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Date:   Tue, 26 Feb 2019 05:37:55 +0000
From:   Martin Lau <kafai@...com>
To:     Lorenz Bauer <lmb@...udflare.com>
CC:     "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>
Subject: Re: [PATCH 1/3] bpf: add helper to check for a valid SYN cookie

On Mon, Feb 25, 2019 at 06:26:42PM +0000, Lorenz Bauer wrote:
> On Sat, 23 Feb 2019 at 00:44, Martin Lau <kafai@...com> wrote:
> >
> > On Fri, Feb 22, 2019 at 09:50:55AM +0000, Lorenz Bauer wrote:
> > > Using bpf_sk_lookup_tcp it's possible to ascertain whether a packet belongs
> > > to a known connection. However, there is one corner case: no sockets are
> > > created if SYN cookies are active. This means that the final ACK in the
> > > 3WHS is misclassified.
> > >
> > > Using the helper, we can look up the listening socket via bpf_sk_lookup_tcp
> > > and then check whether a packet is a valid SYN cookie ACK.
> > >
> > > Signed-off-by: Lorenz Bauer <lmb@...udflare.com>
> > > ---
> > >  include/uapi/linux/bpf.h | 18 ++++++++++-
> > >  net/core/filter.c        | 68 ++++++++++++++++++++++++++++++++++++++++
> > >  2 files changed, 85 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > > index bcdd2474eee7..bc2af87e9621 100644
> > > --- a/include/uapi/linux/bpf.h
> > > +++ b/include/uapi/linux/bpf.h
> > > @@ -2359,6 +2359,21 @@ union bpf_attr {
> > >   *   Return
> > >   *           A **struct bpf_tcp_sock** pointer on success, or NULL in
> > >   *           case of failure.
> > > + *
> > > + * int bpf_sk_check_syncookie(struct bpf_sock *sk, void *iph, u32 iph_len, struct tcphdr *th, u32 th_len)
> > > + *   Description
> > > + *           Check whether iph and th contain a valid SYN cookie ACK for
> > > + *           the listening socket in sk.
> > > + *
> > > + *           iph points to the start of the IPv4 or IPv6 header, while
> > > + *           iph_len contains sizeof(struct iphdr) or sizeof(struct ip6hdr).
> > > + *
> > > + *           th points to the start of the TCP header, while th_len contains
> > > + *           sizeof(struct tcphdr).
> > > + *
> > > + *   Return
> > > + *           0 if iph and th are a valid SYN cookie ACK, or a negative error
> > > + *           otherwise.
> > >   */
> > >  #define __BPF_FUNC_MAPPER(FN)                \
> > >       FN(unspec),                     \
> > > @@ -2457,7 +2472,8 @@ union bpf_attr {
> > >       FN(spin_lock),                  \
> > >       FN(spin_unlock),                \
> > >       FN(sk_fullsock),                \
> > > -     FN(tcp_sock),
> > > +     FN(tcp_sock),                   \
> > > +     FN(sk_check_syncookie),
> > >
> > >  /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> > >   * function eBPF program intends to call
> > > diff --git a/net/core/filter.c b/net/core/filter.c
> > > index 85749f6ec789..9e68897cc7ed 100644
> > > --- a/net/core/filter.c
> > > +++ b/net/core/filter.c
> > > @@ -5426,6 +5426,70 @@ static const struct bpf_func_proto bpf_tcp_sock_proto = {
> > >       .arg1_type      = ARG_PTR_TO_SOCK_COMMON,
> > >  };
> > >
> > > +BPF_CALL_5(bpf_sk_check_syncookie, struct sock *, sk, void *, iph, u32, iph_len,
> > s/bpf_sk_check_syncookie/bpf_tcp_check_syncookie/>
> >
> > > +        struct tcphdr *, th, u32, th_len)
> > > +{
> > > +#if IS_ENABLED(CONFIG_SYN_COOKIES)
> > nit. "#ifdef CONFIG_SYN_COOKIES" such that it is clear it is a bool kconfig.
> >
> > > +     u32 cookie;
> > > +     int ret;
> > > +
> > > +     if (unlikely(th_len < sizeof(*th)))
> > > +             return -EINVAL;
> > > +
> > > +     /* sk_listener() allows TCP_NEW_SYN_RECV, which makes no sense here. */
> > > +     if (sk->sk_protocol != IPPROTO_TCP || sk->sk_state != TCP_LISTEN)
> > From the test program in patch 3, the "sk" here is obtained from
> > bpf_sk_lookup_tcp() which does a sk_to_full_sk() before returning.
> > AFAICT, meaning bpf_sk_lookup_tcp() will return the listening sk
> > even if there is a request_sock.  Does it make sense to check
> > syncookie if there is already a request_sock?
> 
> No, that doesn't make a lot of sense. I hadn't realised that
> sk_lookup_tcp only returns full sockets.
> This means we need a way to detect that there is a request sock for a
> given tuple.
> 
> * adding a reqsk_exists(tuple) helper means we have to pay the lookup cost twice
> * drop the sk argument and do the necessary lookups in the helper
> itself, but that also
>   wastes a call to __inet_lookup_listener
> * skip sk_to_full_sk() in a helper and return RET_PTR_TO_SOCK_COMMON,
>   but that violates a bunch of assumptions (e.g. calling bpf_sk_release on them)
How about creating a new lookup helper, bpf_sk"c"_lookup_tcp,
that does not call sk_to_full_sk() before returning.
Its ".ret_type" will be RET_PTR_TO_SOCK_COMMON_OR_NULL which its
reference(-counting) state has to be tracked in the verifier also.
Mainly in check_helper_call(), iirc.

