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

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Date:   Thu, 8 Oct 2020 16:19:10 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        syzbot <syzbot+4a2c52677a8a1aa283cb@...kaller.appspotmail.com>,
        William Tu <u9012063@...il.com>,
        Willem de Bruijn <willemdebruijn.kernel@...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().
>
> 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.
>
> 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>

Acked-by: Willem de Bruijn <willemb@...gle.com>

> The syzbot report has the information for both of your questions:
> https://syzkaller.appspot.com/text?tag=CrashReport&x=11845568500000
>
> It clearly shows packet_snd() and ipgre_xmit().

Thanks. I hadn't thought to check that (clearly).