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v3.15
  1/*
  2 *  Syncookies implementation for the Linux kernel
  3 *
  4 *  Copyright (C) 1997 Andi Kleen
  5 *  Based on ideas by D.J.Bernstein and Eric Schenk.
  6 *
  7 *	This program is free software; you can redistribute it and/or
  8 *      modify it under the terms of the GNU General Public License
  9 *      as published by the Free Software Foundation; either version
 10 *      2 of the License, or (at your option) any later version.
 11 */
 12
 13#include <linux/tcp.h>
 14#include <linux/slab.h>
 15#include <linux/random.h>
 16#include <linux/cryptohash.h>
 17#include <linux/kernel.h>
 18#include <linux/export.h>
 
 19#include <net/tcp.h>
 20#include <net/route.h>
 21
 22/* Timestamps: lowest bits store TCP options */
 23#define TSBITS 6
 24#define TSMASK (((__u32)1 << TSBITS) - 1)
 25
 26extern int sysctl_tcp_syncookies;
 27
 28static u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS];
 29
 30#define COOKIEBITS 24	/* Upper bits store count */
 31#define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
 32
 33static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS],
 34		      ipv4_cookie_scratch);
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 35
 36static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
 37		       u32 count, int c)
 38{
 39	__u32 *tmp;
 40
 41	net_get_random_once(syncookie_secret, sizeof(syncookie_secret));
 42
 43	tmp  = __get_cpu_var(ipv4_cookie_scratch);
 44	memcpy(tmp + 4, syncookie_secret[c], sizeof(syncookie_secret[c]));
 45	tmp[0] = (__force u32)saddr;
 46	tmp[1] = (__force u32)daddr;
 47	tmp[2] = ((__force u32)sport << 16) + (__force u32)dport;
 48	tmp[3] = count;
 49	sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);
 50
 51	return tmp[17];
 52}
 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 */
 62__u32 cookie_init_timestamp(struct request_sock *req)
 63{
 64	struct inet_request_sock *ireq;
 65	u32 ts, ts_now = tcp_time_stamp;
 66	u32 options = 0;
 67
 68	ireq = inet_rsk(req);
 69
 70	options = ireq->wscale_ok ? ireq->snd_wscale : 0xf;
 71	options |= ireq->sack_ok << 4;
 72	options |= ireq->ecn_ok << 5;
 
