Re: [PATCH bpf-next v5 4/6] bpf: Add helpers to issue and check SYN cookies in XDP

[Maxim Mikityanskiy <maximmi@xxxxxxxxxx>]

On 2022-04-14 01:48, Daniel Borkmann wrote:
> On 4/13/22 3:41 PM, Maxim Mikityanskiy wrote:
> [...]
> >   /* integer value in 'imm' field of BPF_CALL instruction selects 
> > which helper
> > diff --git a/net/core/filter.c b/net/core/filter.c
> > index 7446b0ba4e38..428cc63ecdf7 100644
> > --- a/net/core/filter.c
> > +++ b/net/core/filter.c
> > @@ -7425,6 +7425,124 @@ static const struct bpf_func_proto 
> > bpf_skb_set_tstamp_proto = {
> >       .arg3_type      = ARG_ANYTHING,
> >   };
> > +BPF_CALL_3(bpf_tcp_raw_gen_syncookie_ipv4, struct iphdr *, iph,
> > +       struct tcphdr *, th, u32, th_len)
> > +{
> > +#ifdef CONFIG_SYN_COOKIES
> > +    u32 cookie;
> > +    u16 mss;
> > +
> > +    if (unlikely(th_len < sizeof(*th) || th_len != th->doff * 4))
> > +        return -EINVAL;
> > +
> > +    mss = tcp_parse_mss_option(th, 0) ?: TCP_MSS_DEFAULT;
> > +    cookie = __cookie_v4_init_sequence(iph, th, &mss);
> > +
> > +    return cookie | ((u64)mss << 32);
> > +#else
> > +    return -EOPNOTSUPP;
> > +#endif /* CONFIG_SYN_COOKIES */
>
> This (and for other added helpers below) will be rather tricky to probe 
> for availability
> e.g. via `bpftool feature probe [...]`. Much better if you wrap the 
> ifdef CONFIG_SYN_COOKIES
> around the {xdp,tc_cls_act}_func_proto() instead as we do elsewhere.

Just for the record, I copied this pattern from the existing SYN cookie 
helpers, but what you suggest makes sense, so I'll fix it for my helpers 
and resubmit.

> > +}
> > +
> > +static const struct bpf_func_proto 
> > bpf_tcp_raw_gen_syncookie_ipv4_proto = {
> > +    .func        = bpf_tcp_raw_gen_syncookie_ipv4,
> > +    .gpl_only    = true, /* __cookie_v4_init_sequence() is GPL */
> > +    .pkt_access    = true,
> > +    .ret_type    = RET_INTEGER,
> > +    .arg1_type    = ARG_PTR_TO_MEM,
> > +    .arg1_size    = sizeof(struct iphdr),
> > +    .arg2_type    = ARG_PTR_TO_MEM,
> > +    .arg3_type    = ARG_CONST_SIZE,
> > +};
> > +
> > +BPF_CALL_3(bpf_tcp_raw_gen_syncookie_ipv6, struct ipv6hdr *, iph,
> > +       struct tcphdr *, th, u32, th_len)
> > +{
> > +#ifndef CONFIG_SYN_COOKIES
> > +    return -EOPNOTSUPP;
> > +#elif !IS_BUILTIN(CONFIG_IPV6)
> > +    return -EPROTONOSUPPORT;
> > +#else
> > +    const u16 mss_clamp = IPV6_MIN_MTU - sizeof(struct tcphdr) -
> > +        sizeof(struct ipv6hdr);
> > +    u32 cookie;
> > +    u16 mss;
> > +
> > +    if (unlikely(th_len < sizeof(*th) || th_len != th->doff * 4))
> > +        return -EINVAL;
> > +
> > +    mss = tcp_parse_mss_option(th, 0) ?: mss_clamp;
> > +    cookie = __cookie_v6_init_sequence(iph, th, &mss);
> > +
> > +    return cookie | ((u64)mss << 32);
> > +#endif
> > +}
> > +
> > +static const struct bpf_func_proto 
> > bpf_tcp_raw_gen_syncookie_ipv6_proto = {
> > +    .func        = bpf_tcp_raw_gen_syncookie_ipv6,
> > +    .gpl_only    = true, /* __cookie_v6_init_sequence() is GPL */
> > +    .pkt_access    = true,
> > +    .ret_type    = RET_INTEGER,
> > +    .arg1_type    = ARG_PTR_TO_MEM,
> > +    .arg1_size    = sizeof(struct ipv6hdr),
> > +    .arg2_type    = ARG_PTR_TO_MEM,
> > +    .arg3_type    = ARG_CONST_SIZE,
> > +};
> [...]
> >   bool bpf_helper_changes_pkt_data(void *func)
> > @@ -7837,6 +7955,14 @@ xdp_func_proto(enum bpf_func_id func_id, const 
> > struct bpf_prog *prog)
> >           return &bpf_tcp_check_syncookie_proto;
> >       case BPF_FUNC_tcp_gen_syncookie:
> >           return &bpf_tcp_gen_syncookie_proto;
> > +    case BPF_FUNC_tcp_raw_gen_syncookie_ipv4:
> > +        return &bpf_tcp_raw_gen_syncookie_ipv4_proto;
> > +    case BPF_FUNC_tcp_raw_gen_syncookie_ipv6:
> > +        return &bpf_tcp_raw_gen_syncookie_ipv6_proto;
> > +    case BPF_FUNC_tcp_raw_check_syncookie_ipv4:
> > +        return &bpf_tcp_raw_check_syncookie_ipv4_proto;
> > +    case BPF_FUNC_tcp_raw_check_syncookie_ipv6:
> > +        return &bpf_tcp_raw_check_syncookie_ipv6_proto;
> >   #endif
> >       default:
> >           return bpf_sk_base_func_proto(func_id);