Re: [PATCH v3 1/2] vhost: fix vhost_vq_access_ok() log check

[Michael S. Tsirkin]

On Wed, Apr 11, 2018 at 10:35:40AM +0800, Stefan Hajnoczi wrote:
> 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 
> Signed-off-by: Stefan Hajnoczi 

Acked-by: Michael S. Tsirkin 

> ---
>  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

[PATCH v3 1/2] vhost: fix vhost_vq_access_ok() log check

[Stefan Hajnoczi]

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 
Signed-off-by: Stefan Hajnoczi 
---
 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