vhost: fix vhost_vq_access_ok() log check

Commit d65026c6 ("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: NStefan Hajnoczi <stefanha@redhat.com>
Acked-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/vhost/vhost.c

@@ -1244,10 +1244,12 @@ static int vq_log_access_ok(struct vhost_virtqueue *vq,
 /* 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);
 }