1 // SPDX-License-Identifier: GPL-2.0
2 /* Copyright Amazon.com Inc. or its affiliates. */
3
4 #include "vmlinux.h"
5
6 #include <bpf/bpf_helpers.h>
7 #include <bpf/bpf_endian.h>
8 #include "bpf_tracing_net.h"
9 #include "bpf_kfuncs.h"
10 #include "test_siphash.h"
11 #include "test_tcp_custom_syncookie.h"
12 #include "bpf_misc.h"
13
14 #define MAX_PACKET_OFF 0xffff
15
16 /* Hash is calculated for each client and split into ISN and TS.
17 *
18 * MSB LSB
19 * ISN: | 31 ... 8 | 7 6 | 5 | 4 | 3 2 1 0 |
20 * | Hash_1 | MSS | ECN | SACK | WScale |
21 *
22 * TS: | 31 ... 8 | 7 ... 0 |
23 * | Random | Hash_2 |
24 */
25 #define COOKIE_BITS 8
26 #define COOKIE_MASK (((__u32)1 << COOKIE_BITS) - 1)
27
28 enum {
29 /* 0xf is invalid thus means that SYN did not have WScale. */
30 BPF_SYNCOOKIE_WSCALE_MASK = (1 << 4) - 1,
31 BPF_SYNCOOKIE_SACK = (1 << 4),
32 BPF_SYNCOOKIE_ECN = (1 << 5),
33 };
34
35 #define MSS_LOCAL_IPV4 65495
36 #define MSS_LOCAL_IPV6 65476
37
38 const __u16 msstab4[] = {
39 536,
40 1300,
41 1460,
42 MSS_LOCAL_IPV4,
43 };
44
45 const __u16 msstab6[] = {
46 1280 - 60, /* IPV6_MIN_MTU - 60 */
47 1480 - 60,
48 9000 - 60,
49 MSS_LOCAL_IPV6,
50 };
51
52 static siphash_key_t test_key_siphash = {
53 { 0x0706050403020100ULL, 0x0f0e0d0c0b0a0908ULL }
54 };
55
56 struct tcp_syncookie {
57 struct __sk_buff *skb;
58 void *data;
59 void *data_end;
60 struct ethhdr *eth;
61 struct iphdr *ipv4;
62 struct ipv6hdr *ipv6;
63 struct tcphdr *tcp;
64 __be32 *ptr32;
65 struct bpf_tcp_req_attrs attrs;
66 u32 off;
67 u32 cookie;
68 u64 first;
69 };
70
71 bool handled_syn, handled_ack;
72
73 static int tcp_load_headers(struct tcp_syncookie *ctx)
74 {
75 ctx->data = (void *)(long)ctx->skb->data;
76 ctx->data_end = (void *)(long)ctx->skb->data_end;
77 ctx->eth = (struct ethhdr *)(long)ctx->skb->data;
78
79 if (ctx->eth + 1 > ctx->data_end)
80 goto err;
81
82 switch (bpf_ntohs(ctx->eth->h_proto)) {
83 case ETH_P_IP:
84 ctx->ipv4 = (struct iphdr *)(ctx->eth + 1);
85
86 if (ctx->ipv4 + 1 > ctx->data_end)
87 goto err;
88
89 if (ctx->ipv4->ihl != sizeof(*ctx->ipv4) / 4)
90 goto err;
91
92 if (ctx->ipv4->version != 4)
93 goto err;
94
95 if (ctx->ipv4->protocol != IPPROTO_TCP)
96 goto err;
97
98 ctx->tcp = (struct tcphdr *)(ctx->ipv4 + 1);
99 break;
100 case ETH_P_IPV6:
101 ctx->ipv6 = (struct ipv6hdr *)(ctx->eth + 1);
102
103 if (ctx->ipv6 + 1 > ctx->data_end)
104 goto err;
105
106 if (ctx->ipv6->version != 6)
107 goto err;
108
109 if (ctx->ipv6->nexthdr != NEXTHDR_TCP)
110 goto err;
111
112 ctx->tcp = (struct tcphdr *)(ctx->ipv6 + 1);
113 break;
114 default:
115 goto err;
116 }
117
118 if (ctx->tcp + 1 > ctx->data_end)
119 goto err;
120
121 return 0;
122 err:
123 return -1;
124 }
125
126 static int tcp_reload_headers(struct tcp_syncookie *ctx)
127 {
128 /* Without volatile,
129 * R3 32-bit pointer arithmetic prohibited
130 */
131 volatile u64 data_len = ctx->skb->data_end - ctx->skb->data;
132
