1 files changed, 449 insertions, 0 deletions

diff --git a/net/ipv4/syncookies.c b/net/ipv4/syncookies.c
new file mode 100644
index 000000000..f9514cf87
--- /dev/null
+++ b/net/ipv4/syncookies.c
@@ -0,0 +1,449 @@
+// 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;
+}