Re: [PATCH 1/1] vhost: Added pad cleanup if vnet_hdr is not present.

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

On Mon, Jan 15, 2024 at 05:32:25PM -0500, Michael S. Tsirkin wrote:
> 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@daynix.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 ...


Could you post this analysis pls?

> > -- 
> > 2.43.0