133 if (ctx->tcp->doff < sizeof(*ctx->tcp) / 4)
134 goto err;
135
136 /* Needed to calculate csum and parse TCP options. */
137 if (bpf_skb_change_tail(ctx->skb, data_len + 60 - ctx->tcp->doff * 4, 0))
138 goto err;
139
140 ctx->data = (void *)(long)ctx->skb->data;
141 ctx->data_end = (void *)(long)ctx->skb->data_end;
142 ctx->eth = (struct ethhdr *)(long)ctx->skb->data;
143 if (ctx->ipv4) {
144 ctx->ipv4 = (struct iphdr *)(ctx->eth + 1);
145 ctx->ipv6 = NULL;
146 ctx->tcp = (struct tcphdr *)(ctx->ipv4 + 1);
147 } else {
148 ctx->ipv4 = NULL;
149 ctx->ipv6 = (struct ipv6hdr *)(ctx->eth + 1);
150 ctx->tcp = (struct tcphdr *)(ctx->ipv6 + 1);
151 }
152
153 if ((void *)ctx->tcp + 60 > ctx->data_end)
154 goto err;
155
156 return 0;
157 err:
158 return -1;
159 }
160
161 static __sum16 tcp_v4_csum(struct tcp_syncookie *ctx, __wsum csum)
162 {
163 return csum_tcpudp_magic(ctx->ipv4->saddr, ctx->ipv4->daddr,
164 ctx->tcp->doff * 4, IPPROTO_TCP, csum);
165 }
166
167 static __sum16 tcp_v6_csum(struct tcp_syncookie *ctx, __wsum csum)
168 {
169 return csum_ipv6_magic(&ctx->ipv6->saddr, &ctx->ipv6->daddr,
170 ctx->tcp->doff * 4, IPPROTO_TCP, csum);
171 }
172
173 static int tcp_validate_header(struct tcp_syncookie *ctx)
174 {
175 s64 csum;
176
177 if (tcp_reload_headers(ctx))
178 goto err;
179
180 csum = bpf_csum_diff(0, 0, (void *)ctx->tcp, ctx->tcp->doff * 4, 0);
181 if (csum < 0)
182 goto err;
183
184 if (ctx->ipv4) {
185 /* check tcp_v4_csum(csum) is 0 if not on lo. */
186
187 csum = bpf_csum_diff(0, 0, (void *)ctx->ipv4, ctx->ipv4->ihl * 4, 0);
188 if (csum < 0)
189 goto err;
190
191 if (csum_fold(csum) != 0)
192 goto err;
193 } else if (ctx->ipv6) {
194 /* check tcp_v6_csum(csum) is 0 if not on lo. */
195 }
196
197 return 0;
198 err:
199 return -1;
200 }
201
202 static __always_inline void *next(struct tcp_syncookie *ctx, __u32 sz)
203 {
204 __u64 off = ctx->off;
205 __u8 *data;
206
207 /* Verifier forbids access to packet when offset exceeds MAX_PACKET_OFF */
208 if (off > MAX_PACKET_OFF - sz)
209 return NULL;
210
211 data = ctx->data + off;
212 barrier_var(data);
213 if (data + sz >= ctx->data_end)
214 return NULL;
215
216 ctx->off += sz;
217 return data;
218 }
219
220 static int tcp_parse_option(__u32 index, struct tcp_syncookie *ctx)
221 {
222 __u8 *opcode, *opsize, *wscale;
223 __u32 *tsval, *tsecr;
224 __u16 *mss;
225 __u32 off;
226
227 off = ctx->off;
228 opcode = next(ctx, 1);
229 if (!opcode)
230 goto stop;
231
232 if (*opcode == TCPOPT_EOL)
233 goto stop;
234
235 if (*opcode == TCPOPT_NOP)
236 goto next;
237
238 opsize = next(ctx, 1);
239 if (!opsize)
240 goto stop;
241
242 if (*opsize < 2)
243 goto stop;
244
245 switch (*opcode) {
246 case TCPOPT_MSS:
247 mss = next(ctx, 2);
248 if (*opsize == TCPOLEN_MSS && ctx->tcp->syn && mss)
249 ctx->attrs.mss = get_unaligned_be16(mss);
250 break;
251 case TCPOPT_WINDOW:
252 wscale = next(ctx, 1);
253 if (*opsize == TCPOLEN_WINDOW && ctx->tcp->syn && wscale) {