 
 73
 74	ts = ts_now & ~TSMASK;
 75	ts |= options;
 76	if (ts > ts_now) {
 77		ts >>= TSBITS;
 78		ts--;
 79		ts <<= TSBITS;
 80		ts |= options;
 81	}
 82	return ts;
 83}
 84
 85
 86static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport,
 87				   __be16 dport, __u32 sseq, __u32 data)
 88{
 89	/*
 90	 * Compute the secure sequence number.
 91	 * The output should be:
 92	 *   HASH(sec1,saddr,sport,daddr,dport,sec1) + sseq + (count * 2^24)
 93	 *      + (HASH(sec2,saddr,sport,daddr,dport,count,sec2) % 2^24).
 94	 * Where sseq is their sequence number and count increases every
 95	 * minute by 1.
 96	 * As an extra hack, we add a small "data" value that encodes the
 97	 * MSS into the second hash value.
 98	 */
 99	u32 count = tcp_cookie_time();
100	return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
101		sseq + (count << COOKIEBITS) +
102		((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
103		 & COOKIEMASK));
104}
105
106/*
107 * This retrieves the small "data" value from the syncookie.
108 * If the syncookie is bad, the data returned will be out of
109 * range.  This must be checked by the caller.
110 *
111 * The count value used to generate the cookie must be less than
112 * MAX_SYNCOOKIE_AGE minutes in the past.
113 * The return value (__u32)-1 if this test fails.
114 */
115static __u32 check_tcp_syn_cookie(__u32 cookie, __be32 saddr, __be32 daddr,
116				  __be16 sport, __be16 dport, __u32 sseq)
117{
118	u32 diff, count = tcp_cookie_time();
119
120	/* Strip away the layers from the cookie */
121	cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
122
123	/* Cookie is now reduced to (count * 2^24) ^ (hash % 2^24) */
124	diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
125	if (diff >= MAX_SYNCOOKIE_AGE)
126		return (__u32)-1;
127
128	return (cookie -
129		cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
130		& COOKIEMASK;	/* Leaving the data behind */
131}
132
133/*
134 * MSS Values are chosen based on the 2011 paper
135 * 'An Analysis of TCP Maximum Segement Sizes' by S. Alcock and R. Nelson.
136 * Values ..
137 *  .. lower than 536 are rare (< 0.2%)
138 *  .. between 537 and 1299 account for less than < 1.5% of observed values
139 *  .. in the 1300-1349 range account for about 15 to 20% of observed mss values
140 *  .. exceeding 1460 are very rare (< 0.04%)
141 *
142 *  1460 is the single most frequently announced mss value (30 to 46% depending
143 *  on monitor location).  Table must be sorted.
144 */
145static __u16 const msstab[] = {
146	536,
147	1300,
148	1440,	/* 1440, 1452: PPPoE */
149	1460,
150};
151
152/*
153 * Generate a syncookie.  mssp points to the mss, which is returned
154 * rounded down to the value encoded in the cookie.
155 */
156u32 __cookie_v4_init_sequence(const struct iphdr *iph, const struct tcphdr *th,
157			      u16 *mssp)
158{
159	int mssind;
160	const __u16 mss = *mssp;
161
162	for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
163		if (mss >= msstab[mssind])
164			break;
165	*mssp = msstab[mssind];
166
167	return secure_tcp_syn_cookie(iph->saddr, iph->daddr,
168				     th->source, th->dest, ntohl(th->seq),
169				     mssind);
170}
171EXPORT_SYMBOL_GPL(__cookie_v4_init_sequence);
172
173__u32 cookie_v4_init_sequence(struct sock *sk, struct sk_buff *skb, __u16 *mssp)
174{
175	const struct iphdr *iph = ip_hdr(skb);
176	const struct tcphdr *th = tcp_hdr(skb);
177
178	tcp_synq_overflow(sk);
179	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT);
180
181	return __cookie_v4_init_sequence(iph, th, mssp);
182}
183
184/*
185 * Check if a ack sequence number is a valid syncookie.
186 * Return the decoded mss if it is, or 0 if not.
187 */
188int __cookie_v4_check(const struct iphdr *iph, const struct tcphdr *th,
189		      u32 cookie)
190{
191	__u32 seq = ntohl(th->seq) - 1;
192	__u32 mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr,
193					    th->source, th->dest, seq);
194
195	return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
196}
197EXPORT_SYMBOL_GPL(__cookie_v4_check);
198
199static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb,
200					   struct request_sock *req,
201					   struct dst_entry *dst)
202{
203	struct inet_connection_sock *icsk = inet_csk(sk);
204	struct sock *child;
 
205
206	child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst);
207	if (child)
 
 
 
 
208		inet_csk_reqsk_queue_add(sk, req, child);
209	else
210		reqsk_free(req);
211
212	return child;
213}
214
215
216/*
217 * when syncookies are in effect and tcp timestamps are enabled we stored
218 * additional tcp options in the timestamp.
219 * This extracts these options from the timestamp echo.
220 *
221 * The lowest 4 bits store snd_wscale.
222 * next 2 bits indicate SACK and ECN support.
223 *
224 * return false if we decode an option that should not be.
225 */
226bool cookie_check_timestamp(struct tcp_options_received *tcp_opt,
227			struct net *net, bool *ecn_ok)
228{
229	/* echoed timestamp, lowest bits contain options */
230	u32 options = tcp_opt->rcv_tsecr & TSMASK;
231
232	if (!tcp_opt->saw_tstamp)  {
233		tcp_clear_options(tcp_opt);
234		return true;
235	}
236
237	if (!sysctl_tcp_timestamps)
238		return false;
239
240	tcp_opt->sack_ok = (options & (1 << 4)) ? TCP_SACK_SEEN : 0;
241	*ecn_ok = (options >> 5) & 1;
242	if (*ecn_ok && !net->ipv4.sysctl_tcp_ecn)
243		return false;
244
245	if (tcp_opt->sack_ok && !sysctl_tcp_sack)
246		return false;
247
248	if ((options & 0xf) == 0xf)
249		return true; /* no window scaling */
250
251	tcp_opt->wscale_ok = 1;
252	tcp_opt->snd_wscale = options & 0xf;
253	return sysctl_tcp_window_scaling != 0;
 
