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

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Date:   Mon, 17 Jan 2022 03:26:21 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Halil Pasic <pasic@...ux.ibm.com>,
        Cornelia Huck <cohuck@...hat.com>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] virtio: acknowledge all features before access

On Mon, Jan 17, 2022 at 02:31:49PM +0800, Jason Wang wrote:
> 
> 在 2022/1/15 上午4:09, Michael S. Tsirkin 写道:
> > 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.
> > 
> > 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.
> > 
> > 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>
> > ---
> > 
> > Halil, I thought hard about our situation with transitional and
> > today I finally thought of something I am happy with.
> > Pls let me know what you think. Testing on big endian would
> > also be much appreciated!
> > 
> >   drivers/virtio/virtio.c | 31 +++++++++++++++++--------------
> >   1 file changed, 17 insertions(+), 14 deletions(-)
> > 
> > diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> > index d891b0a354b0..2ed6e2451fd8 100644
> > --- a/drivers/virtio/virtio.c
> > +++ b/drivers/virtio/virtio.c
> > @@ -168,12 +168,10 @@ EXPORT_SYMBOL_GPL(virtio_add_status);
> >   static int virtio_finalize_features(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 +242,6 @@ 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
> > @@ -265,10 +252,22 @@ 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;
> > +
> > +		if (features != dev->features) {
> > +			err = dev->config->finalize_features(dev);
> > +			if (err)
> > +				goto err;
> > +		}
> >   	}
> >   	err = virtio_finalize_features(dev);
> > @@ -495,6 +494,10 @@ int virtio_device_restore(struct virtio_device *dev)
> >   	/* We have a driver! */
> >   	virtio_add_status(dev, VIRTIO_CONFIG_S_DRIVER);
> > +	ret = dev->config->finalize_features(dev);
> > +	if (ret)
> > +		goto err;
> 
> 
> Is this part of code related?
> 
> Thanks
> 

Yes. virtio_finalize_features no longer calls dev->config->finalize_features.

I think the dev->config->finalize_features callback is actually
a misnomer now, it just sends the features to device,
finalize is FEATURES_OK. Renaming that is a bigger
patch though, and I'd like this one to be cherry-pickable
to stable.

> > +
> >   	ret = virtio_finalize_features(dev);
> >   	if (ret)
> >   		goto err;