254 ctx->attrs.wscale_ok = 1;
255 ctx->attrs.snd_wscale = *wscale;
256 }
257 break;
258 case TCPOPT_TIMESTAMP:
259 tsval = next(ctx, 4);
260 tsecr = next(ctx, 4);
261 if (*opsize == TCPOLEN_TIMESTAMP && tsval && tsecr) {
262 ctx->attrs.rcv_tsval = get_unaligned_be32(tsval);
263 ctx->attrs.rcv_tsecr = get_unaligned_be32(tsecr);
264
265 if (ctx->tcp->syn && ctx->attrs.rcv_tsecr)
266 ctx->attrs.tstamp_ok = 0;
267 else
268 ctx->attrs.tstamp_ok = 1;
269 }
270 break;
271 case TCPOPT_SACK_PERM:
272 if (*opsize == TCPOLEN_SACK_PERM && ctx->tcp->syn)
273 ctx->attrs.sack_ok = 1;
274 break;
275 }
276
277 ctx->off = off + *opsize;
278 next:
279 return 0;
280 stop:
281 return 1;
282 }
283
284 static void tcp_parse_options(struct tcp_syncookie *ctx)
285 {
286 ctx->off = (__u8 *)(ctx->tcp + 1) - (__u8 *)ctx->data,
287
288 bpf_loop(40, tcp_parse_option, ctx, 0);
289 }
290
291 static int tcp_validate_sysctl(struct tcp_syncookie *ctx)
292 {
293 if ((ctx->ipv4 && ctx->attrs.mss != MSS_LOCAL_IPV4) ||
294 (ctx->ipv6 && ctx->attrs.mss != MSS_LOCAL_IPV6))
295 goto err;
296
297 if (!ctx->attrs.wscale_ok || ctx->attrs.snd_wscale != 7)
298 goto err;
299
300 if (!ctx->attrs.tstamp_ok)
301 goto err;
302
303 if (!ctx->attrs.sack_ok)
304 goto err;
305
306 if (!ctx->tcp->ece || !ctx->tcp->cwr)
307 goto err;
308
309 return 0;
310 err:
311 return -1;
312 }
313
314 static void tcp_prepare_cookie(struct tcp_syncookie *ctx)
315 {
316 u32 seq = bpf_ntohl(ctx->tcp->seq);
317 u64 first = 0, second;
318 int mssind = 0;
319 u32 hash;
320
321 if (ctx->ipv4) {
322 for (mssind = ARRAY_SIZE(msstab4) - 1; mssind; mssind--)
323 if (ctx->attrs.mss >= msstab4[mssind])
324 break;
325
326 ctx->attrs.mss = msstab4[mssind];
327
328 first = (u64)ctx->ipv4->saddr << 32 | ctx->ipv4->daddr;
329 } else if (ctx->ipv6) {
330 for (mssind = ARRAY_SIZE(msstab6) - 1; mssind; mssind--)
331 if (ctx->attrs.mss >= msstab6[mssind])
332 break;
333
334 ctx->attrs.mss = msstab6[mssind];
335
336 first = (u64)ctx->ipv6->saddr.in6_u.u6_addr8[0] << 32 |
337 ctx->ipv6->daddr.in6_u.u6_addr32[0];
338 }
339
340 second = (u64)seq << 32 | ctx->tcp->source << 16 | ctx->tcp->dest;
341 hash = siphash_2u64(first, second, &test_key_siphash);
342
343 if (ctx->attrs.tstamp_ok) {
344 ctx->attrs.rcv_tsecr = bpf_get_prandom_u32();
345 ctx->attrs.rcv_tsecr &= ~COOKIE_MASK;
346 ctx->attrs.rcv_tsecr |= hash & COOKIE_MASK;
347 }
348
349 hash &= ~COOKIE_MASK;
350 hash |= mssind << 6;
351
352 if (ctx->attrs.wscale_ok)
353 hash |= ctx->attrs.snd_wscale & BPF_SYNCOOKIE_WSCALE_MASK;
354
355 if (ctx->attrs.sack_ok)
356 hash |= BPF_SYNCOOKIE_SACK;
357
358 if (ctx->attrs.tstamp_ok && ctx->tcp->ece && ctx->tcp->cwr)
359 hash |= BPF_SYNCOOKIE_ECN;
360
361 ctx->cookie = hash;
362 }
363
364 static void tcp_write_options(struct tcp_syncookie *ctx)
365 {
366 ctx->ptr32 = (__be32 *)(ctx->tcp + 1);
367
368 *ctx->ptr32++ = bpf_htonl(TCPOPT_MSS << 24 | TCPOLEN_MSS << 16 |
369 ctx->attrs.mss);
370
