// SPDX-License-Identifier: GPL-2.0
/* Copyright Amazon.com Inc. or its affiliates. */
#include "vmlinux.h"
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_endian.h>
#include "bpf_tracing_net.h"
#include "bpf_kfuncs.h"
#include "test_siphash.h"
#include "test_tcp_custom_syncookie.h"
#include "bpf_misc.h"
#define MAX_PACKET_OFF 0xffff
/* Hash is calculated for each client and split into ISN and TS.
*
* MSB LSB
* ISN: | 31 ... 8 | 7 6 | 5 | 4 | 3 2 1 0 |
* | Hash_1 | MSS | ECN | SACK | WScale |
*
* TS: | 31 ... 8 | 7 ... 0 |
* | Random | Hash_2 |
*/
#define COOKIE_BITS 8
#define COOKIE_MASK (((__u32)1 << COOKIE_BITS) - 1)
enum {
/* 0xf is invalid thus means that SYN did not have WScale. */
BPF_SYNCOOKIE_WSCALE_MASK = (1 << 4) - 1,
BPF_SYNCOOKIE_SACK = (1 << 4),
BPF_SYNCOOKIE_ECN = (1 << 5),
};
#define MSS_LOCAL_IPV4 65495
#define MSS_LOCAL_IPV6 65476
const __u16 msstab4[] = {
536,
1300,
1460,
MSS_LOCAL_IPV4,
};
const __u16 msstab6[] = {
1280 - 60, /* IPV6_MIN_MTU - 60 */
1480 - 60,
9000 - 60,
MSS_LOCAL_IPV6,
};
static siphash_key_t test_key_siphash = {
{ 0x0706050403020100ULL, 0x0f0e0d0c0b0a0908ULL }
};
struct tcp_syncookie {
struct __sk_buff *skb;
void *data;
void *data_end;
struct ethhdr *eth;
struct iphdr *ipv4;
struct ipv6hdr *ipv6;
struct tcphdr *tcp;
__be32 *ptr32;
struct bpf_tcp_req_attrs attrs;
u32 off;
u32 cookie;
u64 first;
};
bool handled_syn, handled_ack;
static int tcp_load_headers(struct tcp_syncookie *ctx)
{
ctx->data = (void *)(long)ctx->skb->data;
ctx->data_end = (void *)(long)ctx->skb->data_end;
ctx->eth = (struct ethhdr *)(long)ctx->skb->data;
if (ctx->eth + 1 > ctx->data_end)
goto err;
switch (bpf_ntohs(ctx->eth->h_proto)) {
case ETH_P_IP:
ctx->ipv4 = (struct iphdr *)(ctx->eth + 1);
if (ctx->ipv4 + 1 > ctx->data_end)
goto err;
if (ctx->ipv4->ihl != sizeof(*ctx->ipv4) / 4)
goto err;
if (ctx->ipv4->version != 4)
goto err;
if (ctx->ipv4->protocol != IPPROTO_TCP)
goto err;
ctx->tcp = (struct tcphdr *)(ctx->ipv4 + 1);
break;
case ETH_P_IPV6:
ctx->ipv6 = (struct ipv6hdr *)(ctx->eth + 1);
if (ctx->ipv6 + 1 > ctx->data_end)
goto err;
if (ctx->ipv6->version != 6)
goto err;
if (ctx->ipv6->nexthdr != NEXTHDR_TCP)
goto err;
ctx->tcp = (struct tcphdr *)(ctx->ipv6 + 1);
break;
default:
goto err;
}
if (ctx->tcp + 1 > ctx->data_end)
goto err;
return 0;
err:
return -1;
}
static int tcp_reload_headers(struct tcp_syncookie *ctx)
{
/* Without volatile,
* R3 32-bit pointer arithmetic prohibited
*/