254}
255EXPORT_SYMBOL(cookie_check_timestamp);
256
257struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
258			     struct ip_options *opt)
259{
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
260	struct tcp_options_received tcp_opt;
261	struct inet_request_sock *ireq;
262	struct tcp_request_sock *treq;
263	struct tcp_sock *tp = tcp_sk(sk);
264	const struct tcphdr *th = tcp_hdr(skb);
265	__u32 cookie = ntohl(th->ack_seq) - 1;
266	struct sock *ret = sk;
267	struct request_sock *req;
268	int mss;
269	struct rtable *rt;
270	__u8 rcv_wscale;
271	bool ecn_ok = false;
272	struct flowi4 fl4;
 
 
 
 
273
274	if (!sysctl_tcp_syncookies || !th->ack || th->rst)
275		goto out;
276
277	if (tcp_synq_no_recent_overflow(sk) ||
278	    (mss = __cookie_v4_check(ip_hdr(skb), th, cookie)) == 0) {
279		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
280		goto out;
281	}
282
283	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
284
285	/* check for timestamp cookie support */
286	memset(&tcp_opt, 0, sizeof(tcp_opt));
287	tcp_parse_options(skb, &tcp_opt, 0, NULL);
 
 
 
 
 
 
 
288
289	if (!cookie_check_timestamp(&tcp_opt, sock_net(sk), &ecn_ok))
290		goto out;
291
292	ret = NULL;
293	req = inet_reqsk_alloc(&tcp_request_sock_ops); /* for safety */
294	if (!req)
295		goto out;
296
297	ireq = inet_rsk(req);
298	treq = tcp_rsk(req);
299	treq->rcv_isn		= ntohl(th->seq) - 1;
300	treq->snt_isn		= cookie;
 
 
301	req->mss		= mss;
302	ireq->ir_num		= ntohs(th->dest);
303	ireq->ir_rmt_port	= th->source;
304	ireq->ir_loc_addr	= ip_hdr(skb)->daddr;
305	ireq->ir_rmt_addr	= ip_hdr(skb)->saddr;
306	ireq->ecn_ok		= ecn_ok;
307	ireq->snd_wscale	= tcp_opt.snd_wscale;
308	ireq->sack_ok		= tcp_opt.sack_ok;
309	ireq->wscale_ok		= tcp_opt.wscale_ok;
310	ireq->tstamp_ok		= tcp_opt.saw_tstamp;
311	req->ts_recent		= tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
312	treq->snt_synack	= tcp_opt.saw_tstamp ? tcp_opt.rcv_tsecr : 0;
313	treq->listener		= NULL;
 
 
 
 
314
315	/* We throwed the options of the initial SYN away, so we hope
316	 * the ACK carries the same options again (see RFC1122 4.2.3.8)
317	 */
318	if (opt && opt->optlen) {
319		int opt_size = sizeof(struct ip_options_rcu) + opt->optlen;
320
321		ireq->opt = kmalloc(opt_size, GFP_ATOMIC);
322		if (ireq->opt != NULL && ip_options_echo(&ireq->opt->opt, skb)) {
323			kfree(ireq->opt);
324			ireq->opt = NULL;
325		}
326	}
327
328	if (security_inet_conn_request(sk, skb, req)) {
329		reqsk_free(req);
330		goto out;
331	}
332
333	req->expires	= 0UL;
334	req->num_retrans = 0;
335
336	/*
337	 * We need to lookup the route here to get at the correct
338	 * window size. We should better make sure that the window size
339	 * hasn't changed since we received the original syn, but I see
340	 * no easy way to do this.
341	 */
342	flowi4_init_output(&fl4, sk->sk_bound_dev_if, sk->sk_mark,
343			   RT_CONN_FLAGS(sk), RT_SCOPE_UNIVERSE, IPPROTO_TCP,
344			   inet_sk_flowi_flags(sk),
345			   (opt && opt->srr) ? opt->faddr : ireq->ir_rmt_addr,
346			   ireq->ir_loc_addr, th->source, th->dest);
347	security_req_classify_flow(req, flowi4_to_flowi(&fl4));
348	rt = ip_route_output_key(sock_net(sk), &fl4);
349	if (IS_ERR(rt)) {
350		reqsk_free(req);
351		goto out;
352	}
353
354	/* Try to redo what tcp_v4_send_synack did. */
355	req->window_clamp = tp->window_clamp ? :dst_metric(&rt->dst, RTAX_WINDOW);
356
357	tcp_select_initial_window(tcp_full_space(sk), req->mss,
358				  &req->rcv_wnd, &req->window_clamp,
359				  ireq->wscale_ok, &rcv_wscale,
360				  dst_metric(&rt->dst, RTAX_INITRWND));
361
362	ireq->rcv_wscale  = rcv_wscale;
 
