linux-kernel - Re: [PATCH V5 1/4] virtio_ring: validate used buffer length

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Date:   Tue, 23 Nov 2021 10:25:20 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Halil Pasic <pasic@...ux.ibm.com>
Cc:     mst <mst@...hat.com>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        "Hetzelt, Felicitas" <f.hetzelt@...berlin.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "kaplan, david" <david.kaplan@....com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Stefano Garzarella <sgarzare@...hat.com>
Subject: Re: [PATCH V5 1/4] virtio_ring: validate used buffer length

On Tue, Nov 23, 2021 at 4:24 AM Halil Pasic <pasic@...ux.ibm.com> wrote:
>
> On Mon, 22 Nov 2021 14:25:26 +0800
> Jason Wang <jasowang@...hat.com> wrote:
>
> > I think the fixes are:
> >
> > 1) fixing the vhost vsock
> > 2) use suppress_used_validation=true to let vsock driver to validate
> > the in buffer length
> > 3) probably a new feature so the driver can only enable the validation
> > when the feature is enabled.
>
> I'm not sure, I would consider a F_DEV_Y_FIXED_BUG_X a perfectly good
> feature. Frankly the set of such bugs is device implementation
> specific and it makes little sense to specify a feature bit
> that says the device implementation claims to adhere to some
> aspect of the specification. Also what would be the semantic
> of not negotiating F_DEV_Y_FIXED_BUG_X?

Yes, I agree. Rethink of the feature bit, it seems unnecessary,
especially considering the driver should not care about the used
length for tx.

>
> On the other hand I see no other way to keep the validation
> permanently enabled for fixed implementations, and get around the problem
> with broken implementations. So we could have something like
> VHOST_USED_LEN_STRICT.

It's more about a choice of the driver's knowledge. For vsock TX it
should be fine. If we introduce a parameter and disable it by default,
it won't be very useful.

>
> Maybe, we can also think of 'warn and don't alter behavior' instead of
> 'warn' and alter behavior. Or maybe even not having such checks on in
> production, but only when testing.

I think there's an agreement that virtio drivers need more hardening,
that's why a lot of patches were merged. Especially considering the
new requirements came from confidential computing, smart NIC and
VDUSE. For virtio drivers, enabling the validation may help to

1) protect the driver from the buggy and malicious device
2) uncover the bugs of the devices (as vsock did, and probably rpmsg)
3) force the have a smart driver that can do the validation itself
then we can finally remove the validation in the core

So I'd like to keep it enabled.

Thanks

>
> Regards,
> Halil
>