Re: [PATCH net] vhost-net: fix TX stall when vhost owns virtio-net header

[Michael S. Tsirkin]

On Wed, Jul 08, 2026 at 04:22:42PM +0100, enrico.zanda@xxxxxxx wrote:
> From: Enrico Zanda <enrico.zanda@xxxxxxx>
> 
> When vhost owns the virtio-net header, i.e. when
> VHOST_NET_F_VIRTIO_NET_HDR is negotiated, sock_hlen is 0,
> meaning that no header will be forwarded to the TAP device.
> 
> In the current vhost_net_build_xdp() implementation,
> when sock_hlen == 0, the gso pointer can point at the start of the
> Ethernet frame instead of a virtio-net header.
> This results in a wrong interpretation of the destination MAC address
> bytes as struct virtio_net_hdr fields.
> 
> This can, for some MAC addresses, trigger -EINVAL and return early
> before the TX descriptor is completed, which can stall vhost-net TX.
> 
> Before 97b2409f28e0, the gso pointer was set to the zeroed padding area,
> using it as a synthetic virtio-net header. Restore that behavior.
> 
> Fixes: 97b2409f28e0 ("vhost-net: reduce one userspace copy when building XDP buff")
> Signed-off-by: Enrico Zanda <enrico.zanda@xxxxxxx>


The fix looks good:
Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx>

Sashiko thinks there's something something security here, but I think
it is misguided. It's just guest hurting itself. driver breaks the
device it gets to keep both pieces.

> ---
>  drivers/vhost/net.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 77b59f49bddb..3e72b9c6af0c 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -731,10 +731,12 @@ static int vhost_net_build_xdp(struct vhost_net_virtqueue *nvq,
>  		goto err;
>  	}
>  
> -	gso = buf + pad - sock_hlen;
> -
> -	if (!sock_hlen)
> +	if (!sock_hlen) {
>  		memset(buf, 0, pad);
> +		gso = buf;
> +	} else {
> +		gso = buf + pad - sock_hlen;
> +	}
>  
>  	if ((gso->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) &&
>  	    vhost16_to_cpu(vq, gso->csum_start) +
> -- 
> 2.43.0