linux-kernel - [PATCH 3.16 284/305] vhost: make sure used idx is seen before log in vhost_add_used

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Message-ID: <lsq.1549201508.780249684@decadent.org.uk>
Date:   Sun, 03 Feb 2019 14:45:08 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "David S. Miller" <davem@...emloft.net>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        "Jason Wang" <jasowang@...hat.com>
Subject: [PATCH 3.16 284/305] vhost: make sure used idx is seen before log
 in vhost_add_used_n()

3.16.63-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Jason Wang <jasowang@...hat.com>

commit 841df922417eb82c835e93d4b93eb6a68c99d599 upstream.

We miss a write barrier that guarantees used idx is updated and seen
before log. This will let userspace sync and copy used ring before
used idx is update. Fix this by adding a barrier before log_write().

Fixes: 8dd014adfea6f ("vhost-net: mergeable buffers support")
Acked-by: Michael S. Tsirkin <mst@...hat.com>
Signed-off-by: Jason Wang <jasowang@...hat.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/vhost/vhost.c | 2 ++
 1 file changed, 2 insertions(+)

--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -1411,6 +1411,8 @@ int vhost_add_used_n(struct vhost_virtqu
 		return -EFAULT;
 	}
 	if (unlikely(vq->log_used)) {
+		/* Make sure used idx is seen before log. */
+		smp_wmb();
 		/* Log used index update. */
 		log_write(vq->log_base,
 			  vq->log_addr + offsetof(struct vring_used, idx),