volatile u64 data_len = ctx->skb->data_end - ctx->skb->data;
if (ctx->tcp->doff < sizeof(*ctx->tcp) / 4)
goto err;
/* Needed to calculate csum and parse TCP options. */
if (bpf_skb_change_tail(ctx->skb, data_len + 60 - ctx->tcp->doff * 4, 0))
goto err;
ctx->data = (void *)(long)ctx->skb->data;
ctx->data_end = (void *)(long)ctx->skb->data_end;
ctx->eth = (struct ethhdr *)(long)ctx->skb->data;
if (ctx->ipv4) {
ctx->ipv4 = (struct iphdr *)(ctx->eth + 1);
ctx->ipv6 = NULL;
ctx->tcp = (struct tcphdr *)(ctx->ipv4 + 1);
} else {
ctx->ipv4 = NULL;
ctx->ipv6 = (struct ipv6hdr *)(ctx->eth + 1);
ctx->tcp = (struct tcphdr *)(ctx->ipv6 + 1);
}
if ((void *)ctx->tcp + 60 > ctx->data_end)
goto err;
return 0;
err:
return -1;
}
static __sum16 tcp_v4_csum(struct tcp_syncookie *ctx, __wsum csum)
{
return csum_tcpudp_magic(ctx->ipv4->saddr, ctx->ipv4->daddr,
ctx->tcp->doff * 4, IPPROTO_TCP, csum);
}
static __sum16 tcp_v6_csum(struct tcp_syncookie *ctx, __wsum csum)
{
return csum_ipv6_magic(&ctx->ipv6->saddr, &ctx->ipv6->daddr,
ctx->tcp->doff * 4, IPPROTO_TCP, csum);
}
static int tcp_validate_header(struct tcp_syncookie *ctx)
{
s64 csum;
if (tcp_reload_headers(ctx))
goto err;
csum = bpf_csum_diff(0, 0, (void *)ctx->tcp, ctx->tcp->doff * 4, 0);
if (csum < 0)
goto err;
if (ctx->ipv4) {
/* check tcp_v4_csum(csum) is 0 if not on lo. */
csum = bpf_csum_diff(0, 0, (void *)ctx->ipv4, ctx->ipv4->ihl * 4, 0);
if (csum < 0)
goto err;
if (csum_fold(csum) != 0)
goto err;
} else if (ctx->ipv6) {
/* check tcp_v6_csum(csum) is 0 if not on lo. */
}
return 0;
err:
return -1;
}
static __always_inline void *next(struct tcp_syncookie *ctx, __u32 sz)
{
__u64 off = ctx->off;
__u8 *data;
/* Verifier forbids access to packet when offset exceeds MAX_PACKET_OFF */
if (off > MAX_PACKET_OFF - sz)
return NULL;
data = ctx->data + off;
barrier_var(data);
if (data + sz >= ctx->data_end)
return NULL;
ctx->off += sz;
return data;
}
static int tcp_parse_option(__u32 index, struct tcp_syncookie *ctx)
{
__u8 *opcode, *opsize, *wscale;
__u32 *tsval, *tsecr;
__u16 *mss;
__u32 off;
off = ctx->off;
opcode = next(ctx, 1);
if (!opcode)
goto stop;
if (*opcode == TCPOPT_EOL)
goto stop;
if (*opcode == TCPOPT_NOP)
goto next;
opsize = next(ctx, 1);
if (!opsize)
goto stop;
if (*opsize < 2)
goto stop;
switch (*opcode) {
case TCPOPT_MSS:
mss = next(ctx, 2);
if (*opsize == TCPOLEN_MSS && ctx->tcp->syn && mss)
ctx->attrs.mss = get_unaligned_be16(mss);
break;
case TCPOPT_WINDOW:
wscale = next(ctx, 1);
