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+65a84dde0214b0387...@syzkaller.appspotmail.com
Cc: Jason Wang <jasow...@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com>
---
 drivers/vhost/vhost.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index bec722e41f58..fc805b7fad9d 100644
--- a/drivers/vhost/vhost.c
+++ b/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);
 }
-- 
2.14.3

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