syncookies.c

  1// SPDX-License-Identifier: GPL-2.0-or-later
  2/*
  3 *  Syncookies implementation for the Linux kernel
  4 *
  5 *  Copyright (C) 1997 Andi Kleen
  6 *  Based on ideas by D.J.Bernstein and Eric Schenk.
  7 */
  8
  9#include <linux/tcp.h>
 10#include <linux/siphash.h>
 11#include <linux/kernel.h>
 12#include <linux/export.h>
 13#include <net/secure_seq.h>
 14#include <net/tcp.h>
 15#include <net/tcp_ecn.h>
 16#include <net/route.h>
 17
 18static siphash_aligned_key_t syncookie_secret[2];
 19
 20#define COOKIEBITS 24	/* Upper bits store count */
 21#define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
 22
 23/* TCP Timestamp: 6 lowest bits of timestamp sent in the cookie SYN-ACK
 24 * stores TCP options:
 25 *
 26 * MSB                               LSB
 27 * | 31 ...   6 |  5  |  4   | 3 2 1 0 |
 28 * |  Timestamp | ECN | SACK | WScale  |
 29 *
 30 * When we receive a valid cookie-ACK, we look at the echoed tsval (if
 31 * any) to figure out which TCP options we should use for the rebuilt
 32 * connection.
 33 *
 34 * A WScale setting of '0xf' (which is an invalid scaling value)
 35 * means that original syn did not include the TCP window scaling option.
 36 */
 37#define TS_OPT_WSCALE_MASK	0xf
 38#define TS_OPT_SACK		BIT(4)
 39#define TS_OPT_ECN		BIT(5)
 40/* There is no TS_OPT_TIMESTAMP:
 41 * if ACK contains timestamp option, we already know it was
 42 * requested/supported by the syn/synack exchange.
 43 */
 44#define TSBITS	6
 45
 46static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
 47		       u32 count, int c)
 48{
 49	net_get_random_once(syncookie_secret, sizeof(syncookie_secret));
 50	return siphash_4u32((__force u32)saddr, (__force u32)daddr,
 51			    (__force u32)sport << 16 | (__force u32)dport,
 52			    count, &syncookie_secret[c]);
 53}
 54
 55/*
 56 * when syncookies are in effect and tcp timestamps are enabled we encode
 57 * tcp options in the lower bits of the timestamp value that will be
 58 * sent in the syn-ack.
 59 * Since subsequent timestamps use the normal tcp_time_stamp value, we
 60 * must make sure that the resulting initial timestamp is <= tcp_time_stamp.
 61 */
 62u64 cookie_init_timestamp(struct request_sock *req, u64 now)
 63{
 64	const struct inet_request_sock *ireq = inet_rsk(req);
 65	u64 ts, ts_now = tcp_ns_to_ts(false, now);
 66	u32 options = 0;
 67
 68	options = ireq->wscale_ok ? ireq->snd_wscale : TS_OPT_WSCALE_MASK;
 69	if (ireq->sack_ok)
 70		options |= TS_OPT_SACK;
 71	if (ireq->ecn_ok)
 72		options |= TS_OPT_ECN;
 73
 74	ts = (ts_now >> TSBITS) << TSBITS;
 75	ts |= options;
 76	if (ts > ts_now)
 77		ts -= (1UL << TSBITS);
 78
 79	if (tcp_rsk(req)->req_usec_ts)
 80		return ts * NSEC_PER_USEC;
 81	return ts * NSEC_PER_MSEC;
 82}
 83
 84
 85static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport,
 86				   __be16 dport, __u32 sseq, __u32 data)
 87{
 88	/*
 89	 * Compute the secure sequence number.
 90	 * The output should be:
 91	 *   HASH(sec1,saddr,sport,daddr,dport,sec1) + sseq + (count * 2^24)
 92	 *      + (HASH(sec2,saddr,sport,daddr,dport,count,sec2) % 2^24).
 93	 * Where sseq is their sequence number and count increases every
 94	 * minute by 1.
