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

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Message-ID: <CA+55aFzJ_JPtU5SjAHHv--p3ZcAw4gVidxvrLLZF7w+kbHCNdg@mail.gmail.com>
Date:   Mon, 9 Apr 2018 09:52:13 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Stefan Hajnoczi <stefanha@...hat.com>
Cc:     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>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH] vhost: fix vhost_vq_access_ok() log check

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