Linux Virtualization - Apr 2018 - [PATCH] vhost: fix vhost_vq_access_ok() log check

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 at syzkaller.appspotmail.com
Cc: Jason Wang <jasowang at redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha at 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 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+65a84dde0214b0387ccd at syzkaller.appspotmail.com
> Cc: Jason Wang <jasowang at redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha at redhat.com>
Acked-by: Michael S. Tsirkin <mst at 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 Mon, Apr 9, 2018 at 6:10 AM, Stefan Hajnoczi <stefanha at redhat.com>
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

From: Stefan Hajnoczi <stefanha at 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 at syzkaller.appspotmail.com
Cc: Jason Wang <jasowang at redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha at redhat.com>
Acked-by: Michael S. Tsirkin <mst at 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 Mon, Apr 09, 2018 at 09:52:13AM -0700, Linus Torvalds
wrote:
> On Mon, Apr 9, 2018 at 6:10 AM, Stefan Hajnoczi <stefanha at
redhat.com> 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

I agree it's cleaner.

Stefan, I reposted your patch on netdev (since the breakage got applied
there too).

Would you like to self-nak it and post v2? Pls remember to CC netdev.

-- 
MST

On Tue, Apr 10, 2018 at 3:40 AM, Michael S. Tsirkin <mst at redhat.com>
wrote:
> From: Stefan Hajnoczi <stefanha at 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 at syzkaller.appspotmail.com
> Cc: Jason Wang <jasowang at redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha at redhat.com>
> Acked-by: Michael S. Tsirkin <mst at 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