363
364	ret = get_cookie_sock(sk, skb, req, &rt->dst);
365	/* ip_queue_xmit() depends on our flow being setup
366	 * Normal sockets get it right from inet_csk_route_child_sock()
367	 */
368	if (ret)
369		inet_sk(ret)->cork.fl.u.ip4 = fl4;
370out:	return ret;
371}
v4.17
  1/*
  2 *  Syncookies implementation for the Linux kernel
  3 *
  4 *  Copyright (C) 1997 Andi Kleen
  5 *  Based on ideas by D.J.Bernstein and Eric Schenk.
  6 *
  7 *	This program is free software; you can redistribute it and/or
  8 *      modify it under the terms of the GNU General Public License
  9 *      as published by the Free Software Foundation; either version
 10 *      2 of the License, or (at your option) any later version.
 11 */
 12
 13#include <linux/tcp.h>
 14#include <linux/slab.h>
 15#include <linux/random.h>
 16#include <linux/siphash.h>
 17#include <linux/kernel.h>
 18#include <linux/export.h>
 19#include <net/secure_seq.h>
 20#include <net/tcp.h>
 21#include <net/route.h>
 22
 23static siphash_key_t syncookie_secret[2] __read_mostly;
 
 
 
 
 
 
 24
 25#define COOKIEBITS 24	/* Upper bits store count */
 26#define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
 27
 28/* TCP Timestamp: 6 lowest bits of timestamp sent in the cookie SYN-ACK
 29 * stores TCP options:
 30 *
 31 * MSB                               LSB
 32 * | 31 ...   6 |  5  |  4   | 3 2 1 0 |
 33 * |  Timestamp | ECN | SACK | WScale  |
 34 *
 35 * When we receive a valid cookie-ACK, we look at the echoed tsval (if
 36 * any) to figure out which TCP options we should use for the rebuilt
 37 * connection.
 38 *
 39 * A WScale setting of '0xf' (which is an invalid scaling value)
 40 * means that original syn did not include the TCP window scaling option.
 41 */
 42#define TS_OPT_WSCALE_MASK	0xf
 43#define TS_OPT_SACK		BIT(4)
 44#define TS_OPT_ECN		BIT(5)
 45/* There is no TS_OPT_TIMESTAMP:
 46 * if ACK contains timestamp option, we already know it was
 47 * requested/supported by the syn/synack exchange.
 48 */
 49#define TSBITS	6
 50#define TSMASK	(((__u32)1 << TSBITS) - 1)
 51
 52static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
 53		       u32 count, int c)
 54{
 
 
 55	net_get_random_once(syncookie_secret, sizeof(syncookie_secret));
 56	return siphash_4u32((__force u32)saddr, (__force u32)daddr,
 57			    (__force u32)sport << 16 | (__force u32)dport,
 58			    count, &syncookie_secret[c]);
 
 
 
 
 
 
 
