netdev - Re: [bpf-next 6/6] selftests/bpf: add test for bpf_tcp_gen

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Date:   Tue, 23 Jul 2019 13:46:16 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Lorenz Bauer <lmb@...udflare.com>
Cc:     Petar Penkov <ppenkov.kernel@...il.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        davem@...emloft.net, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Stanislav Fomichev <sdf@...gle.com>,
        Petar Penkov <ppenkov@...gle.com>, yhs@...com
Subject: Re: [bpf-next 6/6] selftests/bpf: add test for bpf_tcp_gen_syncookie

On Tue, Jul 23, 2019 at 10:37:29AM +0100, Lorenz Bauer wrote:
> On Tue, 23 Jul 2019 at 01:20, Petar Penkov <ppenkov.kernel@...il.com> wrote:
> > +static __always_inline __s64 gen_syncookie(void *data_end, struct bpf_sock *sk,
> > +                                          void *iph, __u32 ip_size,
> > +                                          struct tcphdr *tcph)
> > +{
> > +       __u32 thlen = tcph->doff * 4;
> > +
> > +       if (tcph->syn && !tcph->ack) {
> > +               // packet should only have an MSS option
> > +               if (thlen != 24)
> > +                       return 0;
> 
> Just for my own understanding: without this the verifier complains since
> thlen is not a known value, even though it is in bounds due to the check below?

the verifier understands only constant part of the packet pointer.
Without additional 'if' above the statement:
if ((void *)tcph + thlen > data_end)
will add variables length 'thlen' to pkt pointer which will become
another pkt pointer (with different id).
That pointer would need 'pkt + const_range > data_end' to have valid access.

We hit this issue in the past when folks wanted to use bpf_csum_diff() helper
with variable size.
It's possible to extend the verifier to support that but it's intrusive,
since variable part would need to passed around to a bunch of check* functions.
I think it's tricky, but doable. Looking forward to patches :)

> > +
> > +               if ((void *)tcph + thlen > data_end)
> > +                       return 0;
> > +
> > +               return bpf_tcp_gen_syncookie(sk, iph, ip_size, tcph, thlen);
> > +       }
> > +       return 0;
> > +}
> > +
> 
> -- 
> Lorenz Bauer  |  Systems Engineer
> 6th Floor, County Hall/The Riverside Building, SE1 7PB, UK
> 
> www.cloudflare.com