371 if (ctx->attrs.wscale_ok)
372 *ctx->ptr32++ = bpf_htonl(TCPOPT_NOP << 24 |
373 TCPOPT_WINDOW << 16 |
374 TCPOLEN_WINDOW << 8 |
375 ctx->attrs.snd_wscale);
376
377 if (ctx->attrs.tstamp_ok) {
378 if (ctx->attrs.sack_ok)
379 *ctx->ptr32++ = bpf_htonl(TCPOPT_SACK_PERM << 24 |
380 TCPOLEN_SACK_PERM << 16 |
381 TCPOPT_TIMESTAMP << 8 |
382 TCPOLEN_TIMESTAMP);
383 else
384 *ctx->ptr32++ = bpf_htonl(TCPOPT_NOP << 24 |
385 TCPOPT_NOP << 16 |
386 TCPOPT_TIMESTAMP << 8 |
387 TCPOLEN_TIMESTAMP);
388
389 *ctx->ptr32++ = bpf_htonl(ctx->attrs.rcv_tsecr);
390 *ctx->ptr32++ = bpf_htonl(ctx->attrs.rcv_tsval);
391 } else if (ctx->attrs.sack_ok) {
392 *ctx->ptr32++ = bpf_htonl(TCPOPT_NOP << 24 |
393 TCPOPT_NOP << 16 |
394 TCPOPT_SACK_PERM << 8 |
395 TCPOLEN_SACK_PERM);
396 }
397 }
398
399 static int tcp_handle_syn(struct tcp_syncookie *ctx)
400 {
401 s64 csum;
402
403 if (tcp_validate_header(ctx))
404 goto err;
405
406 tcp_parse_options(ctx);
407
408 if (tcp_validate_sysctl(ctx))
409 goto err;
410
411 tcp_prepare_cookie(ctx);
412 tcp_write_options(ctx);
413
414 swap(ctx->tcp->source, ctx->tcp->dest);
415 ctx->tcp->check = 0;
416 ctx->tcp->ack_seq = bpf_htonl(bpf_ntohl(ctx->tcp->seq) + 1);
417 ctx->tcp->seq = bpf_htonl(ctx->cookie);
418 ctx->tcp->doff = ((long)ctx->ptr32 - (long)ctx->tcp) >> 2;
419 ctx->tcp->ack = 1;
420 if (!ctx->attrs.tstamp_ok || !ctx->tcp->ece || !ctx->tcp->cwr)
421 ctx->tcp->ece = 0;
422 ctx->tcp->cwr = 0;
423
424 csum = bpf_csum_diff(0, 0, (void *)ctx->tcp, ctx->tcp->doff * 4, 0);
425 if (csum < 0)
426 goto err;
427
428 if (ctx->ipv4) {
429 swap(ctx->ipv4->saddr, ctx->ipv4->daddr);
430 ctx->tcp->check = tcp_v4_csum(ctx, csum);
431
432 ctx->ipv4->check = 0;
433 ctx->ipv4->tos = 0;
434 ctx->ipv4->tot_len = bpf_htons((long)ctx->ptr32 - (long)ctx->ipv4);
435 ctx->ipv4->id = 0;
436 ctx->ipv4->ttl = 64;
437
438 csum = bpf_csum_diff(0, 0, (void *)ctx->ipv4, sizeof(*ctx->ipv4), 0);
439 if (csum < 0)
440 goto err;
441
442 ctx->ipv4->check = csum_fold(csum);
443 } else if (ctx->ipv6) {
444 swap(ctx->ipv6->saddr, ctx->ipv6->daddr);
445 ctx->tcp->check = tcp_v6_csum(ctx, csum);
446
447 *(__be32 *)ctx->ipv6 = bpf_htonl(0x60000000);
448 ctx->ipv6->payload_len = bpf_htons((long)ctx->ptr32 - (long)ctx->tcp);
449 ctx->ipv6->hop_limit = 64;
450 }
451
452 swap_array(ctx->eth->h_source, ctx->eth->h_dest);
453
454 if (bpf_skb_change_tail(ctx->skb, (long)ctx->ptr32 - (long)ctx->eth, 0))
455 goto err;
456
457 return bpf_redirect(ctx->skb->ifindex, 0);
458 err:
459 return TC_ACT_SHOT;
460 }
461
462 static int tcp_validate_cookie(struct tcp_syncookie *ctx)
463 {
464 u32 cookie = bpf_ntohl(ctx->tcp->ack_seq) - 1;
465 u32 seq = bpf_ntohl(ctx->tcp->seq) - 1;
466 u64 first = 0, second;
467 int mssind;
468 u32 hash;
469
470 if (ctx->ipv4)
471 first = (u64)ctx->ipv4->saddr << 32 | ctx->ipv4->daddr;
472 else if (ctx->ipv6)
473 first = (u64)ctx->ipv6->saddr.in6_u.u6_addr8[0] << 32 |
474 ctx->ipv6->daddr.in6_u.u6_addr32[0];