 95	 * As an extra hack, we add a small "data" value that encodes the
 96	 * MSS into the second hash value.
 97	 */
 98	u32 count = tcp_cookie_time();
 99	return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
100		sseq + (count << COOKIEBITS) +
101		((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
102		 & COOKIEMASK));
103}
104
105/*
106 * This retrieves the small "data" value from the syncookie.
107 * If the syncookie is bad, the data returned will be out of
108 * range.  This must be checked by the caller.
109 *
110 * The count value used to generate the cookie must be less than
111 * MAX_SYNCOOKIE_AGE minutes in the past.
112 * The return value (__u32)-1 if this test fails.
113 */
114static __u32 check_tcp_syn_cookie(__u32 cookie, __be32 saddr, __be32 daddr,
115				  __be16 sport, __be16 dport, __u32 sseq)
116{
117	u32 diff, count = tcp_cookie_time();
118
119	/* Strip away the layers from the cookie */
120	cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
121
122	/* Cookie is now reduced to (count * 2^24) ^ (hash % 2^24) */
123	diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
124	if (diff >= MAX_SYNCOOKIE_AGE)
125		return (__u32)-1;
126
127	return (cookie -
128		cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
129		& COOKIEMASK;	/* Leaving the data behind */
130}
131
132/*
133 * MSS Values are chosen based on the 2011 paper
134 * 'An Analysis of TCP Maximum Segement Sizes' by S. Alcock and R. Nelson.
135 * Values ..
136 *  .. lower than 536 are rare (< 0.2%)
137 *  .. between 537 and 1299 account for less than < 1.5% of observed values
138 *  .. in the 1300-1349 range account for about 15 to 20% of observed mss values
139 *  .. exceeding 1460 are very rare (< 0.04%)
140 *
141 *  1460 is the single most frequently announced mss value (30 to 46% depending
142 *  on monitor location).  Table must be sorted.
143 */
144static __u16 const msstab[] = {
145	536,
146	1300,
147	1440,	/* 1440, 1452: PPPoE */
148	1460,
149};
150
151/*
152 * Generate a syncookie.  mssp points to the mss, which is returned
153 * rounded down to the value encoded in the cookie.
154 */
155u32 __cookie_v4_init_sequence(const struct iphdr *iph, const struct tcphdr *th,
156			      u16 *mssp)
157{
158	int mssind;
159	const __u16 mss = *mssp;
160
161	for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
162		if (mss >= msstab[mssind])
163			break;
164	*mssp = msstab[mssind];
165
166	return secure_tcp_syn_cookie(iph->saddr, iph->daddr,
167				     th->source, th->dest, ntohl(th->seq),
168				     mssind);
169}
170EXPORT_SYMBOL_GPL(__cookie_v4_init_sequence);
171
172__u32 cookie_v4_init_sequence(const struct sk_buff *skb, __u16 *mssp)
173{
174	const struct iphdr *iph = ip_hdr(skb);
175	const struct tcphdr *th = tcp_hdr(skb);
176
177	return __cookie_v4_init_sequence(iph, th, mssp);
178}
179
180/*
181 * Check if a ack sequence number is a valid syncookie.
182 * Return the decoded mss if it is, or 0 if not.
183 */
184int __cookie_v4_check(const struct iphdr *iph, const struct tcphdr *th)
185{
186	__u32 cookie = ntohl(th->ack_seq) - 1;
187	__u32 seq = ntohl(th->seq) - 1;
188	__u32 mssind;
189
190	mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr,
191				      th->source, th->dest, seq);
192
193	return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
194}
195EXPORT_SYMBOL_GPL(__cookie_v4_check);
196
197struct sock *tcp_get_cookie_sock(struct sock *sk, struct sk_buff *skb,
198				 struct request_sock *req,
199				 struct dst_entry *dst)
200{
201	struct inet_connection_sock *icsk = inet_csk(sk);
202	struct sock *child;
203	bool own_req;
204
205	child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst,
206						 NULL, &own_req, NULL);
207	if (child) {
208		refcount_set(&req->rsk_refcnt, 1);
209		sock_rps_save_rxhash(child, skb);
210
211		if (rsk_drop_req(req)) {
212			reqsk_put(req);
213			return child;
214		}
215
216		if (inet_csk_reqsk_queue_add(sk, req, child))
217			return child;
218
219		bh_unlock_sock(child);
220		sock_put(child);
221	}
222	__reqsk_free(req);
223
224	return NULL;
225}
226EXPORT_IPV6_MOD(tcp_get_cookie_sock);
227
228/*
229 * when syncookies are in effect and tcp timestamps are enabled we stored
230 * additional tcp options in the timestamp.
