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

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Date:	Tue, 18 Nov 2014 11:03:32 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	"Michael S. Tsirkin" <mst@...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 11/17/2014 06:08 PM, Michael S. Tsirkin wrote:
> 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.

We can't guarantee that hypervisor's implementation are 100% correct.
> 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.
>

So we still need something like this patch to detect the wrong
dependencies. And the features fixing like this is not something new,
see how net device fix the features through ndo_fix_features().
>
>
>
> 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);
> 	}
>

This is similar to what I did in v1 and v2. Either are ok for me.
>
>
>> ---
>> 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.
>

I see ,and this kind of check could be added. But we're really safe
since qemu handle such cases and won't advertise any offload feature is
csum is not supported.
>
>> +	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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