Re: [PATCH net] virtio_net: do not allow tunnel csum offload for non GSO packets

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jun 09, 2026 at 04:44:26PM +0200, Paolo Abeni wrote:
> Fiona reports broken connectivity for virtio net setup using UDP tunnel
> inside the guest and NIC with not UDP tunnel TSO support in the host.
> 
> Currently the virtio_net driver exposes csum offload for UDP-tunneled,
> TCP non GSO packets. Such packet reach the host as CSUM_PARTIAL ones
> with the 'encapsulation' flag cleared, as the virtio specification do
> not support this specific kind of offload.
> 
> HW NICs with UDP tunnel TSO support - and those drivers directly
> accessing skb->csum_start/csum_offset - are still capable of computing
> the needed csum correctly, but otherwise the packets reach the wire with
> bad csum on both the inner and outer transport header.
> 
> Address the issue explicitly disabling csum offload for UDP tunneled,
> non GSO packets via the ndo_features_check op.
> 
> Fixes: 56a06bd40fab ("virtio_net: enable gso over UDP tunnel support.")
> Reported-by: Fiona Ebner <f.ebner@proxmox.com>
> Closes: https://bugzilla.proxmox.com/show_bug.cgi?id=7627
> Tested-by: Fiona Ebner <f.ebner@proxmox.com>
> Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Acked-by: Michael S. Tsirkin <mst@redhat.com>

> ---
>  drivers/net/virtio_net.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index f4adcfee7a80..07b8710639f9 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -6222,6 +6222,18 @@ static void virtnet_free_irq_moder(struct virtnet_info *vi)
>  	rtnl_unlock();
>  }
>  
> +static netdev_features_t virtnet_features_check(struct sk_buff *skb,
> +						struct net_device *dev,
> +						netdev_features_t features)
> +{
> +	/* Inner csum offload is only available for GSO packets. */
> +	if (skb->encapsulation && !skb_is_gso(skb))
> +		return features & ~NETIF_F_CSUM_MASK;
> +
> +	/* Passthru. */
> +	return features;
> +}
> +
>  static const struct net_device_ops virtnet_netdev = {
>  	.ndo_open            = virtnet_open,
>  	.ndo_stop   	     = virtnet_close,
> @@ -6235,7 +6247,7 @@ static const struct net_device_ops virtnet_netdev = {
>  	.ndo_bpf		= virtnet_xdp,
>  	.ndo_xdp_xmit		= virtnet_xdp_xmit,
>  	.ndo_xsk_wakeup         = virtnet_xsk_wakeup,
> -	.ndo_features_check	= passthru_features_check,
> +	.ndo_features_check	= virtnet_features_check,
>  	.ndo_get_phys_port_name	= virtnet_get_phys_port_name,
>  	.ndo_set_features	= virtnet_set_features,
>  	.ndo_tx_timeout		= virtnet_tx_timeout,
> -- 
> 2.54.0