if (*opsize == TCPOLEN_WINDOW && ctx->tcp->syn && wscale) {
ctx->attrs.wscale_ok = 1;
ctx->attrs.snd_wscale = *wscale;
}
break;
case TCPOPT_TIMESTAMP:
tsval = next(ctx, 4);
tsecr = next(ctx, 4);
if (*opsize == TCPOLEN_TIMESTAMP && tsval && tsecr) {
ctx->attrs.rcv_tsval = get_unaligned_be32(tsval);
ctx->attrs.rcv_tsecr = get_unaligned_be32(tsecr);
if (ctx->tcp->syn && ctx->attrs.rcv_tsecr)
ctx->attrs.tstamp_ok = 0;
else
ctx->attrs.tstamp_ok = 1;
}
break;
case TCPOPT_SACK_PERM:
if (*opsize == TCPOLEN_SACK_PERM && ctx->tcp->syn)
ctx->attrs.sack_ok = 1;
break;
}
ctx->off = off + *opsize;
next:
return 0;
stop:
return 1;
}
static void tcp_parse_options(struct tcp_syncookie *ctx)
{
ctx->off = (__u8 *)(ctx->tcp + 1) - (__u8 *)ctx->data,
bpf_loop(40, tcp_parse_option, ctx, 0);
}
static int tcp_validate_sysctl(struct tcp_syncookie *ctx)
{
if ((ctx->ipv4 && ctx->attrs.mss != MSS_LOCAL_IPV4) ||
(ctx->ipv6 && ctx->attrs.mss != MSS_LOCAL_IPV6))
goto err;
if (!ctx->attrs.wscale_ok || ctx->attrs.snd_wscale != 7)
goto err;
if (!ctx->attrs.tstamp_ok)
goto err;
if (!ctx->attrs.sack_ok)
goto err;
if (!ctx->tcp->ece || !ctx->tcp->cwr)
goto err;
return 0;
err:
return -1;
}
static void tcp_prepare_cookie(struct tcp_syncookie *ctx)
{
u32 seq = bpf_ntohl(ctx->tcp->seq);
u64 first = 0, second;
int mssind = 0;
u32 hash;
if (ctx->ipv4) {
for (mssind = ARRAY_SIZE(msstab4) - 1; mssind; mssind--)
if (ctx->attrs.mss >= msstab4[mssind])
break;
ctx->attrs.mss = msstab4[mssind];
first = (u64)ctx->ipv4->saddr << 32 | ctx->ipv4->daddr;
} else if (ctx->ipv6) {
for (mssind = ARRAY_SIZE(msstab6) - 1; mssind; mssind--)
if (ctx->attrs.mss >= msstab6[mssind])
break;
ctx->attrs.mss = msstab6[mssind];
first = (u64)ctx->ipv6->saddr.in6_u.u6_addr8[0] << 32 |
ctx->ipv6->daddr.in6_u.u6_addr32[0];
}
second = (u64)seq << 32 | ctx->tcp->source << 16 | ctx->tcp->dest;
hash = siphash_2u64(first, second, &test_key_siphash);
if (ctx->attrs.tstamp_ok) {
ctx->attrs.rcv_tsecr = bpf_get_prandom_u32();
ctx->attrs.rcv_tsecr &= ~COOKIE_MASK;
ctx->attrs.rcv_tsecr |= hash & COOKIE_MASK;
}
hash &= ~COOKIE_MASK;
hash |= mssind << 6;
if (ctx->attrs.wscale_ok)
hash |= ctx->attrs.snd_wscale & BPF_SYNCOOKIE_WSCALE_MASK;
if (ctx->attrs.sack_ok)
hash |= BPF_SYNCOOKIE_SACK;
if (ctx->attrs.tstamp_ok && ctx->tcp->ece && ctx->tcp->cwr)
hash |= BPF_SYNCOOKIE_ECN;
ctx->cookie = hash;
}
static void tcp_write_options(struct tcp_syncookie *ctx)
{
ctx->ptr32 = (__be32 *)(ctx->tcp + 1);
