[PATCH RESEND net] vhost: fix vhost_vq_access_ok() log check

From: Stefan Hajnoczi <stefanha@redhat.com>

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;

The correct logic is:

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

Reported-by: syzbot+65a84dde0214b0387ccd@syzkaller.appspotmail.com
Cc: Jason Wang <redacted>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>

---
 drivers/vhost/vhost.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 5320039671b7..f6af4210679a 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c

@@ -1246,7 +1246,7 @@ int vhost_vq_access_ok(struct vhost_virtqueue *vq)
 {
 	int ret = vq_log_access_ok(vq, vq->log_base);
 
-	if (ret || vq->iotlb)
+	if (!ret || vq->iotlb)
 		return ret;
 
 	return vq_access_ok(vq, vq->num, vq->desc, vq->avail, vq->used);

-- 
2.14.3

On Tue, Apr 10, 2018 at 3:40 AM, Michael S. Tsirkin [off-list ref] wrote:

From: Stefan Hajnoczi <stefanha@redhat.com>

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;

The correct logic is:

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

Reported-by: syzbot+65a84dde0214b0387ccd@syzkaller.appspotmail.com
Cc: Jason Wang <redacted>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>

---
 drivers/vhost/vhost.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

NACK

I will send a v2 with cleaner logic as suggested by Linus.

Stefan