linux-kernel - Re: [PATCH v2 2/2] virtio: acknowledge all features before access

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Message-ID: <87h79ycw6i.fsf@redhat.com>
Date:   Thu, 20 Jan 2022 15:35:01 +0100
From:   Cornelia Huck <cohuck@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>, linux-kernel@...r.kernel.org
Cc:     Jason Wang <jasowang@...hat.com>,
        virtualization@...ts.linux-foundation.org, stable@...r.kernel.org,
        Halil Pasic <pasic@...ux.ibm.com>
Subject: Re: [PATCH v2 2/2] virtio: acknowledge all features before access

On Tue, Jan 18 2022, "Michael S. Tsirkin" <mst@...hat.com> wrote:

> The feature negotiation was designed in a way that
> makes it possible for devices to know which config
> fields will be accessed by drivers.
>
> This is broken since commit 404123c2db79 ("virtio: allow drivers to
> validate features") with fallout in at least block and net.  We have a
> partial work-around in commit 2f9a174f918e ("virtio: write back
> F_VERSION_1 before validate") which at least lets devices find out which
> format should config space have, but this is a partial fix: guests
> should not access config space without acknowledging features since
> otherwise we'll never be able to change the config space format.
>
> To fix, split finalize_features from virtio_finalize_features and
> call finalize_features with all feature bits before validation,
> and then - if validation changed any bits - once again after.
>
> Since virtio_finalize_features no longer writes out features
> rename it to virtio_features_ok - since that is what it does:
> checks that features are ok with the device.
>
> As a side effect, this also reduces the amount of hypervisor accesses -
> we now only acknowledge features once unless we are clearing any
> features when validating (which is uncommon).
>
> Cc: stable@...r.kernel.org
> Fixes: 404123c2db79 ("virtio: allow drivers to validate features")
> Fixes: 2f9a174f918e ("virtio: write back F_VERSION_1 before validate")
> Cc: "Halil Pasic" <pasic@...ux.ibm.com>
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
>
> fixup! virtio: acknowledge all features before access

Leftover from rebasing?

> ---
>  drivers/virtio/virtio.c       | 39 ++++++++++++++++++++---------------
>  include/linux/virtio_config.h |  3 ++-
>  2 files changed, 24 insertions(+), 18 deletions(-)

Reviewed-by: Cornelia Huck <cohuck@...hat.com>

Would like to see a quick sanity test from Halil, though.