linux-kernel - Re: [PATCH V3 2/2] virtio-net: sanitize buggy features advertised by host

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Date:	Mon, 17 Nov 2014 12:08:38 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	rusty@...tcorp.com.au, virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Wanlong Gao <gaowanlong@...fujitsu.com>
Subject: Re: [PATCH V3 2/2] virtio-net: sanitize buggy features advertised by
 host

On Mon, Nov 17, 2014 at 05:17:18PM +0800, Jason Wang wrote:
> This patch tries to detect the possible buggy features advertised by host
> and sanitize them. One example is booting virtio-net with only ctrl_vq
> disabled, qemu may still advertise many features which depends on it. This
> will trigger several BUG()s in virtnet_send_command().
> 
> This patch utilizes the sanitize_features() method, and disables all
> features that depends on ctrl_vq if it was not advertised.
> 
> This fixes the crash when booting with ctrl_vq=off using qemu.
> 
> Cc: Rusty Russell <rusty@...tcorp.com.au>
> Cc: Michael S. Tsirkin <mst@...hat.com>
> Cc: Cornelia Huck <cornelia.huck@...ibm.com>
> Cc: Wanlong Gao <gaowanlong@...fujitsu.com>
> Signed-off-by: Jason Wang <jasowang@...hat.com>


So I'm not sure this is useful.
The spec says:
	The device MUST NOT offer a feature which requires another feature which
	was not offered.
So this is a buggy hypervisor, and I believe we should just fail probe.
This can be done without crashing, and is generally a better
idea that second-guessing what hypervisor wants us to do.





However, assuming that we do want this change:
This can be replaced with a table driven design in virtio core, but
since you chose to open code it, I would drop table below altogether.


Just make it
	if (!virtio_has_feature(dev, VIRTIO_NET_F_CTRL_VQ)) {
		virtio_disable_feature(dev, VIRTIO_NET_F_CTRL_RX);
		virtio_disable_feature(dev, VIRTIO_NET_F_CTRL_VLAN);
		virtio_disable_feature(dev, VIRTIO_NET_F_GUEST_ANNOUNCE);
		virtio_disable_feature(dev, VIRTIO_NET_F_MQ);
		virtio_disable_feature(dev, VIRTIO_NET_F_CTRL_MAC_ADDR);
	}




> ---
> Changes from V1:
> - fix the cut-and-paste error
> Changes from V2:
> - loop through an array of feature bits
> - switch to use dev_warn()
> ---
>  drivers/net/virtio_net.c | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index ec2a8b4..6fadd8c 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1948,6 +1948,31 @@ static int virtnet_restore(struct virtio_device *vdev)
>  }
>  #endif
>  
> +static void virtnet_sanitize_features(struct virtio_device *dev)
> +{
> +	unsigned int features_for_ctrl_vq[] = {
> +		VIRTIO_NET_F_CTRL_RX,
> +		VIRTIO_NET_F_CTRL_VLAN,
> +		VIRTIO_NET_F_GUEST_ANNOUNCE,
> +		VIRTIO_NET_F_MQ,
> +		VIRTIO_NET_F_CTRL_MAC_ADDR
> +	};

This is not the only dependency: checksums
have dependencies too. See virtio 1.0 spec.



> +	int i;
> +
> +	if (!virtio_has_feature(dev, VIRTIO_NET_F_CTRL_VQ)) {
> +		for (i = 0; i < ARRAY_SIZE(features_for_ctrl_vq); i++) {
> +			unsigned int f = features_for_ctrl_vq[i];
> +			if (virtio_has_feature(dev, f)) {
> +				virtio_disable_feature(dev, f);
> +				dev_warn(&dev->dev,
> +					 "buggy hyperviser: disable feature "
> +					 "0x%x since VIRTIO_NET_F_CTRL_VQ was "
> +					 "not advertised.\n", f);
> +			}
> +		}
> +	}
> +}
> +
>  static struct virtio_device_id id_table[] = {
>  	{ VIRTIO_ID_NET, VIRTIO_DEV_ANY_ID },
>  	{ 0 },
> @@ -1975,6 +2000,7 @@ static struct virtio_driver virtio_net_driver = {
>  	.probe =	virtnet_probe,
>  	.remove =	virtnet_remove,
>  	.config_changed = virtnet_config_changed,
> +	.sanitize_features = virtnet_sanitize_features,
>  #ifdef CONFIG_PM_SLEEP
>  	.freeze =	virtnet_freeze,
>  	.restore =	virtnet_restore,
> -- 
> 1.9.1
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