*ctx->ptr32++ = bpf_htonl(TCPOPT_MSS << 24 | TCPOLEN_MSS << 16 |
ctx->attrs.mss);
if (ctx->attrs.wscale_ok)
*ctx->ptr32++ = bpf_htonl(TCPOPT_NOP << 24 |
TCPOPT_WINDOW << 16 |
TCPOLEN_WINDOW << 8 |
ctx->attrs.snd_wscale);
if (ctx->attrs.tstamp_ok) {
if (ctx->attrs.sack_ok)
*ctx->ptr32++ = bpf_htonl(TCPOPT_SACK_PERM << 24 |
TCPOLEN_SACK_PERM << 16 |
TCPOPT_TIMESTAMP << 8 |
TCPOLEN_TIMESTAMP);
else
*ctx->ptr32++ = bpf_htonl(TCPOPT_NOP << 24 |
TCPOPT_NOP << 16 |
TCPOPT_TIMESTAMP << 8 |
TCPOLEN_TIMESTAMP);
*ctx->ptr32++ = bpf_htonl(ctx->attrs.rcv_tsecr);
*ctx->ptr32++ = bpf_htonl(ctx->attrs.rcv_tsval);
} else if (ctx->attrs.sack_ok) {
*ctx->ptr32++ = bpf_htonl(TCPOPT_NOP << 24 |
TCPOPT_NOP << 16 |
TCPOPT_SACK_PERM << 8 |
TCPOLEN_SACK_PERM);
}
}
static int tcp_handle_syn(struct tcp_syncookie *ctx)
{
s64 csum;
if (tcp_validate_header(ctx))
goto err;
tcp_parse_options(ctx);
if (tcp_validate_sysctl(ctx))
goto err;
tcp_prepare_cookie(ctx);
tcp_write_options(ctx);
swap(ctx->tcp->source, ctx->tcp->dest);
ctx->tcp->check = 0;
ctx->tcp->ack_seq = bpf_htonl(bpf_ntohl(ctx->tcp->seq) + 1);
ctx->tcp->seq = bpf_htonl(ctx->cookie);
ctx->tcp->doff = ((long)ctx->ptr32 - (long)ctx->tcp) >> 2;
ctx->tcp->ack = 1;
if (!ctx->attrs.tstamp_ok || !ctx->tcp->ece || !ctx->tcp->cwr)
ctx->tcp->ece = 0;
ctx->tcp->cwr = 0;
csum = bpf_csum_diff(0, 0, (void *)ctx->tcp, ctx->tcp->doff * 4, 0);
if (csum < 0)
goto err;
if (ctx->ipv4) {
swap(ctx->ipv4->saddr, ctx->ipv4->daddr);
ctx->tcp->check = tcp_v4_csum(ctx, csum);
ctx->ipv4->check = 0;
ctx->ipv4->tos = 0;
ctx->ipv4->tot_len = bpf_htons((long)ctx->ptr32 - (long)ctx->ipv4);
ctx->ipv4->id = 0;
ctx->ipv4->ttl = 64;
csum = bpf_csum_diff(0, 0, (void *)ctx->ipv4, sizeof(*ctx->ipv4), 0);
if (csum < 0)
goto err;
ctx->ipv4->check = csum_fold(csum);
} else if (ctx->ipv6) {
swap(ctx->ipv6->saddr, ctx->ipv6->daddr);
ctx->tcp->check = tcp_v6_csum(ctx, csum);
*(__be32 *)ctx->ipv6 = bpf_htonl(0x60000000);
ctx->ipv6->payload_len = bpf_htons((long)ctx->ptr32 - (long)ctx->tcp);
ctx->ipv6->hop_limit = 64;
}
swap_array(ctx->eth->h_source, ctx->eth->h_dest);
if (bpf_skb_change_tail(ctx->skb, (long)ctx->ptr32 - (long)ctx->eth, 0))
goto err;
return bpf_redirect(ctx->skb->ifindex, 0);
err:
return TC_ACT_SHOT;
}
static int tcp_validate_cookie(struct tcp_syncookie *ctx)
{
u32 cookie = bpf_ntohl(ctx->tcp->ack_seq) - 1;
u32 seq = bpf_ntohl(ctx->tcp->seq) - 1;
u64 first = 0, second;
int mssind;
u32 hash;
if (ctx->ipv4)
