linux-kernel - Re: [PATCH v2 1/1] vhost: Added pad cleanup if vnet

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Message-ID: <CACGkMEuW8jLvje0_oqCT=-ih9JEgxOrWRsvjvfwQXw=OWT_RtQ@mail.gmail.com>
Date: Thu, 28 Mar 2024 12:02:54 +0800
From: Jason Wang <jasowang@...hat.com>
To: Andrew Melnychenko <andrew@...nix.com>
Cc: mst@...hat.com, ast@...nel.org, daniel@...earbox.net, davem@...emloft.net, 
	kuba@...nel.org, hawk@...nel.org, john.fastabend@...il.com, 
	kvm@...r.kernel.org, virtualization@...ts.linux.dev, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org, 
	yuri.benditovich@...nix.com, yan@...nix.com
Subject: Re: [PATCH v2 1/1] vhost: Added pad cleanup if vnet_hdr is not present.

On Thu, Mar 28, 2024 at 7:44 AM Andrew Melnychenko <andrew@...nix.com> 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,...
>
> From security point of view, wrong values on field used later
> tap's tap_get_user_xdp() and will affect skb gso and options.
> Later the header(and data in headroom) should not be used by the stack.
> Using custom socket as a backend to vhost_net can reveal some data
> in the vnet_hdr, although it would require kernel access to implement.
>
> The issue happens because the value of sock_len in virtqueue is 0.
> That value is set at vhost_net_set_features() with
> VHOST_NET_F_VIRTIO_NET_HDR, also it's set to zero at device open()
> and reset() routine.
> So, currently, to trigger the issue, we need to set up qemu with
> vhost=on,vnet_hdr=off, or do not configure vhost in the custom program.
>
> Signed-off-by: Andrew Melnychenko <andrew@...nix.com>

Acked-by: Jason Wang <jasowang@...hat.com>

It seems it has been merged by Michael.

Thanks

> ---
>  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 >
> --
> 2.43.0
>