eBPF program to filter prefixes in IPv6 RA

ra_prefix_filter.c

/**
 * ra_prefix_filter.c - eBPF program to filter prefixes in IPv6 RA
 * Copyright (C) 2023  Haowei Wen <yushijinhun@gmail.com>
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 */

/**
 * This program discards specific Prefix Information options in IPv6
 * Router Advertisement message, which allows you to blacklist specific
 * prefixes in SLAAC process while keeping other prefixes untouched.
 *
 * Steps to use this program:
 *
 * (1) Compile the program:
 *   clang -O2 -g -Wall -target bpf -c ra_prefix_filter.c -o ra_prefix_filter.o
 * 
 * (2) Load the program to the interface that receives RA messages:
 *   ip link set eth0 xdpgeneric obj ra_prefix_filter.o sec ra_prefix_filter
 * 
 * (3) Unload the program:
 *   ip link set eth0 xdpgeneric off
 *
 * To turn on debug logging, uncomment the DEBUG macro defined in this file.
 * Debug output is available in '/sys/kernel/debug/tracing/trace_pipe'.
 *
 * To specify the prefixes to discard, edit the 'on_prefix_information' function
 * defined in this file.
 */

#include <linux/bpf.h>
#include <linux/icmpv6.h>
#include <linux/if_ether.h>
#include <linux/ipv6.h>
#include <linux/pkt_cls.h>
#include <linux/types.h>

#include <bpf/bpf_endian.h>
#include <bpf/bpf_helpers.h>

// ==== UNCOMMENT THIS TO TURN ON DEBUG ====
// #define DEBUG 1

#ifdef DEBUG
#define bpf_debug(fmt, ...)                                                    \
    ({                                                                         \
        char ____fmt[] = fmt;                                                  \
        bpf_trace_printk(____fmt, sizeof(____fmt), ##__VA_ARGS__);             \
    })
#else
#define bpf_debug(fmt, ...)                                                    \
    {}                                                                         \
    while (0)
#endif

#define MAX_ND_OPTIONS 256
#define MAX_ND_OPTION_LEN 256

#define ICMPV6_ROUTER_ADVERTISEMENT 134
#define NDP_OPTION_PREFIX_INFORMATION 3

struct ra_msg {
    struct icmp6hdr icmph;
    __be32 reachable_time;
    __be32 retrans_timer;
} __attribute__((packed));

struct nd_opt_hdr {
    __u8 nd_opt_type;
    __u8 nd_opt_len;
} __attribute__((packed));

struct prefix_info {
    __u8 type;
    __u8 length;
    __u8 prefix_len;

#if defined(__BIG_ENDIAN_BITFIELD)
    __u8 onlink : 1, autoconf : 1, reserved : 6;
#elif defined(__LITTLE_ENDIAN_BITFIELD)
    __u8 reserved : 6, autoconf : 1, onlink : 1;
#else
#error "Please fix <asm/byteorder.h>"
#endif
    __be32 valid;
    __be32 prefered;
    __be32 reserved2;

    struct in6_addr prefix;
} __attribute__((packed));

#define IPv6(x0, x1, x2, x3, x4, x5, x6, x7)                                   \
    ((((__uint128_t)0x##x0) << 112) | (((__uint128_t)0x##x1) << 96) |          \
     (((__uint128_t)0x##x2) << 80) | (((__uint128_t)0x##x3) << 64) |           \
     (((__uint128_t)0x##x4) << 48) | (((__uint128_t)0x##x5) << 32) |           \
     (((__uint128_t)0x##x6) << 16) | ((__uint128_t)0x##x7))

static void __always_inline discard_prefix(struct prefix_info *prefix_info,
                                           __sum16 *csum, __uint128_t prefix,
                                           int prefixlen) {

    if (prefix_info->prefix_len >= prefixlen) {

        __uint128_t mask = (~(__uint128_t)0) << (128 - prefixlen);

        __uint128_t addr = 0;
#pragma unroll
        for (int i = 0; i < 16; i++) {
            addr <<= 8;
            addr |= prefix_info->prefix.in6_u.u6_addr8[i];
        }

        if ((addr & mask) == (prefix & mask)) {
            // Disallow

            // Set option type to 0 to make kernel ignore it
            prefix_info->type = 0;