 59}
 60
 61
 62/*
 63 * when syncookies are in effect and tcp timestamps are enabled we encode
 64 * tcp options in the lower bits of the timestamp value that will be
 65 * sent in the syn-ack.
 66 * Since subsequent timestamps use the normal tcp_time_stamp value, we
 67 * must make sure that the resulting initial timestamp is <= tcp_time_stamp.
 68 */
 69u64 cookie_init_timestamp(struct request_sock *req)
 70{
 71	struct inet_request_sock *ireq;
 72	u32 ts, ts_now = tcp_time_stamp_raw();
 73	u32 options = 0;
 74
 75	ireq = inet_rsk(req);
 76
 77	options = ireq->wscale_ok ? ireq->snd_wscale : TS_OPT_WSCALE_MASK;
 78	if (ireq->sack_ok)
 79		options |= TS_OPT_SACK;
 80	if (ireq->ecn_ok)
 81		options |= TS_OPT_ECN;
 82
 83	ts = ts_now & ~TSMASK;
 84	ts |= options;
 85	if (ts > ts_now) {
 86		ts >>= TSBITS;
 87		ts--;
 88		ts <<= TSBITS;
 89		ts |= options;
 90	}
 91	return (u64)ts * (USEC_PER_SEC / TCP_TS_HZ);
 92}
 93
 94
 95static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport,
 96				   __be16 dport, __u32 sseq, __u32 data)
 97{
 98	/*
 99	 * Compute the secure sequence number.
100	 * The output should be:
101	 *   HASH(sec1,saddr,sport,daddr,dport,sec1) + sseq + (count * 2^24)
102	 *      + (HASH(sec2,saddr,sport,daddr,dport,count,sec2) % 2^24).
103	 * Where sseq is their sequence number and count increases every
104	 * minute by 1.
105	 * As an extra hack, we add a small "data" value that encodes the
106	 * MSS into the second hash value.
107	 */
108	u32 count = tcp_cookie_time();
109	return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
110		sseq + (count << COOKIEBITS) +
111		((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
112		 & COOKIEMASK));
113}
114
115/*
116 * This retrieves the small "data" value from the syncookie.
117 * If the syncookie is bad, the data returned will be out of
118 * range.  This must be checked by the caller.
119 *
120 * The count value used to generate the cookie must be less than
121 * MAX_SYNCOOKIE_AGE minutes in the past.
122 * The return value (__u32)-1 if this test fails.
123 */
124static __u32 check_tcp_syn_cookie(__u32 cookie, __be32 saddr, __be32 daddr,
125				  __be16 sport, __be16 dport, __u32 sseq)
126{
127	u32 diff, count = tcp_cookie_time();
128
129	/* Strip away the layers from the cookie */
130	cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
131
132	/* Cookie is now reduced to (count * 2^24) ^ (hash % 2^24) */
133	diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
134	if (diff >= MAX_SYNCOOKIE_AGE)
135		return (__u32)-1;
136
137	return (cookie -
138		cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
139		& COOKIEMASK;	/* Leaving the data behind */
140}
141
142/*
143 * MSS Values are chosen based on the 2011 paper
144 * 'An Analysis of TCP Maximum Segement Sizes' by S. Alcock and R. Nelson.
145 * Values ..
146 *  .. lower than 536 are rare (< 0.2%)
147 *  .. between 537 and 1299 account for less than < 1.5% of observed values
148 *  .. in the 1300-1349 range account for about 15 to 20% of observed mss values
149 *  .. exceeding 1460 are very rare (< 0.04%)
150 *
151 *  1460 is the single most frequently announced mss value (30 to 46% depending
152 *  on monitor location).  Table must be sorted.
153 */
154static __u16 const msstab[] = {
155	536,
156	1300,
157	1440,	/* 1440, 1452: PPPoE */
158	1460,
159};
160
161/*
162 * Generate a syncookie.  mssp points to the mss, which is returned
163 * rounded down to the value encoded in the cookie.
164 */
165u32 __cookie_v4_init_sequence(const struct iphdr *iph, const struct tcphdr *th,
166			      u16 *mssp)
167{
168	int mssind;
169	const __u16 mss = *mssp;
170
171	for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
172		if (mss >= msstab[mssind])
173			break;
174	*mssp = msstab[mssind];
175
176	return secure_tcp_syn_cookie(iph->saddr, iph->daddr,
177				     th->source, th->dest, ntohl(th->seq),
178				     mssind);
179}
180EXPORT_SYMBOL_GPL(__cookie_v4_init_sequence);
181
182__u32 cookie_v4_init_sequence(const struct sk_buff *skb, __u16 *mssp)
183{
184	const struct iphdr *iph = ip_hdr(skb);
185	const struct tcphdr *th = tcp_hdr(skb);
186
 
 
 