475
476 second = (u64)seq << 32 | ctx->tcp->source << 16 | ctx->tcp->dest;
477 hash = siphash_2u64(first, second, &test_key_siphash);
478
479 if (ctx->attrs.tstamp_ok)
480 hash -= ctx->attrs.rcv_tsecr & COOKIE_MASK;
481 else
482 hash &= ~COOKIE_MASK;
483
484 hash -= cookie & ~COOKIE_MASK;
485 if (hash)
486 goto err;
487
488 mssind = (cookie & (3 << 6)) >> 6;
489 if (ctx->ipv4) {
490 if (mssind > ARRAY_SIZE(msstab4))
491 goto err;
492
493 ctx->attrs.mss = msstab4[mssind];
494 } else {
495 if (mssind > ARRAY_SIZE(msstab6))
496 goto err;
497
498 ctx->attrs.mss = msstab6[mssind];
499 }
500
501 ctx->attrs.snd_wscale = cookie & BPF_SYNCOOKIE_WSCALE_MASK;
502 ctx->attrs.rcv_wscale = ctx->attrs.snd_wscale;
503 ctx->attrs.wscale_ok = ctx->attrs.snd_wscale == BPF_SYNCOOKIE_WSCALE_MASK;
504 ctx->attrs.sack_ok = cookie & BPF_SYNCOOKIE_SACK;
505 ctx->attrs.ecn_ok = cookie & BPF_SYNCOOKIE_ECN;
506
507 return 0;
508 err:
509 return -1;
510 }
511
512 static int tcp_handle_ack(struct tcp_syncookie *ctx)
513 {
514 struct bpf_sock_tuple tuple;
515 struct bpf_sock *skc;
516 int ret = TC_ACT_OK;
517 struct sock *sk;
518 u32 tuple_size;
519
520 if (ctx->ipv4) {
521 tuple.ipv4.saddr = ctx->ipv4->saddr;
522 tuple.ipv4.daddr = ctx->ipv4->daddr;
523 tuple.ipv4.sport = ctx->tcp->source;
524 tuple.ipv4.dport = ctx->tcp->dest;
525 tuple_size = sizeof(tuple.ipv4);
526 } else if (ctx->ipv6) {
527 __builtin_memcpy(tuple.ipv6.saddr, &ctx->ipv6->saddr, sizeof(tuple.ipv6.saddr));
528 __builtin_memcpy(tuple.ipv6.daddr, &ctx->ipv6->daddr, sizeof(tuple.ipv6.daddr));
529 tuple.ipv6.sport = ctx->tcp->source;
530 tuple.ipv6.dport = ctx->tcp->dest;
531 tuple_size = sizeof(tuple.ipv6);
532 } else {
533 goto out;
534 }
535
536 skc = bpf_skc_lookup_tcp(ctx->skb, &tuple, tuple_size, -1, 0);
537 if (!skc)
538 goto out;
539
540 if (skc->state != TCP_LISTEN)
541 goto release;
542
543 sk = (struct sock *)bpf_skc_to_tcp_sock(skc);
544 if (!sk)
545 goto err;
546
547 if (tcp_validate_header(ctx))
548 goto err;
549
550 tcp_parse_options(ctx);
551
552 if (tcp_validate_cookie(ctx))
553 goto err;
554
555 ret = bpf_sk_assign_tcp_reqsk(ctx->skb, sk, &ctx->attrs, sizeof(ctx->attrs));
556 if (ret < 0)
557 goto err;
558
559 release:
560 bpf_sk_release(skc);
561 out:
562 return ret;
563
564 err:
565 ret = TC_ACT_SHOT;
566 goto release;
567 }
568
569 SEC("tc")
570 int tcp_custom_syncookie(struct __sk_buff *skb)
571 {
572 struct tcp_syncookie ctx = {
573 .skb = skb,
574 };
575
576 if (tcp_load_headers(&ctx))
577 return TC_ACT_OK;
578
579 if (ctx.tcp->rst)
580 return TC_ACT_OK;
581
582 if (ctx.tcp->syn) {
583 if (ctx.tcp->ack)
584 return TC_ACT_OK;
585
586 handled_syn = true;
587
588 return tcp_handle_syn(&ctx);
589 }
590
591 handled_ack = true;
592
593 return tcp_handle_ack(&ctx);
594 }
595
596 char _license[] SEC("license") = "GPL";
597
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
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