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- // SPDX-License-Identifier: GPL-2.0-or-later
- /*
- * Syncookies implementation for the Linux kernel
- *
- * Copyright (C) 1997 Andi Kleen
- * Based on ideas by D.J.Bernstein and Eric Schenk.
- */
- #include <linux/tcp.h>
- #include <linux/siphash.h>
- #include <linux/kernel.h>
- #include <linux/export.h>
- #include <net/secure_seq.h>
- #include <net/tcp.h>
- #include <net/route.h>
- static siphash_aligned_key_t syncookie_secret[2];
- #define COOKIEBITS 24 /* Upper bits store count */
- #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
- /* TCP Timestamp: 6 lowest bits of timestamp sent in the cookie SYN-ACK
- * stores TCP options:
- *
- * MSB LSB
- * | 31 ... 6 | 5 | 4 | 3 2 1 0 |
- * | Timestamp | ECN | SACK | WScale |
- *
- * When we receive a valid cookie-ACK, we look at the echoed tsval (if
- * any) to figure out which TCP options we should use for the rebuilt
- * connection.
- *
- * A WScale setting of '0xf' (which is an invalid scaling value)
- * means that original syn did not include the TCP window scaling option.
- */
- #define TS_OPT_WSCALE_MASK 0xf
- #define TS_OPT_SACK BIT(4)
- #define TS_OPT_ECN BIT(5)
- /* There is no TS_OPT_TIMESTAMP:
- * if ACK contains timestamp option, we already know it was
- * requested/supported by the syn/synack exchange.
- */
- #define TSBITS 6
- static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
- u32 count, int c)
- {
- net_get_random_once(syncookie_secret, sizeof(syncookie_secret));
- return siphash_4u32((__force u32)saddr, (__force u32)daddr,
- (__force u32)sport << 16 | (__force u32)dport,
- count, &syncookie_secret[c]);
- }
- /*
- * when syncookies are in effect and tcp timestamps are enabled we encode
- * tcp options in the lower bits of the timestamp value that will be
- * sent in the syn-ack.
- * Since subsequent timestamps use the normal tcp_time_stamp value, we
- * must make sure that the resulting initial timestamp is <= tcp_time_stamp.
- */
- u64 cookie_init_timestamp(struct request_sock *req, u64 now)
- {
- const struct inet_request_sock *ireq = inet_rsk(req);
- u64 ts, ts_now = tcp_ns_to_ts(now);
- u32 options = 0;
- options = ireq->wscale_ok ? ireq->snd_wscale : TS_OPT_WSCALE_MASK;
- if (ireq->sack_ok)
- options |= TS_OPT_SACK;
- if (ireq->ecn_ok)
- options |= TS_OPT_ECN;
- ts = (ts_now >> TSBITS) << TSBITS;
- ts |= options;
- if (ts > ts_now)
- ts -= (1UL << TSBITS);
- return ts * (NSEC_PER_SEC / TCP_TS_HZ);
- }
- static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport,
- __be16 dport, __u32 sseq, __u32 data)
- {
- /*
- * Compute the secure sequence number.
- * The output should be:
- * HASH(sec1,saddr,sport,daddr,dport,sec1) + sseq + (count * 2^24)
- * + (HASH(sec2,saddr,sport,daddr,dport,count,sec2) % 2^24).
- * Where sseq is their sequence number and count increases every
- * minute by 1.
- * As an extra hack, we add a small "data" value that encodes the
- * MSS into the second hash value.
- */
- u32 count = tcp_cookie_time();
- return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
- sseq + (count << COOKIEBITS) +
- ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
- & COOKIEMASK));
- }
- /*
- * This retrieves the small "data" value from the syncookie.
- * If the syncookie is bad, the data returned will be out of
- * range. This must be checked by the caller.
- *
- * The count value used to generate the cookie must be less than
- * MAX_SYNCOOKIE_AGE minutes in the past.
- * The return value (__u32)-1 if this test fails.
