/*
 *  Syncookies implementation for the Linux kernel
 *
 *  Copyright (C) 1997 Andi Kleen
 *  Based on ideas by D.J.Bernstein and Eric Schenk.
 *
 *      This program is free software; you can redistribute it and/or
 *      modify it under the terms of the GNU General Public License
 *      as published by the Free Software Foundation; either version
 *      2 of the License, or (at your option) any later version.
 */

#include <linux/tcp.h>
#include <linux/slab.h>
#include <linux/random.h>
#include <linux/cryptohash.h>
#include <linux/kernel.h>
#include <linux/export.h>
#include <net/tcp.h>
#include <net/route.h>

extern int sysctl_tcp_syncookies;

static u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS] __read_mostly;

#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
#define TSMASK  (((__u32)1 << TSBITS) - 1)

static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS],
                      ipv4_cookie_scratch);

static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
                       u32 count, int c)
{
        __u32 *tmp;

        net_get_random_once(syncookie_secret, sizeof(syncookie_secret));

        tmp  = this_cpu_ptr(ipv4_cookie_scratch);
        memcpy(tmp + 4, syncookie_secret[c], sizeof(syncookie_secret[c]));
        tmp[0] = (__force u32)saddr;
        tmp[1] = (__force u32)daddr;
        tmp[2] = ((__force u32)sport << 16) + (__force u32)dport;
        tmp[3] = count;
        sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);

        return tmp[17];
}


/*
 * 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.
 */
__u32 cookie_init_timestamp(struct request_sock *req)
{
        struct inet_request_sock *ireq;
        u32 ts, ts_now = tcp_time_stamp;
        u32 options = 0;

        ireq = inet_rsk(req);

        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 & ~TSMASK;
        ts |= options;
        if (ts > ts_now) {
                ts >>= TSBITS;
                ts--;
                ts <<= TSBITS;
                ts |= options;
        }
        return ts;
}


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(struct sock *sk, const struct sk_buff *skb,
                              __u16 *mssp)
{
        const struct iphdr *iph = ip_hdr(skb);
        const struct tcphdr *th = tcp_hdr(skb);

        tcp_synq_overflow(sk);
        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT);

        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);

static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb,
                                           struct request_sock *req,
                                           struct dst_entry *dst)
{
        struct inet_connection_sock *icsk = inet_csk(sk);
        struct sock *child;

        child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst);
        if (child)
                inet_csk_reqsk_queue_add(sk, req, child);
        else
                reqsk_free(req);

        return child;
}


/*
 * 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(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 (!sysctl_tcp_timestamps)
                return false;

        tcp_opt->sack_ok = (options & TS_OPT_SACK) ? TCP_SACK_SEEN : 0;

        if (tcp_opt->sack_ok && !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 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 (net->ipv4.sysctl_tcp_ecn)
                return true;

        return dst_feature(dst, RTAX_FEATURE_ECN);
}
EXPORT_SYMBOL(cookie_ecn_ok);

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 mss;
        struct rtable *rt;
        __u8 rcv_wscale;
        struct flowi4 fl4;

        if (!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_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
                goto out;
        }

        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);

        /* check for timestamp cookie support */
        memset(&tcp_opt, 0, sizeof(tcp_opt));
        tcp_parse_options(skb, &tcp_opt, 0, NULL);

        if (!cookie_timestamp_decode(&tcp_opt))
                goto out;

        ret = NULL;
        req = inet_reqsk_alloc(&tcp_request_sock_ops); /* for safety */
        if (!req)
                goto out;

        ireq = inet_rsk(req);
        treq = tcp_rsk(req);
        treq->rcv_isn           = ntohl(th->seq) - 1;
        treq->snt_isn           = cookie;
        req->mss                = mss;
        ireq->ir_num            = ntohs(th->dest);
        ireq->ir_rmt_port       = th->source;
        ireq->ir_loc_addr       = ip_hdr(skb)->daddr;
        ireq->ir_rmt_addr       = 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        = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsecr : 0;
        treq->listener          = NULL;

        /* 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)
         */
        ireq->opt = tcp_v4_save_options(skb);

        if (security_inet_conn_request(sk, skb, req)) {
                reqsk_free(req);
                goto out;
        }

        req->expires    = 0UL;
        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, sk->sk_bound_dev_if, 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);
        security_req_classify_flow(req, flowi4_to_flowi(&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->window_clamp = tp->window_clamp ? :dst_metric(&rt->dst, RTAX_WINDOW);

        tcp_select_initial_window(tcp_full_space(sk), req->mss,
                                  &req->rcv_wnd, &req->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 = get_cookie_sock(sk, skb, req, &rt->dst);
        /* 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;
}