Subject: Re: [PATCH net,v4,1/2] virtio_net: Improve RSS key size validation and use NETDEV_RSS_KEY_LEN

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 25 Feb 2026 05:03:50 -0500

On Tue, Feb 24, 2026 at 12:28:49PM +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.
> 
> Cc: [email-masked]
> Fixes: 3f7d9c1964fc ("virtio_net: Add hash_key_length check")
> Signed-off-by: Srujana Challa <[email-masked]>

--- should come here before changelog.

> v3:
> - Moved RSS key validation checks to virtnet_validate.
> - Add fixes: tag and CC -stable
> v4:
> - Use NETDEV_RSS_KEY_LEN instead of type_max for the maximum rss key size.
> ---
>  drivers/net/virtio_net.c | 34 ++++++++++++++++++++++++----------
>  1 file changed, 24 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index db88dcaefb20..eeefe8abc122 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -381,8 +381,6 @@ struct receive_queue {
>  	struct xdp_buff **xsk_buffs;
>  };
>  
> -#define VIRTIO_NET_RSS_MAX_KEY_SIZE     40
> -
>  /* Control VQ buffers: protected by the rtnl lock */
>  struct control_buf {
>  	struct virtio_net_ctrl_hdr hdr;
> @@ -486,7 +484,7 @@ struct virtnet_info {
>  
>  	/* Must be last as it ends in a flexible-array member. */
>  	TRAILING_OVERLAP(struct virtio_net_rss_config_trailer, rss_trailer, hash_key_data,
> -		u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];
> +		u8 rss_hash_key_data[NETDEV_RSS_KEY_LEN];
>  	);
>  };
>  static_assert(offsetof(struct virtnet_info, rss_trailer.hash_key_data) ==
> @@ -6627,6 +6625,29 @@ static int virtnet_validate(struct virtio_device *vdev)
>  		__virtio_clear_bit(vdev, VIRTIO_NET_F_STANDBY);
>  	}
>  
> +	if (virtio_has_feature(vdev, VIRTIO_NET_F_RSS) ||
> +	    virtio_has_feature(vdev, VIRTIO_NET_F_HASH_REPORT)) {
> +		u8 key_sz = virtio_cread8(vdev,
> +					  offsetof(struct virtio_net_config,
> +						   rss_max_key_size));
> +		/* Spec requires at least 40 bytes */
> +#define VIRTIO_NET_RSS_MIN_KEY_SIZE 40
> +		if (key_sz < VIRTIO_NET_RSS_MIN_KEY_SIZE) {
> +			dev_warn(&vdev->dev,
> +				 "rss_max_key_size=%u is less than spec minimum %u, disabling RSS\n",
> +				 key_sz, VIRTIO_NET_RSS_MIN_KEY_SIZE);
> +			__virtio_clear_bit(vdev, VIRTIO_NET_F_RSS);
> +			__virtio_clear_bit(vdev, VIRTIO_NET_F_HASH_REPORT);
> +		}
> +		if (key_sz > NETDEV_RSS_KEY_LEN) {
> +			dev_warn(&vdev->dev,
> +				 "rss_max_key_size=%u exceeds driver limit %u, disabling RSS\n",
> +				 key_sz, NETDEV_RSS_KEY_LEN);
> +			__virtio_clear_bit(vdev, VIRTIO_NET_F_RSS);
> +			__virtio_clear_bit(vdev, VIRTIO_NET_F_HASH_REPORT);

you flipped the logic here and it makes no sense now.

Did you test this path?

So if device is powerful and supports a very big key size then...
we disable the feature? how does this make sense?

> +		}
> +	}
> +
>  	return 0;
>  }
>  
> @@ -6839,13 +6860,6 @@ static int virtnet_probe(struct virtio_device *vdev)
>  	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 &=
> -- 
> 2.25.1