netdev - Re: [PATCH bpf v2 4/4] bpf: Fix documentation of th_len in bpf_tcp_{gen,check}

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Message-ID: <5090da78-305c-dc42-65a6-ef0b2927db51@nvidia.com>
Date:   Mon, 31 Jan 2022 15:37:49 +0200
From:   Maxim Mikityanskiy <maximmi@...dia.com>
To:     Lorenz Bauer <lmb@...udflare.com>
CC:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        "Daniel Borkmann" <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Networking <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...dia.com>,
        "Martin KaFai Lau" <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Petar Penkov <ppenkov@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH bpf v2 4/4] bpf: Fix documentation of th_len in
 bpf_tcp_{gen,check}_syncookie

On 2022-01-26 11:45, Lorenz Bauer wrote:
> On Mon, 24 Jan 2022 at 15:13, Maxim Mikityanskiy <maximmi@...dia.com> wrote:
>>
>> bpf_tcp_gen_syncookie and bpf_tcp_check_syncookie expect the full length
>> of the TCP header (with all extensions). Fix the documentation that says
>> it should be sizeof(struct tcphdr).
> 
> I don't understand this change, sorry. Are you referring to the fact
> that the check is len < sizeof(*th) instead of len != sizeof(*th)?
> 
> Your commit message makes me think that the helpers will access data
> in the extension headers, which isn't true as far as I can tell.

Yes, they will. See bpf_tcp_gen_syncookie -> tcp_v4_get_syncookie -> 
tcp_get_syncookie_mss -> tcp_parse_mss_option, which iterates over the 
TCP options ("extensions" wasn't the best word I used here). Moreover, 
bpf_tcp_gen_syncookie even checks that th_len == th->doff * 4.

Although bpf_tcp_check_syncookie doesn't need the TCP options and 
doesn't enforce them to be passed, it's still allowed.

> That
> would be a problem in fact, since it could be used to read memory that
> the verifier hasn't deemed safe.

It's safe, because bpf_tcp_gen_syncookie reads up to th_len, which is 
ARG_CONST_SIZE for the TCP header.