231 * This extracts these options from the timestamp echo.
232 *
233 * return false if we decode a tcp option that is disabled
234 * on the host.
235 */
236bool cookie_timestamp_decode(const struct net *net,
237			     struct tcp_options_received *tcp_opt)
238{
239	/* echoed timestamp, lowest bits contain options */
240	u32 options = tcp_opt->rcv_tsecr;
241
242	if (!tcp_opt->saw_tstamp)  {
243		tcp_clear_options(tcp_opt);
244		return true;
245	}
246
247	if (!READ_ONCE(net->ipv4.sysctl_tcp_timestamps))
248		return false;
249
250	tcp_opt->sack_ok = (options & TS_OPT_SACK) ? TCP_SACK_SEEN : 0;
251
252	if (tcp_opt->sack_ok && !READ_ONCE(net->ipv4.sysctl_tcp_sack))
253		return false;
254
255	if ((options & TS_OPT_WSCALE_MASK) == TS_OPT_WSCALE_MASK)
256		return true; /* no window scaling */
257
258	tcp_opt->wscale_ok = 1;
259	tcp_opt->snd_wscale = options & TS_OPT_WSCALE_MASK;
260
261	return READ_ONCE(net->ipv4.sysctl_tcp_window_scaling) != 0;
262}
263EXPORT_IPV6_MOD(cookie_timestamp_decode);
264
265static int cookie_tcp_reqsk_init(struct sock *sk, struct sk_buff *skb,
266				 struct request_sock *req)
267{
268	struct inet_request_sock *ireq = inet_rsk(req);
269	struct tcp_request_sock *treq = tcp_rsk(req);
270	const struct tcphdr *th = tcp_hdr(skb);
271
272	req->num_retrans = 0;
273
274	ireq->ir_num = ntohs(th->dest);
275	ireq->ir_rmt_port = th->source;
276	ireq->ir_iif = inet_request_bound_dev_if(sk, skb);
277	ireq->ir_mark = inet_request_mark(sk, skb);
278
279	if (IS_ENABLED(CONFIG_SMC))
280		ireq->smc_ok = 0;
281
282	treq->snt_synack = 0;
283	treq->snt_tsval_first = 0;
284	treq->tfo_listener = false;
285	treq->txhash = net_tx_rndhash();
286	treq->rcv_isn = ntohl(th->seq) - 1;
287	treq->snt_isn = ntohl(th->ack_seq) - 1;
288	treq->syn_tos = TCP_SKB_CB(skb)->ip_dsfield;
289	treq->req_usec_ts = false;
290
291#if IS_ENABLED(CONFIG_MPTCP)
292	treq->is_mptcp = sk_is_mptcp(sk);
293	if (treq->is_mptcp)
294		return mptcp_subflow_init_cookie_req(req, sk, skb);
295#endif
296
297	return 0;
298}
299
300#if IS_ENABLED(CONFIG_BPF)
301struct request_sock *cookie_bpf_check(struct sock *sk, struct sk_buff *skb)
302{
303	struct request_sock *req = inet_reqsk(skb->sk);
304
305	skb->sk = NULL;
306	skb->destructor = NULL;
307
308	if (cookie_tcp_reqsk_init(sk, skb, req)) {
309		reqsk_free(req);
310		req = NULL;
311	}
312
313	return req;
314}
315EXPORT_IPV6_MOD_GPL(cookie_bpf_check);
