Subject: Re: [PATCH v2 net-next] virtio_net: Improve RSS key size validation and use NETDEV_RSS_KEY_LEN

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 6 Feb 2026 10:02:45 -0500

On Fri, Feb 06, 2026 at 05:31:54PM +0530, Srujana Challa wrote:
> Replace hardcoded RSS max key size limit with NETDEV_RSS_KEY_LEN to
> align with kernel's standard RSS key length. Add validation for RSS
> key size against spec minimum (40 bytes) and driver maximum. When
> validation fails, gracefully disable RSS features and continue
> initialization rather than failing completely.
> 
> Signed-off-by: Srujana Challa <[email-masked]>

I'd CC stable on this actually. failing when device is more capable
was a bad idea.
can you add a Fixes: tag, too? Thanks!

> ---
>  drivers/net/virtio_net.c | 62 +++++++++++++++++++++++-----------------
>  1 file changed, 36 insertions(+), 26 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index db88dcaefb20..1fa4197dffc3 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -381,7 +381,8 @@ struct receive_queue {
>  	struct xdp_buff **xsk_buffs;
>  };
>  
> -#define VIRTIO_NET_RSS_MAX_KEY_SIZE     40
> +#define VIRTIO_NET_RSS_MAX_KEY_SIZE     NETDEV_RSS_KEY_LEN
> +#define VIRTIO_NET_RSS_MIN_KEY_SIZE     40
>  
>  /* Control VQ buffers: protected by the rtnl lock */
>  struct control_buf {
> @@ -6823,38 +6824,47 @@ static int virtnet_probe(struct virtio_device *vdev)
>  	if (virtio_has_feature(vdev, VIRTIO_NET_F_HASH_REPORT))
>  		vi->has_rss_hash_report = true;
>  
> -	if (virtio_has_feature(vdev, VIRTIO_NET_F_RSS)) {
> +	if (virtio_has_feature(vdev, VIRTIO_NET_F_RSS))
>  		vi->has_rss = true;
>  
> -		vi->rss_indir_table_size =
> -			virtio_cread16(vdev, offsetof(struct virtio_net_config,
> -				rss_max_indirection_table_length));
> -	}
> -	vi->rss_hdr = devm_kzalloc(&vdev->dev, virtnet_rss_hdr_size(vi), GFP_KERNEL);
> -	if (!vi->rss_hdr) {
> -		err = -ENOMEM;
> -		goto free;
> -	}
> -
>  	if (vi->has_rss || vi->has_rss_hash_report) {
>  		vi->rss_key_size =
>  			virtio_cread8(vdev, offsetof(struct virtio_net_config, rss_max_key_size));
> -		if (vi->rss_key_size > VIRTIO_NET_RSS_MAX_KEY_SIZE) {
> -			dev_err(&vdev->dev, "rss_max_key_size=%u exceeds the limit %u.\n",
> -				vi->rss_key_size, VIRTIO_NET_RSS_MAX_KEY_SIZE);
> -			err = -EINVAL;
> -			goto free;
> -		}
>  
> -		vi->rss_hash_types_supported =
> -		    virtio_cread32(vdev, offsetof(struct virtio_net_config, supported_hash_types));
> -		vi->rss_hash_types_supported &=
> -				~(VIRTIO_NET_RSS_HASH_TYPE_IP_EX |
> -				  VIRTIO_NET_RSS_HASH_TYPE_TCP_EX |
> -				  VIRTIO_NET_RSS_HASH_TYPE_UDP_EX);
> +		/* Spec requires at least 40 bytes */
> +		if (vi->rss_key_size < VIRTIO_NET_RSS_MIN_KEY_SIZE) {
> +			dev_warn(&vdev->dev,
> +				 "rss_max_key_size=%u is less than spec minimum %u, disabling RSS\n",
> +				 vi->rss_key_size, VIRTIO_NET_RSS_MIN_KEY_SIZE);
> +			vi->has_rss = false;
> +			vi->has_rss_hash_report = false;
> +		} else if (vi->rss_key_size > VIRTIO_NET_RSS_MAX_KEY_SIZE) {
> +			dev_warn(&vdev->dev,
> +				 "rss_max_key_size=%u exceeds driver limit %u, disabling RSS\n",
> +				 vi->rss_key_size, VIRTIO_NET_RSS_MAX_KEY_SIZE);
> +			vi->has_rss = false;
> +			vi->has_rss_hash_report = false;
> +		} else {
> +			vi->rss_indir_table_size =
> +				virtio_cread16(vdev, offsetof(struct virtio_net_config,
> +							      rss_max_indirection_table_length));
> +			vi->rss_hash_types_supported =
> +			    virtio_cread32(vdev, offsetof(struct virtio_net_config,
> +							  supported_hash_types));
> +			vi->rss_hash_types_supported &=
> +					~(VIRTIO_NET_RSS_HASH_TYPE_IP_EX |
> +					  VIRTIO_NET_RSS_HASH_TYPE_TCP_EX |
> +					  VIRTIO_NET_RSS_HASH_TYPE_UDP_EX);
> +
> +			dev->hw_features |= NETIF_F_RXHASH;
> +			dev->xdp_metadata_ops = &virtnet_xdp_metadata_ops;
> +		}
> +	}
>  
> -		dev->hw_features |= NETIF_F_RXHASH;
> -		dev->xdp_metadata_ops = &virtnet_xdp_metadata_ops;
> +	vi->rss_hdr = devm_kzalloc(&vdev->dev, virtnet_rss_hdr_size(vi), GFP_KERNEL);
> +	if (!vi->rss_hdr) {
> +		err = -ENOMEM;
> +		goto free;
>  	}

I feel these checks belong in virtnet_validate. Then we can disable
features e.g. by clearing VIRTIO_NET_F_RSS and host will have some
visibility that RSS can't be used.

>  
>  	if (virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_UDP_TUNNEL_GSO) ||
> -- 
> 2.25.1