Re: [PATCH] virtio: acknowledge all features before access

[Halil Pasic]

On Fri, 14 Jan 2022 15:09:14 -0500
"Michael S. Tsirkin" <mst@xxxxxxxxxx> 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.
> 
> 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@xxxxxxxxxxxxxxx
> Fixes: 404123c2db79 ("virtio: allow drivers to validate features")
> Fixes: 2f9a174f918e ("virtio: write back F_VERSION_1 before validate")
> Cc: "Halil Pasic" <pasic@xxxxxxxxxxxxx>
> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> ---
> 
> 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!
 
Hi Michael!

I was just about to have a look into this. But it does not apply
cleanly to Linus master (fetched a couple of minutes ago). I also tride
with d9679d0013a66849~1 but no luck. What is a suitable base for this
patch?

Regards,
Halil


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