316#endif
317
318struct request_sock *cookie_tcp_reqsk_alloc(const struct request_sock_ops *ops,
319					    struct sock *sk, struct sk_buff *skb,
320					    struct tcp_options_received *tcp_opt,
321					    int mss, u32 tsoff)
322{
323	struct inet_request_sock *ireq;
324	struct tcp_request_sock *treq;
325	struct request_sock *req;
326
327	if (sk_is_mptcp(sk))
328		req = mptcp_subflow_reqsk_alloc(ops, sk, false);
329	else
330		req = inet_reqsk_alloc(ops, sk, false);
331
332	if (!req)
333		return NULL;
334
335	if (cookie_tcp_reqsk_init(sk, skb, req)) {
336		reqsk_free(req);
337		return NULL;
338	}
339
340	ireq = inet_rsk(req);
341	treq = tcp_rsk(req);
342
343	req->mss = mss;
344	req->ts_recent = tcp_opt->saw_tstamp ? tcp_opt->rcv_tsval : 0;
345
346	ireq->snd_wscale = tcp_opt->snd_wscale;
347	ireq->tstamp_ok = tcp_opt->saw_tstamp;
348	ireq->sack_ok = tcp_opt->sack_ok;
349	ireq->wscale_ok = tcp_opt->wscale_ok;
350	ireq->ecn_ok = !!(tcp_opt->rcv_tsecr & TS_OPT_ECN);
351
352	treq->ts_off = tsoff;
353
354	return req;
355}
356EXPORT_IPV6_MOD_GPL(cookie_tcp_reqsk_alloc);
357
358static struct request_sock *cookie_tcp_check(struct net *net, struct sock *sk,
359					     struct sk_buff *skb)
360{
361	struct tcp_options_received tcp_opt;
362	u32 tsoff = 0;
363	int mss;
364
365	if (tcp_synq_no_recent_overflow(sk))
366		goto out;
367
368	mss = __cookie_v4_check(ip_hdr(skb), tcp_hdr(skb));
369	if (!mss) {
370		__NET_INC_STATS(net, LINUX_MIB_SYNCOOKIESFAILED);
371		goto out;
372	}
373
374	__NET_INC_STATS(net, LINUX_MIB_SYNCOOKIESRECV);
375
376	/* check for timestamp cookie support */
377	memset(&tcp_opt, 0, sizeof(tcp_opt));
378	tcp_parse_options(net, skb, &tcp_opt, 0, NULL);
379
380	if (tcp_opt.saw_tstamp && tcp_opt.rcv_tsecr) {
381		union tcp_seq_and_ts_off st;
382
383		st = secure_tcp_seq_and_ts_off(net,
384					       ip_hdr(skb)->daddr,
385					       ip_hdr(skb)->saddr,
386					       tcp_hdr(skb)->dest,
387					       tcp_hdr(skb)->source);
388		tsoff = st.ts_off;
389		tcp_opt.rcv_tsecr -= tsoff;
390	}
391
392	if (!cookie_timestamp_decode(net, &tcp_opt))
393		goto out;
394
395	return cookie_tcp_reqsk_alloc(&tcp_request_sock_ops, sk, skb,
396				      &tcp_opt, mss, tsoff);
397out:
398	return ERR_PTR(-EINVAL);
399}
400
401/* On input, sk is a listener.
402 * Output is listener if incoming packet would not create a child
403 *           NULL if memory could not be allocated.