187	return __cookie_v4_init_sequence(iph, th, mssp);
188}
189
190/*
191 * Check if a ack sequence number is a valid syncookie.
192 * Return the decoded mss if it is, or 0 if not.
193 */
194int __cookie_v4_check(const struct iphdr *iph, const struct tcphdr *th,
195		      u32 cookie)
196{
197	__u32 seq = ntohl(th->seq) - 1;
198	__u32 mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr,
199					    th->source, th->dest, seq);
200
201	return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
202}
203EXPORT_SYMBOL_GPL(__cookie_v4_check);
204
205struct sock *tcp_get_cookie_sock(struct sock *sk, struct sk_buff *skb,
206				 struct request_sock *req,
207				 struct dst_entry *dst, u32 tsoff)
208{
209	struct inet_connection_sock *icsk = inet_csk(sk);
210	struct sock *child;
211	bool own_req;
212
213	child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst,
214						 NULL, &own_req);
215	if (child) {
216		refcount_set(&req->rsk_refcnt, 1);
217		tcp_sk(child)->tsoffset = tsoff;
218		sock_rps_save_rxhash(child, skb);
219		inet_csk_reqsk_queue_add(sk, req, child);
220	} else {
221		reqsk_free(req);
222	}
223	return child;
224}
225EXPORT_SYMBOL(tcp_get_cookie_sock);
226
227/*
228 * when syncookies are in effect and tcp timestamps are enabled we stored
229 * additional tcp options in the timestamp.
230 * This extracts these options from the timestamp echo.
231 *
232 * return false if we decode a tcp option that is disabled
233 * on the host.
 
 
234 */
235bool cookie_timestamp_decode(const struct net *net,
236			     struct tcp_options_received *tcp_opt)
237{
238	/* echoed timestamp, lowest bits contain options */
239	u32 options = tcp_opt->rcv_tsecr;
240
241	if (!tcp_opt->saw_tstamp)  {
242		tcp_clear_options(tcp_opt);
243		return true;
244	}
245
246	if (!net->ipv4.sysctl_tcp_timestamps)
247		return false;
248
249	tcp_opt->sack_ok = (options & TS_OPT_SACK) ? TCP_SACK_SEEN : 0;
 
 
 
250
251	if (tcp_opt->sack_ok && !net->ipv4.sysctl_tcp_sack)
252		return false;
253
254	if ((options & TS_OPT_WSCALE_MASK) == TS_OPT_WSCALE_MASK)
255		return true; /* no window scaling */
256
257	tcp_opt->wscale_ok = 1;
258	tcp_opt->snd_wscale = options & TS_OPT_WSCALE_MASK;
259
260	return net->ipv4.sysctl_tcp_window_scaling != 0;
261}
262EXPORT_SYMBOL(cookie_timestamp_decode);
263
264bool cookie_ecn_ok(const struct tcp_options_received *tcp_opt,
265		   const struct net *net, const struct dst_entry *dst)
266{
267	bool ecn_ok = tcp_opt->rcv_tsecr & TS_OPT_ECN;
268
269	if (!ecn_ok)
270		return false;
271
272	if (net->ipv4.sysctl_tcp_ecn)
273		return true;
274
275	return dst_feature(dst, RTAX_FEATURE_ECN);
276}
277EXPORT_SYMBOL(cookie_ecn_ok);
278
279/* On input, sk is a listener.
280 * Output is listener if incoming packet would not create a child
281 *           NULL if memory could not be allocated.
282 */
283struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb)
284{
285	struct ip_options *opt = &TCP_SKB_CB(skb)->header.h4.opt;
286	struct tcp_options_received tcp_opt;
287	struct inet_request_sock *ireq;
288	struct tcp_request_sock *treq;
289	struct tcp_sock *tp = tcp_sk(sk);
290	const struct tcphdr *th = tcp_hdr(skb);
291	__u32 cookie = ntohl(th->ack_seq) - 1;
292	struct sock *ret = sk;
293	struct request_sock *req;
294	int mss;
295	struct rtable *rt;
296	__u8 rcv_wscale;
 
