From: Stefan Hajnoczi <stefanha@redhat.com>
Date: Wed, 11 Apr 2018 10:35:40 +0800
Subject: vhost: fix vhost_vq_access_ok() log check
Git-commit: d14d2b78090c7de0557362b26a4ca591aa6a9faa
Patch-mainline: v4.17-rc1
References: networking-stable-18_04_13

Commit d65026c6c62e7d9616c8ceb5a53b68bcdc050525 ("vhost: validate log
when IOTLB is enabled") introduced a regression.  The logic was
originally:

  if (vq->iotlb)
      return 1;
  return A && B;

After the patch the short-circuit logic for A was inverted:

  if (A || vq->iotlb)
      return A;
  return B;

This patch fixes the regression by rewriting the checks in the obvious
way, no longer returning A when vq->iotlb is non-NULL (which is hard to
understand).

Reported-by: syzbot+65a84dde0214b0387ccd@syzkaller.appspotmail.com
Cc: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
 drivers/vhost/vhost.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -1253,10 +1253,12 @@ static int vq_log_access_ok(struct vhost
 /* Caller should have vq mutex and device mutex */
 int vhost_vq_access_ok(struct vhost_virtqueue *vq)
 {
-	int ret = vq_log_access_ok(vq, vq->log_base);
+	if (!vq_log_access_ok(vq, vq->log_base))
+		return 0;
 
-	if (ret || vq->iotlb)
-		return ret;
+	/* Access validation occurs at prefetch time with IOTLB */
+	if (vq->iotlb)
+		return 1;
 
 	return vq_access_ok(vq, vq->num, vq->desc, vq->avail, vq->used);
 }