first = (u64)ctx->ipv4->saddr << 32 | ctx->ipv4->daddr;
else if (ctx->ipv6)
first = (u64)ctx->ipv6->saddr.in6_u.u6_addr8[0] << 32 |
ctx->ipv6->daddr.in6_u.u6_addr32[0];
second = (u64)seq << 32 | ctx->tcp->source << 16 | ctx->tcp->dest;
hash = siphash_2u64(first, second, &test_key_siphash);
if (ctx->attrs.tstamp_ok)
hash -= ctx->attrs.rcv_tsecr & COOKIE_MASK;
else
hash &= ~COOKIE_MASK;
hash -= cookie & ~COOKIE_MASK;
if (hash)
goto err;
mssind = (cookie & (3 << 6)) >> 6;
if (ctx->ipv4)
ctx->attrs.mss = msstab4[mssind];
else
ctx->attrs.mss = msstab6[mssind];
ctx->attrs.snd_wscale = cookie & BPF_SYNCOOKIE_WSCALE_MASK;
ctx->attrs.rcv_wscale = ctx->attrs.snd_wscale;
ctx->attrs.wscale_ok = ctx->attrs.snd_wscale == BPF_SYNCOOKIE_WSCALE_MASK;
ctx->attrs.sack_ok = cookie & BPF_SYNCOOKIE_SACK;
ctx->attrs.ecn_ok = cookie & BPF_SYNCOOKIE_ECN;
return 0;
err:
return -1;
}
static int tcp_handle_ack(struct tcp_syncookie *ctx)
{
struct bpf_sock_tuple tuple;
struct bpf_sock *skc;
int ret = TC_ACT_OK;
struct sock *sk;
u32 tuple_size;
if (ctx->ipv4) {
tuple.ipv4.saddr = ctx->ipv4->saddr;
tuple.ipv4.daddr = ctx->ipv4->daddr;
tuple.ipv4.sport = ctx->tcp->source;
tuple.ipv4.dport = ctx->tcp->dest;
tuple_size = sizeof(tuple.ipv4);
} else if (ctx->ipv6) {
__builtin_memcpy(tuple.ipv6.saddr, &ctx->ipv6->saddr, sizeof(tuple.ipv6.saddr));
__builtin_memcpy(tuple.ipv6.daddr, &ctx->ipv6->daddr, sizeof(tuple.ipv6.daddr));
tuple.ipv6.sport = ctx->tcp->source;
tuple.ipv6.dport = ctx->tcp->dest;
tuple_size = sizeof(tuple.ipv6);
} else {
goto out;
}
skc = bpf_skc_lookup_tcp(ctx->skb, &tuple, tuple_size, -1, 0);
if (!skc)
goto out;
if (skc->state != TCP_LISTEN)
goto release;
sk = (struct sock *)bpf_skc_to_tcp_sock(skc);
if (!sk)
goto err;
if (tcp_validate_header(ctx))
goto err;
tcp_parse_options(ctx);
if (tcp_validate_cookie(ctx))
goto err;
ret = bpf_sk_assign_tcp_reqsk(ctx->skb, sk, &ctx->attrs, sizeof(ctx->attrs));
if (ret < 0)
goto err;
release:
bpf_sk_release(skc);
out:
return ret;
err:
ret = TC_ACT_SHOT;
goto release;
}
SEC("tc")
int tcp_custom_syncookie(struct __sk_buff *skb)
{
struct tcp_syncookie ctx = {
.skb = skb,
};
if (tcp_load_headers(&ctx))
return TC_ACT_OK;
if (ctx.tcp->rst)
return TC_ACT_OK;
if (ctx.tcp->syn) {
if (ctx.tcp->ack)
return TC_ACT_OK;
handled_syn = true;
return tcp_handle_syn(&ctx);
}
handled_ack = true;
return tcp_handle_ack(&ctx);
}
char _license[] SEC("license") = "GPL";
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