linux-kernel - Re: [PATCH] vhost: fix vhost_vq_access

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Message-ID: <20180409224842-mutt-send-email-mst@kernel.org>
Date:   Mon, 9 Apr 2018 22:54:34 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Stefan Hajnoczi <stefanha@...hat.com>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jason Wang <jasowang@...hat.com>,
        KVM list <kvm@...r.kernel.org>, syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH] vhost: fix vhost_vq_access_ok() log check

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@...hat.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