404 */
405struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb)
406{
407	struct ip_options *opt = &TCP_SKB_CB(skb)->header.h4.opt;
408	const struct tcphdr *th = tcp_hdr(skb);
409	struct tcp_sock *tp = tcp_sk(sk);
410	struct inet_request_sock *ireq;
411	struct net *net = sock_net(sk);
412	struct tcp_request_sock *treq;
413	struct request_sock *req;
414	struct sock *ret = sk;
415	struct flowi4 fl4;
416	struct rtable *rt;
417	__u8 rcv_wscale;
418	int full_space;
419	SKB_DR(reason);
420
421	if (!READ_ONCE(net->ipv4.sysctl_tcp_syncookies) ||
422	    !th->ack || th->rst)
423		goto out;
424
425	if (cookie_bpf_ok(skb)) {
426		req = cookie_bpf_check(sk, skb);
427	} else {
428		req = cookie_tcp_check(net, sk, skb);
429		if (IS_ERR(req))
430			goto out;
431	}
432	if (!req) {
433		SKB_DR_SET(reason, NO_SOCKET);
434		goto out_drop;
435	}
436
437	ireq = inet_rsk(req);
438	treq = tcp_rsk(req);
439
440	sk_rcv_saddr_set(req_to_sk(req), ip_hdr(skb)->daddr);
441	sk_daddr_set(req_to_sk(req), ip_hdr(skb)->saddr);
442
443	/* We throwed the options of the initial SYN away, so we hope
444	 * the ACK carries the same options again (see RFC1122 4.2.3.8)
445	 */
446	RCU_INIT_POINTER(ireq->ireq_opt, tcp_v4_save_options(net, skb));
447
448	if (security_inet_conn_request(sk, skb, req)) {
449		SKB_DR_SET(reason, SECURITY_HOOK);
450		goto out_free;
451	}
452
453	tcp_ao_syncookie(sk, skb, req, AF_INET);
454
455	/*
456	 * We need to lookup the route here to get at the correct
457	 * window size. We should better make sure that the window size
458	 * hasn't changed since we received the original syn, but I see
459	 * no easy way to do this.
460	 */
461	flowi4_init_output(&fl4, ireq->ir_iif, ireq->ir_mark,
462			   ip_sock_rt_tos(sk), ip_sock_rt_scope(sk),
463			   IPPROTO_TCP, inet_sk_flowi_flags(sk),
464			   opt->srr ? opt->faddr : ireq->ir_rmt_addr,
465			   ireq->ir_loc_addr, th->source, th->dest,
466			   sk_uid(sk));
467	security_req_classify_flow(req, flowi4_to_flowi_common(&fl4));
468	rt = ip_route_output_key(net, &fl4);
469	if (IS_ERR(rt)) {
470		SKB_DR_SET(reason, IP_OUTNOROUTES);
471		goto out_free;
472	}
473
474	/* Try to redo what tcp_v4_send_synack did. */
475	req->rsk_window_clamp = READ_ONCE(tp->window_clamp) ? :
476				dst_metric(&rt->dst, RTAX_WINDOW);
477	/* limit the window selection if the user enforce a smaller rx buffer */
478	full_space = tcp_full_space(sk);
479	if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
480	    (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
481		req->rsk_window_clamp = full_space;
482
483	tcp_select_initial_window(sk, full_space, req->mss,
484				  &req->rsk_rcv_wnd, &req->rsk_window_clamp,
485				  ireq->wscale_ok, &rcv_wscale,
486				  dst_metric(&rt->dst, RTAX_INITRWND));
487
488	/* req->syncookie is set true only if ACK is validated
489	 * by BPF kfunc, then, rcv_wscale is already configured.
490	 */
491	if (!req->syncookie)
492		ireq->rcv_wscale = rcv_wscale;
493	ireq->ecn_ok &= cookie_ecn_ok(net, &rt->dst);
494	treq->accecn_ok = ireq->ecn_ok && cookie_accecn_ok(th);
495
496	ret = tcp_get_cookie_sock(sk, skb, req, &rt->dst);
497	/* ip_queue_xmit() depends on our flow being setup
498	 * Normal sockets get it right from inet_csk_route_child_sock()
499	 */
500	if (!ret) {
501		SKB_DR_SET(reason, NO_SOCKET);
502		goto out_drop;
503	}
504	inet_sk(ret)->cork.fl.u.ip4 = fl4;
505out:
506	return ret;
507out_free:
508	reqsk_free(req);
509out_drop:
510	sk_skb_reason_drop(sk, skb, reason);
511	return NULL;
512}