netdev - Re: [PATCH 1/1] vhost: Added pad cleanup if vnet

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Date: Mon, 15 Jan 2024 17:32:21 -0500
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Andrew Melnychenko <andrew@...nix.com>
Cc: jasowang@...hat.com, kvm@...r.kernel.org,
	virtualization@...ts.linux.dev, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, yuri.benditovich@...nix.com,
	yan@...nix.com
Subject: Re: [PATCH 1/1] vhost: Added pad cleanup if vnet_hdr is not present.

On Mon, Jan 15, 2024 at 09:48:40PM +0200, Andrew Melnychenko wrote:
> When the Qemu launched with vhost but without tap vnet_hdr,
> vhost tries to copy vnet_hdr from socket iter with size 0
> to the page that may contain some trash.
> That trash can be interpreted as unpredictable values for
> vnet_hdr.
> That leads to dropping some packets and in some cases to
> stalling vhost routine when the vhost_net tries to process
> packets and fails in a loop.
> 
> Qemu options:
>   -netdev tap,vhost=on,vnet_hdr=off,...
> 
> Signed-off-by: Andrew Melnychenko <andrew@...nix.com>
> ---
>  drivers/vhost/net.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index f2ed7167c848..57411ac2d08b 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -735,6 +735,9 @@ static int vhost_net_build_xdp(struct vhost_net_virtqueue *nvq,
>  	hdr = buf;
>  	gso = &hdr->gso;
>  
> +	if (!sock_hlen)
> +		memset(buf, 0, pad);
> +
>  	if ((gso->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) &&
>  	    vhost16_to_cpu(vq, gso->csum_start) +
>  	    vhost16_to_cpu(vq, gso->csum_offset) + 2 >


Hmm need to analyse it to make sure there are no cases where we leak
some data to guest here in case where sock_hlen is set ...
> -- 
> 2.43.0