Date:   Wed, 19 Jan 2022 04:18:04 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Halil Pasic <pasic@...ux.ibm.com>,
        Cornelia Huck <cohuck@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        stable@...r.kernel.org,
        virtualization <virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH v2 2/2] virtio: acknowledge all features before access

On Wed, Jan 19, 2022 at 10:52:34AM +0800, Jason Wang wrote:
> On Wed, Jan 19, 2022 at 1:04 AM 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.
> 
> So I guess this is for this part of the spec 3.1.1:
> 
> """
> 4. Read device feature bits, and write the subset of feature bits
> understood by the OS and driver to the device. During this step the
> driver MAY read (but MUST NOT write) the device-specific configuration
> fields to check that it can support the device before accepting it.
> """
> 
> If it is, is this better to quote in the change log?

I don't think this spec actually clarifies anything.
Sent some spec patches to improve the situation.

> Other than this,
> 
> Acked-by: Jason Wang <jasowang@...hat.com>
> 
> >
> > 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
> > ---
> >  drivers/virtio/virtio.c       | 39 ++++++++++++++++++++---------------
> >  include/linux/virtio_config.h |  3 ++-
> >  2 files changed, 24 insertions(+), 18 deletions(-)
> >
> > diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> > index d891b0a354b0..d6396be0ea83 100644
> > --- a/drivers/virtio/virtio.c
> > +++ b/drivers/virtio/virtio.c
> > @@ -166,14 +166,13 @@ void virtio_add_status(struct virtio_device *dev, unsigned int status)
> >  }
> >  EXPORT_SYMBOL_GPL(virtio_add_status);
> >
> > -static int virtio_finalize_features(struct virtio_device *dev)
> > +/* Do some validation, then set FEATURES_OK */
> > +static int virtio_features_ok(struct virtio_device *dev)
> >  {
> > -       int ret = dev->config->finalize_features(dev);
> >         unsigned status;
> > +       int ret;
> >
> >         might_sleep();
> > -       if (ret)
> > -               return ret;
> >
> >         ret = arch_has_restricted_virtio_memory_access();
> >         if (ret) {
> > @@ -244,17 +243,6 @@ static int virtio_dev_probe(struct device *_d)
> >                 driver_features_legacy = driver_features;
> >         }
> >
> > -       /*
> > -        * Some devices detect legacy solely via F_VERSION_1. Write
> > -        * F_VERSION_1 to force LE config space accesses before FEATURES_OK for
> > -        * these when needed.
> > -        */
> > -       if (drv->validate && !virtio_legacy_is_little_endian()
> > -                         && device_features & BIT_ULL(VIRTIO_F_VERSION_1)) {
> > -               dev->features = BIT_ULL(VIRTIO_F_VERSION_1);
> > -               dev->config->finalize_features(dev);
> > -       }
> > -
> >         if (device_features & (1ULL << VIRTIO_F_VERSION_1))
> >                 dev->features = driver_features & device_features;
> >         else
> > @@ -265,13 +253,26 @@ static int virtio_dev_probe(struct device *_d)
> >                 if (device_features & (1ULL << i))
> >                         __virtio_set_bit(dev, i);
> >
> > +       err = dev->config->finalize_features(dev);
> > +       if (err)
> > +               goto err;
> > +
> >         if (drv->validate) {
> > +               u64 features = dev->features;
> > +
> >                 err = drv->validate(dev);
> >                 if (err)
> >                         goto err;
> > +
> > +               /* Did validation change any features? Then write them again. */
> > +               if (features != dev->features) {
> > +                       err = dev->config->finalize_features(dev);
> > +                       if (err)
> > +                               goto err;
> > +               }
> >         }
> >
> > -       err = virtio_finalize_features(dev);
> > +       err = virtio_features_ok(dev);
> >         if (err)
> >                 goto err;
> >
> > @@ -495,7 +496,11 @@ int virtio_device_restore(struct virtio_device *dev)
> >         /* We have a driver! */
> >         virtio_add_status(dev, VIRTIO_CONFIG_S_DRIVER);
> >
> > -       ret = virtio_finalize_features(dev);
> > +       ret = dev->config->finalize_features(dev);
> > +       if (ret)
> > +               goto err;
> > +
> > +       ret = virtio_features_ok(dev);
> >         if (ret)
> >                 goto err;
> >
> > diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
> > index 4d107ad31149..dafdc7f48c01 100644
> > --- a/include/linux/virtio_config.h
> > +++ b/include/linux/virtio_config.h
> > @@ -64,8 +64,9 @@ struct virtio_shm_region {
> >   *     Returns the first 64 feature bits (all we currently need).
> >   * @finalize_features: confirm what device features we'll be using.
> >   *     vdev: the virtio_device
> > - *     This gives the final feature bits for the device: it can change
> > + *     This sends the driver feature bits to the device: it can change
> >   *     the dev->feature bits if it wants.
> > + * Note: despite the name this can be called any number of times.
> >   *     Returns 0 on success or error status
> >   * @bus_name: return the bus name associated with the device (optional)
> >   *     vdev: the virtio_device
> > --
> > MST
> >
> 
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