netdev - Re: [Patch net] ip_gre: set dev->hard_header

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Date:   Thu, 8 Oct 2020 07:48:40 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        syzbot+4a2c52677a8a1aa283cb@...kaller.appspotmail.com,
        William Tu <u9012063@...il.com>, Xie He <xie.he.0141@...il.com>
Subject: Re: [Patch net] ip_gre: set dev->hard_header_len properly

On Wed, Oct 7, 2020 at 9:22 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
>
> GRE tunnel has its own header_ops, ipgre_header_ops, and sets it
> conditionally. When it is set, it assumes the outer IP header is
> already created before ipgre_xmit().
>
> This is not true when we send packets through a raw packet socket,
> where L2 headers are supposed to be constructed by user. Packet
> socket calls dev_validate_header() to validate the header. But
> GRE tunnel does not set dev->hard_header_len, so that check can
> be simply bypassed, therefore uninit memory could be passed down
> to ipgre_xmit().

If dev->hard_header_len is zero, the packet socket will not reserve
room for the link layer header, so skb->data points to network_header.
But I don't see any uninitialized packet data?

> Fix this by setting dev->hard_header_len whenever sets header_ops,
> as dev->hard_header_len is supposed to be the length of the header
> created by dev->header_ops->create() anyway.

Agreed. Xie has made similar changes to lapbether recently in
commit c7ca03c216ac.

> Reported-and-tested-by: syzbot+4a2c52677a8a1aa283cb@...kaller.appspotmail.com
> Cc: William Tu <u9012063@...il.com>
> Cc: Willem de Bruijn <willemdebruijn.kernel@...il.com>
> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
> ---
>  net/ipv4/ip_gre.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
> index 4e31f23e4117..43b62095559e 100644
> --- a/net/ipv4/ip_gre.c
> +++ b/net/ipv4/ip_gre.c
> @@ -987,10 +987,12 @@ static int ipgre_tunnel_init(struct net_device *dev)
>                                 return -EINVAL;
>                         dev->flags = IFF_BROADCAST;
>                         dev->header_ops = &ipgre_header_ops;
> +                       dev->hard_header_len = tunnel->hlen + sizeof(*iph);
>                 }
>  #endif
>         } else if (!tunnel->collect_md) {
>                 dev->header_ops = &ipgre_header_ops;
> +               dev->hard_header_len = tunnel->hlen + sizeof(*iph);

Unrelated to this patch, I do wonder if ipgre_header does the
right thing when tunnel->hlen > sizeof(gre_base_hdr),
i.e., for gre tunnels with optional fields.

Currently it appears to not initialize those.

>         }
>
>         return ip_tunnel_init(dev);
> --
> 2.28.0
>