            // Update checksum
            *csum = ~(~*csum + (-NDP_OPTION_PREFIX_INFORMATION) + 0);

            bpf_debug("Prefix %llx:%llx/%d discarded",
                      ((__u64)bpf_ntohl(prefix_info->prefix.in6_u.u6_addr32[0])
                       << 32) |
                          bpf_ntohl(prefix_info->prefix.in6_u.u6_addr32[1]),
                      ((__u64)bpf_ntohl(prefix_info->prefix.in6_u.u6_addr32[2])
                       << 32) |
                          bpf_ntohl(prefix_info->prefix.in6_u.u6_addr32[3]),
                      prefix_info->prefix_len);
            return;
        }
    }
    bpf_debug("Prefix %llx:%llx/%d allowed",
              ((__u64)bpf_ntohl(prefix_info->prefix.in6_u.u6_addr32[0]) << 32) |
                  bpf_ntohl(prefix_info->prefix.in6_u.u6_addr32[1]),
              ((__u64)bpf_ntohl(prefix_info->prefix.in6_u.u6_addr32[2]) << 32) |
                  bpf_ntohl(prefix_info->prefix.in6_u.u6_addr32[3]),
              prefix_info->prefix_len);
}

static void __always_inline
on_prefix_information(struct prefix_info *prefix_info, __sum16 *csum) {
    // ==== MODIFY THIS TO CUSTOMIZE ====
    // Example: discard the prefix if it falls in 2001:da8:1011::/48
    discard_prefix(prefix_info, csum, IPv6(2001, da8, 1011, 0, 0, 0, 0, 0), 48);
}

SEC("ra_prefix_filter")
int ra_prefix_filter_prog(struct xdp_md *xdp) {
    void *data = (void *)(long)xdp->data;
    void *data_end = (void *)(long)xdp->data_end;

    struct ethhdr *eth = data;
    if ((void *)eth + sizeof(*eth) > data_end)
        // Incomplete ethernet header
        return XDP_PASS;

    if (eth->h_proto != bpf_htons(ETH_P_IPV6))
        // Not an IPv6 packet
        return XDP_PASS;

    struct ipv6hdr *ipv6 = (void *)eth + sizeof(*eth);
    if ((void *)ipv6 + sizeof(*ipv6) > data_end)
        // Incomplete IPv6 header
        return XDP_PASS;

    if (ipv6->nexthdr != IPPROTO_ICMPV6)
        // Not ICMPv6
        return XDP_PASS;

    struct ra_msg *ra = (void *)ipv6 + sizeof(*ipv6);
    if ((void *)ra + sizeof(*ra) > data_end)
        // Not an complete RA message
        return XDP_PASS;

    if (ra->icmph.icmp6_type != ICMPV6_ROUTER_ADVERTISEMENT)
        // Not an RA
        return XDP_PASS;

    struct nd_opt_hdr *opt = (void *)ra + sizeof(*ra);

    for (int i = 0; i < MAX_ND_OPTIONS; i++) {

        if ((void *)opt == data_end)
            // End of packet reached
            break;

        if ((void *)opt + sizeof(*opt) > data_end)
            // Incomplete option header
            return XDP_PASS;

        // Length of current option in bytes
        __u16 opt_len = opt->nd_opt_len * 8;

        if (opt_len > MAX_ND_OPTION_LEN)
            return XDP_PASS;

        if ((void *)opt + opt_len > data_end)
            // Incomplete option
            return XDP_PASS;

        if (opt->nd_opt_type == NDP_OPTION_PREFIX_INFORMATION) {

            struct prefix_info *prefix_info = (void *)opt;

            // Ensure the prefix option has a correct length
            if (opt_len == sizeof(*prefix_info)) {

                if ((void *)prefix_info + sizeof(*prefix_info) > data_end)
                    // Redundant check to please eBPF verifier
                    return XDP_PASS;

                __sum16 csum = ra->icmph.icmp6_cksum;
                on_prefix_information(prefix_info, &csum);
                ra->icmph.icmp6_cksum = csum;
            }
        }

        opt = (void *)opt + opt_len;
    }

    return XDP_PASS;
}

char _license[] SEC("license") = "GPL";