Re: [PATCH] vhost: fix vhost_vq_access_ok() log check

[Linus Torvalds]

On Mon, Apr 9, 2018 at 6:10 AM, Stefan Hajnoczi  wrote:
> @@ -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;

That logic is still very non-obvious.

This code already had one bug because of an odd illegible test
sequence. Let's not keep the crazy code.

Why not just do the *obvious* thing, and get rid of "ret" entirely,
and make the damn thing return a boolean, and then just write it all
as

/* Caller should have vq mutex and device mutex */
bool vhost_vq_access_ok(struct vhost_virtqueue *vq)
{
if (!vq_log_access_ok(vq, vq->log_base))
return false;

if (vq->iotlb || vq_access_ok(vq, vq->num, vq->desc,
vq->avail, vq->used);
}

which makes the logic obvious: if vq_log_access_ok() fails, then then
vhost_vq_access_ok() fails unconditionally.

Otherwise, we need to have an iotlb, or a successful vq_access_ok() check.

Doesn't that all make more sense, and avoid the insane "ret" value use
that is really quite subtle?

Linus
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Re: [PATCH] vhost: fix vhost_vq_access_ok() log check

[Michael S. Tsirkin]

On Mon, Apr 09, 2018 at 09:10:21PM +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;
> 
> The correct logic is:
> 
>   if (!A || vq->iotlb)
>   return A;
>   return B;
> 
> Reported-by: syzbot+65a84dde0214b0387...@syzkaller.appspotmail.com
> Cc: Jason Wang 
> Signed-off-by: Stefan Hajnoczi 

Acked-by: Michael S. Tsirkin 

> ---
>  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
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[PATCH] 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;

The correct logic is:

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

Reported-by: syzbot+65a84dde0214b0387...@syzkaller.appspotmail.com
Cc: Jason Wang 
Signed-off-by: Stefan Hajnoczi 
---
 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

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