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References: <20240327231826.1725488-1-andrew@daynix.com>
In-Reply-To: <20240327231826.1725488-1-andrew@daynix.com>
From: Jason Wang <jasowang@redhat.com>
Date: Thu, 28 Mar 2024 12:02:54 +0800
Message-ID: <CACGkMEuW8jLvje0_oqCT=-ih9JEgxOrWRsvjvfwQXw=OWT_RtQ@mail.gmail.com>
Subject: Re: [PATCH v2 1/1] vhost: Added pad cleanup if vnet_hdr is not present.
To: Andrew Melnychenko <andrew@daynix.com>
Cc: mst@redhat.com, ast@kernel.org, daniel@iogearbox.net, davem@davemloft.net, 
	kuba@kernel.org, hawk@kernel.org, john.fastabend@gmail.com, 
	kvm@vger.kernel.org, virtualization@lists.linux.dev, netdev@vger.kernel.org, 
	linux-kernel@vger.kernel.org, bpf@vger.kernel.org, 
	yuri.benditovich@daynix.com, yan@daynix.com
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On Thu, Mar 28, 2024 at 7:44=E2=80=AFAM Andrew Melnychenko <andrew@daynix.c=
om> 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=3Don,vnet_hdr=3Doff,...
>
> 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=3Don,vnet_hdr=3Doff, or do not configure vhost in the custom progra=
m.
>
> Signed-off-by: Andrew Melnychenko <andrew@daynix.com>

Acked-by: Jason Wang <jasowang@redhat.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_virtq=
ueue *nvq,
>         hdr =3D buf;
>         gso =3D &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
>