297	struct flowi4 fl4;
298	u32 tsoff = 0;
299
300	if (!sock_net(sk)->ipv4.sysctl_tcp_syncookies || !th->ack || th->rst)
301		goto out;
302
303	if (tcp_synq_no_recent_overflow(sk))
304		goto out;
305
306	mss = __cookie_v4_check(ip_hdr(skb), th, cookie);
307	if (mss == 0) {
308		__NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
309		goto out;
310	}
311
312	__NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
313
314	/* check for timestamp cookie support */
315	memset(&tcp_opt, 0, sizeof(tcp_opt));
316	tcp_parse_options(sock_net(sk), skb, &tcp_opt, 0, NULL);
317
318	if (tcp_opt.saw_tstamp && tcp_opt.rcv_tsecr) {
319		tsoff = secure_tcp_ts_off(sock_net(sk),
320					  ip_hdr(skb)->daddr,
321					  ip_hdr(skb)->saddr);
322		tcp_opt.rcv_tsecr -= tsoff;
323	}
324
325	if (!cookie_timestamp_decode(sock_net(sk), &tcp_opt))
326		goto out;
327
328	ret = NULL;
329	req = inet_reqsk_alloc(&tcp_request_sock_ops, sk, false); /* for safety */
330	if (!req)
331		goto out;
332
333	ireq = inet_rsk(req);
334	treq = tcp_rsk(req);
335	treq->rcv_isn		= ntohl(th->seq) - 1;
336	treq->snt_isn		= cookie;
337	treq->ts_off		= 0;
338	treq->txhash		= net_tx_rndhash();
339	req->mss		= mss;
340	ireq->ir_num		= ntohs(th->dest);
341	ireq->ir_rmt_port	= th->source;
342	sk_rcv_saddr_set(req_to_sk(req), ip_hdr(skb)->daddr);
343	sk_daddr_set(req_to_sk(req), ip_hdr(skb)->saddr);
344	ireq->ir_mark		= inet_request_mark(sk, skb);
345	ireq->snd_wscale	= tcp_opt.snd_wscale;
346	ireq->sack_ok		= tcp_opt.sack_ok;
347	ireq->wscale_ok		= tcp_opt.wscale_ok;
348	ireq->tstamp_ok		= tcp_opt.saw_tstamp;
349	req->ts_recent		= tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
350	treq->snt_synack	= 0;
351	treq->tfo_listener	= false;
352	if (IS_ENABLED(CONFIG_SMC))
353		ireq->smc_ok = 0;
354
355	ireq->ir_iif = inet_request_bound_dev_if(sk, skb);
356
357	/* We throwed the options of the initial SYN away, so we hope
358	 * the ACK carries the same options again (see RFC1122 4.2.3.8)
359	 */
360	RCU_INIT_POINTER(ireq->ireq_opt, tcp_v4_save_options(sock_net(sk), skb));
 
 
 
 
 
 
 
 
361
362	if (security_inet_conn_request(sk, skb, req)) {
363		reqsk_free(req);
364		goto out;
365	}
366
 
367	req->num_retrans = 0;
368
369	/*
370	 * We need to lookup the route here to get at the correct
371	 * window size. We should better make sure that the window size
372	 * hasn't changed since we received the original syn, but I see
373	 * no easy way to do this.
374	 */
375	flowi4_init_output(&fl4, ireq->ir_iif, ireq->ir_mark,
376			   RT_CONN_FLAGS(sk), RT_SCOPE_UNIVERSE, IPPROTO_TCP,
377			   inet_sk_flowi_flags(sk),
378			   opt->srr ? opt->faddr : ireq->ir_rmt_addr,
379			   ireq->ir_loc_addr, th->source, th->dest, sk->sk_uid);
380	security_req_classify_flow(req, flowi4_to_flowi(&fl4));
381	rt = ip_route_output_key(sock_net(sk), &fl4);
382	if (IS_ERR(rt)) {
383		reqsk_free(req);
384		goto out;
385	}
386
387	/* Try to redo what tcp_v4_send_synack did. */
388	req->rsk_window_clamp = tp->window_clamp ? :dst_metric(&rt->dst, RTAX_WINDOW);
389
390	tcp_select_initial_window(sk, tcp_full_space(sk), req->mss,
391				  &req->rsk_rcv_wnd, &req->rsk_window_clamp,
392				  ireq->wscale_ok, &rcv_wscale,
393				  dst_metric(&rt->dst, RTAX_INITRWND));
394
395	ireq->rcv_wscale  = rcv_wscale;
396	ireq->ecn_ok = cookie_ecn_ok(&tcp_opt, sock_net(sk), &rt->dst);
397
398	ret = tcp_get_cookie_sock(sk, skb, req, &rt->dst, tsoff);
399	/* ip_queue_xmit() depends on our flow being setup
400	 * Normal sockets get it right from inet_csk_route_child_sock()
401	 */
402	if (ret)
403		inet_sk(ret)->cork.fl.u.ip4 = fl4;
404out:	return ret;
405}