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From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Lorenz Bauer <lmb@cloudflare.com>
Cc: Petar Penkov <ppenkov.kernel@gmail.com>,
	Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	davem@davemloft.net, Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Eric Dumazet <edumazet@google.com>,
	Stanislav Fomichev <sdf@google.com>,
	Petar Penkov <ppenkov@google.com>,
	yhs@fb.com
Subject: Re: [bpf-next 6/6] selftests/bpf: add test for bpf_tcp_gen_syncookie
Date: Tue, 23 Jul 2019 13:46:16 -0700	[thread overview]
Message-ID: <20190723204615.a6tia3f6fipdoht2@ast-mbp> (raw)
In-Reply-To: <CACAyw9-qQ8KbQk6Q6dg0+A337ZbSpot-sHpH_tSxFaQmUfhLyQ@mail.gmail.com>

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@gmail.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

  reply	other threads:[~2019-07-23 20:46 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-07-23  0:20 [bpf-next 0/6] Introduce a BPF helper to generate SYN cookies Petar Penkov
2019-07-23  0:20 ` [bpf-next 1/6] tcp: tcp_syn_flood_action read port from socket Petar Penkov
2019-07-23  0:20 ` [bpf-next 2/6] tcp: add skb-less helpers to retrieve SYN cookie Petar Penkov
2019-07-24  6:05   ` kbuild test robot
2019-07-24  6:19   ` kbuild test robot
2019-07-23  0:20 ` [bpf-next 3/6] bpf: add bpf_tcp_gen_syncookie helper Petar Penkov
2019-07-23 12:33   ` Toke Høiland-Jørgensen
2019-07-24  0:15     ` Petar Penkov
2019-07-23  0:20 ` [bpf-next 4/6] bpf: sync bpf.h to tools/ Petar Penkov
2019-07-23  0:20 ` [bpf-next 5/6] selftests/bpf: bpf_tcp_gen_syncookie->bpf_helpers Petar Penkov
2019-07-23  0:20 ` [bpf-next 6/6] selftests/bpf: add test for bpf_tcp_gen_syncookie Petar Penkov
2019-07-23  9:37   ` Lorenz Bauer
2019-07-23 20:46     ` Alexei Starovoitov [this message]
2019-07-23  6:30 ` [bpf-next 0/6] Introduce a BPF helper to generate SYN cookies Eric Dumazet
2019-07-23 10:27 ` Lorenz Bauer

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