- */
- static __u32 check_tcp_syn_cookie(__u32 cookie, __be32 saddr, __be32 daddr,
- __be16 sport, __be16 dport, __u32 sseq)
- {
- u32 diff, count = tcp_cookie_time();
- /* Strip away the layers from the cookie */
- cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
- /* Cookie is now reduced to (count * 2^24) ^ (hash % 2^24) */
- diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
- if (diff >= MAX_SYNCOOKIE_AGE)
- return (__u32)-1;
- return (cookie -
- cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
- & COOKIEMASK; /* Leaving the data behind */
- }
- /*
- * MSS Values are chosen based on the 2011 paper
- * 'An Analysis of TCP Maximum Segement Sizes' by S. Alcock and R. Nelson.
- * Values ..
- * .. lower than 536 are rare (< 0.2%)
- * .. between 537 and 1299 account for less than < 1.5% of observed values
- * .. in the 1300-1349 range account for about 15 to 20% of observed mss values
- * .. exceeding 1460 are very rare (< 0.04%)
- *
- * 1460 is the single most frequently announced mss value (30 to 46% depending
- * on monitor location). Table must be sorted.
- */
- static __u16 const msstab[] = {
- 536,
- 1300,
- 1440, /* 1440, 1452: PPPoE */
- 1460,
- };
- /*
- * Generate a syncookie. mssp points to the mss, which is returned
- * rounded down to the value encoded in the cookie.
- */
- u32 __cookie_v4_init_sequence(const struct iphdr *iph, const struct tcphdr *th,
- u16 *mssp)
- {
- int mssind;
- const __u16 mss = *mssp;
- for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
- if (mss >= msstab[mssind])
- break;
- *mssp = msstab[mssind];
- return secure_tcp_syn_cookie(iph->saddr, iph->daddr,
- th->source, th->dest, ntohl(th->seq),
- mssind);
- }
- EXPORT_SYMBOL_GPL(__cookie_v4_init_sequence);
- __u32 cookie_v4_init_sequence(const struct sk_buff *skb, __u16 *mssp)
- {
- const struct iphdr *iph = ip_hdr(skb);
- const struct tcphdr *th = tcp_hdr(skb);
- return __cookie_v4_init_sequence(iph, th, mssp);
- }
- /*
- * Check if a ack sequence number is a valid syncookie.
- * Return the decoded mss if it is, or 0 if not.
- */
- int __cookie_v4_check(const struct iphdr *iph, const struct tcphdr *th,
- u32 cookie)
- {
- __u32 seq = ntohl(th->seq) - 1;
- __u32 mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr,
- th->source, th->dest, seq);
- return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
- }
- EXPORT_SYMBOL_GPL(__cookie_v4_check);
- struct sock *tcp_get_cookie_sock(struct sock *sk, struct sk_buff *skb,
- struct request_sock *req,
- struct dst_entry *dst, u32 tsoff)
- {
- struct inet_connection_sock *icsk = inet_csk(sk);
- struct sock *child;
- bool own_req;
- child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst,
- NULL, &own_req);
- if (child) {
- refcount_set(&req->rsk_refcnt, 1);
- tcp_sk(child)->tsoffset = tsoff;
- sock_rps_save_rxhash(child, skb);
- if (rsk_drop_req(req)) {
- reqsk_put(req);
- return child;
- }
- if (inet_csk_reqsk_queue_add(sk, req, child))
- return child;
- bh_unlock_sock(child);
- sock_put(child);
- }
- __reqsk_free(req);
- return NULL;
- }
- EXPORT_SYMBOL(tcp_get_cookie_sock);
- /*
- * when syncookies are in effect and tcp timestamps are enabled we stored
- * additional tcp options in the timestamp.
- * This extracts these options from the timestamp echo.
- *
- * return false if we decode a tcp option that is disabled
- * on the host.
