From: Jason Wang <jasowang@redhat.com>
Date: Thu, 13 Dec 2018 10:53:37 +0800
Subject: vhost: make sure used idx is seen before log in vhost_add_used_n()
Patch-mainline: v4.20
Git-commit: 841df922417eb82c835e93d4b93eb6a68c99d599
References: bsc#1109837

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@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
---
 drivers/vhost/vhost.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -2257,6 +2257,8 @@ 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),