date:	Thu, 28 Jul 2016 16:33:38 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	akpm@...ux-foundation.org
Cc:	manfred@...orfullife.com, bigeasy@...utronix.de,
	peterz@...radead.org, tglx@...utronix.de, dave@...olabs.net,
	linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>
Subject: [PATCH 4/5] ipc/msg: Lockless security checks for msgsnd

Just as with msgrcv (along with the rest of sysvipc since a few
years ago), perform the security checks without holding the ipc
object lock. This also reduces the hogging of the lock for the
entire duration of a sender, as we drop the lock upon every
iteration -- and this is exactly why we also check for racing
with RMID in the first place.

Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
---
 ipc/msg.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/ipc/msg.c b/ipc/msg.c
index 5181259e2ff0..fe793304dddb 100644
--- a/ipc/msg.c
+++ b/ipc/msg.c
@@ -623,14 +623,14 @@ 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_unlock0;
+			goto out_unlock1;
+
+		ipc_lock_object(&msq->q_perm);
 
 		/* raced with RMID? */
 		if (!ipc_valid_object(&msq->q_perm)) {
@@ -681,6 +681,7 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext,
 			goto out_unlock0;
 		}
 
+		ipc_unlock_object(&msq->q_perm);
 	}
 	msq->q_lspid = task_tgid_vnr(current);
 	msq->q_stime = get_seconds();
-- 
2.6.6