- */
- bool cookie_timestamp_decode(const struct net *net,
- struct tcp_options_received *tcp_opt)
- {
- /* echoed timestamp, lowest bits contain options */
- u32 options = tcp_opt->rcv_tsecr;
- if (!tcp_opt->saw_tstamp) {
- tcp_clear_options(tcp_opt);
- return true;
- }
- if (!READ_ONCE(net->ipv4.sysctl_tcp_timestamps))
- return false;
- tcp_opt->sack_ok = (options & TS_OPT_SACK) ? TCP_SACK_SEEN : 0;
- if (tcp_opt->sack_ok && !READ_ONCE(net->ipv4.sysctl_tcp_sack))
- return false;
- if ((options & TS_OPT_WSCALE_MASK) == TS_OPT_WSCALE_MASK)
- return true; /* no window scaling */
- tcp_opt->wscale_ok = 1;
- tcp_opt->snd_wscale = options & TS_OPT_WSCALE_MASK;
- return READ_ONCE(net->ipv4.sysctl_tcp_window_scaling) != 0;
- }
- EXPORT_SYMBOL(cookie_timestamp_decode);
- bool cookie_ecn_ok(const struct tcp_options_received *tcp_opt,
- const struct net *net, const struct dst_entry *dst)
- {
- bool ecn_ok = tcp_opt->rcv_tsecr & TS_OPT_ECN;
- if (!ecn_ok)
- return false;
- if (READ_ONCE(net->ipv4.sysctl_tcp_ecn))
- return true;
- return dst_feature(dst, RTAX_FEATURE_ECN);
- }
- EXPORT_SYMBOL(cookie_ecn_ok);
- struct request_sock *cookie_tcp_reqsk_alloc(const struct request_sock_ops *ops,
- const struct tcp_request_sock_ops *af_ops,
- struct sock *sk,
- struct sk_buff *skb)
- {
- struct tcp_request_sock *treq;
- struct request_sock *req;
- if (sk_is_mptcp(sk))
- req = mptcp_subflow_reqsk_alloc(ops, sk, false);
- else
- req = inet_reqsk_alloc(ops, sk, false);
- if (!req)
- return NULL;
- treq = tcp_rsk(req);
- /* treq->af_specific might be used to perform TCP_MD5 lookup */
- treq->af_specific = af_ops;
- treq->syn_tos = TCP_SKB_CB(skb)->ip_dsfield;
- #if IS_ENABLED(CONFIG_MPTCP)
- treq->is_mptcp = sk_is_mptcp(sk);
- if (treq->is_mptcp) {
- int err = mptcp_subflow_init_cookie_req(req, sk, skb);
- if (err) {
- reqsk_free(req);
- return NULL;
- }
- }
- #endif
- return req;
- }
- EXPORT_SYMBOL_GPL(cookie_tcp_reqsk_alloc);
- /* On input, sk is a listener.
- * Output is listener if incoming packet would not create a child
- * NULL if memory could not be allocated.
- */
- struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb)
- {
- struct ip_options *opt = &TCP_SKB_CB(skb)->header.h4.opt;
- struct tcp_options_received tcp_opt;
- struct inet_request_sock *ireq;
- struct tcp_request_sock *treq;
- struct tcp_sock *tp = tcp_sk(sk);
- const struct tcphdr *th = tcp_hdr(skb);
- __u32 cookie = ntohl(th->ack_seq) - 1;
- struct sock *ret = sk;
- struct request_sock *req;
- int full_space, mss;
- struct rtable *rt;
- __u8 rcv_wscale;
- struct flowi4 fl4;
- u32 tsoff = 0;
- if (!READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_syncookies) ||
- !th->ack || th->rst)
- goto out;
- if (tcp_synq_no_recent_overflow(sk))
- goto out;
- mss = __cookie_v4_check(ip_hdr(skb), th, cookie);
- if (mss == 0) {
- __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
- goto out;
- }
- __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
- /* check for timestamp cookie support */
- memset(&tcp_opt, 0, sizeof(tcp_opt));
- tcp_parse_options(sock_net(sk), skb, &tcp_opt, 0, NULL);