The bpf_prog can then check bpf_sock->state for TCP_LISTEN,
call bpf_tcp_sock() to get the TCP listener sock and pass to
the bpf_tcp_check_syncookie()

> 
> For context: ultimately we want use this to answer the question: does
> this (encapsulated)
> packet contain a payload destined to a local socket? Amongst the edge
> cases we need to
> handle are ICMP Packet Too Big messages and SYN cookies. A solution
> would be to hide
> all this in an "uber" helper that takes pointers to the L3 / L4
> headers and returns a verdict,
> but that seems a bit gross.
Please include this use case in the commit message.
It is useful.

> 
> >
> > > +             return -EINVAL;
> > > +
> > > +     if (!sock_net(sk)->ipv4.sysctl_tcp_syncookies)
> > Should tcp_synq_no_recent_overflow(tp) be checked also?
> >
> 
> Yes, not sure how that slipped out.
> 
> > > +             return -EINVAL;
> > > +
> > > +     if (!th->ack || th->rst)
> > How about th->syn?
> >
> 
> Yes, I missed the fact that the callers in tcp_ipv{4,6}.c check this.
> 
> > > +             return -ENOENT;
> > > +
> > > +     cookie = ntohl(th->ack_seq) - 1;
> > > +
> > > +     switch (sk->sk_family) {
> > > +     case AF_INET:
> > > +             if (unlikely(iph_len < sizeof(struct iphdr)))
> > > +                     return -EINVAL;
> > > +
> > > +             ret = __cookie_v4_check((struct iphdr *)iph, th, cookie);
> > > +             break;
> > > +
> > > +#if IS_ENABLED(CONFIG_IPV6)
> > > +     case AF_INET6:
> > > +             if (unlikely(iph_len < sizeof(struct ipv6hdr)))
> > > +                     return -EINVAL;
> > > +
> > > +             ret = __cookie_v6_check((struct ipv6hdr *)iph, th, cookie);
> > > +             break;
> > > +#endif /* CONFIG_IPV6 */
> > > +
> > > +     default:
> > > +             return -EPROTONOSUPPORT;
> > > +     }
> > > +
> > > +     if (ret > 0)
> > > +             return 0;
> > > +
> > > +     return -ENOENT;
> > > +#else
> > > +     return -ENOTSUP;
> > > +#endif
> > > +}
> > > +
> > > +static const struct bpf_func_proto bpf_sk_check_syncookie_proto = {
> > > +     .func           = bpf_sk_check_syncookie,
> > > +     .gpl_only       = true,
> > > +     .pkt_access     = true,
> > > +     .ret_type       = RET_INTEGER,
> > > +     .arg1_type      = ARG_PTR_TO_SOCKET,
> > I think it should be ARG_PTR_TO_TCP_SOCK
> >
> > > +     .arg2_type      = ARG_PTR_TO_MEM,
> > > +     .arg3_type      = ARG_CONST_SIZE,
> > > +     .arg4_type      = ARG_PTR_TO_MEM,
> > > +     .arg5_type      = ARG_CONST_SIZE,
> > > +};
> > > +
> > >  #endif /* CONFIG_INET */
> 
> 
> 
> -- 
> Lorenz Bauer  |  Systems Engineer
> 25 Lavington St., London SE1 0NZ
> 
> https://urldefense.proofpoint.com/v2/url?u=http-3A__www.cloudflare.com&d=DwIBaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=VQnoQ7LvghIj0gVEaiQSUw&m=xhDwvX3iD-mbqSrx-L8XQNaZiYFZzMWNo_2Y38Z9j34&s=I4Ag3HflabFppFv7UtMp8WnMVSqCDW0W28ziWIvuwDE&e=

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