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

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Date:   Thu, 8 Oct 2020 12:16:23 -0700
From:   Xie He <xie.he.0141@...il.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Cong Wang <xiyou.wangcong@...il.com>,
        Network Development <netdev@...r.kernel.org>,
        syzbot <syzbot+4a2c52677a8a1aa283cb@...kaller.appspotmail.com>,
        William Tu <u9012063@...il.com>
Subject: Re: [Patch net] ip_gre: set dev->hard_header_len properly

On Thu, Oct 8, 2020 at 12:04 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> On Thu, Oct 8, 2020 at 1:34 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
> >
> > The uninit data is allocated by packet_alloc_skb(), if dev->hard_header_len
> > is 0 and 'len' is anything between [0, tunnel->hlen + sizeof(struct iphdr)),
> > dev_validate_header() still returns true obviously but only 'len'
> > bytes are copied
> > from user-space by skb_copy_datagram_from_iter(). Therefore, those bytes
> > within range (len, tunnel->hlen + sizeof(struct iphdr)] are uninitialized.
>
> With dev->hard_header_len of zero, packet_alloc_skb() only allocates len bytes.
>
> With SOCK_RAW, the writer is expected to write the ip and gre header
> and include these in the send len argument. The only difference I see
> is that with hard_header_len the data starts reserve bytes before
> skb_network_header, and an additional tail has been allocated that is
> not used.
>
> But this also fixes a potentially more serious bug. With SOCK_DGRAM,
> dev_hard_header/ipgre_header just assumes that there is enough room in
> the packet to skb_push(skb, t->hlen + sizeof(*iph)). Which may be
> false if this header length had not been reserved.
>
> Though I've mainly looked at packet_snd. Perhaps you are referring to
> tpacket_snd?

I think what Cong means is that hard_header_len has to be set properly
to prevent an AF_PACKET/RAW user from sending a frame that is too
short (shorter than the header length). When an AF_PACKET/RAW user
sends a frame shorter than the header length, and the code on the
sending path still expects a full header, it will read uninitialized
data.

If my understanding is right, I agree on this part.

However, there's something I don't understand in the GRE code. The
ipgre_header function only creates an IP header (20 bytes) + a GRE
base header (4 bytes), but pushes and returns "t->hlen +
sizeof(*iph)". What is t->hlen? It seems to me it is the sum of
t->tun_hlen and t->encap_hlen. What are these two?