- if (tcp_opt.saw_tstamp && tcp_opt.rcv_tsecr) {
- tsoff = secure_tcp_ts_off(sock_net(sk),
- ip_hdr(skb)->daddr,
- ip_hdr(skb)->saddr);
- tcp_opt.rcv_tsecr -= tsoff;
- }
- if (!cookie_timestamp_decode(sock_net(sk), &tcp_opt))
- goto out;
- ret = NULL;
- req = cookie_tcp_reqsk_alloc(&tcp_request_sock_ops,
- &tcp_request_sock_ipv4_ops, sk, skb);
- if (!req)
- goto out;
- ireq = inet_rsk(req);
- treq = tcp_rsk(req);
- treq->rcv_isn = ntohl(th->seq) - 1;
- treq->snt_isn = cookie;
- treq->ts_off = 0;
- treq->txhash = net_tx_rndhash();
- req->mss = mss;
- ireq->ir_num = ntohs(th->dest);
- ireq->ir_rmt_port = th->source;
- sk_rcv_saddr_set(req_to_sk(req), ip_hdr(skb)->daddr);
- sk_daddr_set(req_to_sk(req), ip_hdr(skb)->saddr);
- ireq->ir_mark = inet_request_mark(sk, skb);
- ireq->snd_wscale = tcp_opt.snd_wscale;
- ireq->sack_ok = tcp_opt.sack_ok;
- ireq->wscale_ok = tcp_opt.wscale_ok;
- ireq->tstamp_ok = tcp_opt.saw_tstamp;
- req->ts_recent = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
- treq->snt_synack = 0;
- treq->tfo_listener = false;
- if (IS_ENABLED(CONFIG_SMC))
- ireq->smc_ok = 0;
- ireq->ir_iif = inet_request_bound_dev_if(sk, skb);
- /* We throwed the options of the initial SYN away, so we hope
- * the ACK carries the same options again (see RFC1122 4.2.3.8)
- */
- RCU_INIT_POINTER(ireq->ireq_opt, tcp_v4_save_options(sock_net(sk), skb));
- if (security_inet_conn_request(sk, skb, req)) {
- reqsk_free(req);
- goto out;
- }
- req->num_retrans = 0;
- /*
- * We need to lookup the route here to get at the correct
- * window size. We should better make sure that the window size
- * hasn't changed since we received the original syn, but I see
- * no easy way to do this.
- */
- flowi4_init_output(&fl4, ireq->ir_iif, ireq->ir_mark,
- RT_CONN_FLAGS(sk), RT_SCOPE_UNIVERSE, IPPROTO_TCP,
- inet_sk_flowi_flags(sk),
- opt->srr ? opt->faddr : ireq->ir_rmt_addr,
- ireq->ir_loc_addr, th->source, th->dest, sk->sk_uid);
- security_req_classify_flow(req, flowi4_to_flowi_common(&fl4));
- rt = ip_route_output_key(sock_net(sk), &fl4);
- if (IS_ERR(rt)) {
- reqsk_free(req);
- goto out;
- }
- /* Try to redo what tcp_v4_send_synack did. */
- req->rsk_window_clamp = tp->window_clamp ? :dst_metric(&rt->dst, RTAX_WINDOW);
- /* limit the window selection if the user enforce a smaller rx buffer */
- full_space = tcp_full_space(sk);
- if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
- (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
- req->rsk_window_clamp = full_space;
- tcp_select_initial_window(sk, full_space, req->mss,
- &req->rsk_rcv_wnd, &req->rsk_window_clamp,
- ireq->wscale_ok, &rcv_wscale,
- dst_metric(&rt->dst, RTAX_INITRWND));
- ireq->rcv_wscale = rcv_wscale;
- ireq->ecn_ok = cookie_ecn_ok(&tcp_opt, sock_net(sk), &rt->dst);
- ret = tcp_get_cookie_sock(sk, skb, req, &rt->dst, tsoff);
- /* ip_queue_xmit() depends on our flow being setup
- * Normal sockets get it right from inet_csk_route_child_sock()
- */
- if (ret)
- inet_sk(ret)->cork.fl.u.ip4 = fl4;
- out: return ret;
- }
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