diff options
Diffstat (limited to 'net/ipv4/syncookies.c')
| -rw-r--r-- | net/ipv4/syncookies.c | 279 |
1 files changed, 279 insertions, 0 deletions
diff --git a/net/ipv4/syncookies.c b/net/ipv4/syncookies.c new file mode 100644 index 0000000..e923d2f --- /dev/null +++ b/net/ipv4/syncookies.c @@ -0,0 +1,279 @@ +/* + * 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. + * + * $Id: syncookies.c,v 1.18 2002/02/01 22:01:04 davem Exp $ + * + * Missing: IPv6 support. + */ + +#include <linux/tcp.h> +#include <linux/slab.h> +#include <linux/random.h> +#include <linux/cryptohash.h> +#include <linux/kernel.h> +#include <net/tcp.h> + +extern int sysctl_tcp_syncookies; + +static __u32 syncookie_secret[2][16-3+SHA_DIGEST_WORDS]; + +static __init int init_syncookies(void) +{ + get_random_bytes(syncookie_secret, sizeof(syncookie_secret)); + return 0; +} +module_init(init_syncookies); + +#define COOKIEBITS 24 /* Upper bits store count */ +#define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1) + +static u32 cookie_hash(u32 saddr, u32 daddr, u32 sport, u32 dport, + u32 count, int c) +{ + __u32 tmp[16 + 5 + SHA_WORKSPACE_WORDS]; + + memcpy(tmp + 3, syncookie_secret[c], sizeof(syncookie_secret[c])); + tmp[0] = saddr; + tmp[1] = daddr; + tmp[2] = (sport << 16) + dport; + tmp[3] = count; + sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5); + + return tmp[17]; +} + +static __u32 secure_tcp_syn_cookie(__u32 saddr, __u32 daddr, __u16 sport, + __u16 dport, __u32 sseq, __u32 count, + __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. + */ + + 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 within + * "maxdiff" if the current (passed-in) "count". The return value + * is (__u32)-1 if this test fails. + */ +static __u32 check_tcp_syn_cookie(__u32 cookie, __u32 saddr, __u32 daddr, + __u16 sport, __u16 dport, __u32 sseq, + __u32 count, __u32 maxdiff) +{ + __u32 diff; + + /* 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 >= maxdiff) + return (__u32)-1; + + return (cookie - + cookie_hash(saddr, daddr, sport, dport, count - diff, 1)) + & COOKIEMASK; /* Leaving the data behind */ +} + +/* + * This table has to be sorted and terminated with (__u16)-1. + * XXX generate a better table. + * Unresolved Issues: HIPPI with a 64k MSS is not well supported. + */ +static __u16 const msstab[] = { + 64 - 1, + 256 - 1, + 512 - 1, + 536 - 1, + 1024 - 1, + 1440 - 1, + 1460 - 1, + 4312 - 1, + (__u16)-1 +}; +/* The number doesn't include the -1 terminator */ +#define NUM_MSS (ARRAY_SIZE(msstab) - 1) + +/* + * 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(struct sock *sk, struct sk_buff *skb, __u16 *mssp) +{ + struct tcp_sock *tp = tcp_sk(sk); + int mssind; + const __u16 mss = *mssp; + + + tp->last_synq_overflow = jiffies; + + /* XXX sort msstab[] by probability? Binary search? */ + for (mssind = 0; mss > msstab[mssind + 1]; mssind++) + ; + *mssp = msstab[mssind] + 1; + + NET_INC_STATS_BH(LINUX_MIB_SYNCOOKIESSENT); + + return secure_tcp_syn_cookie(skb->nh.iph->saddr, skb->nh.iph->daddr, + skb->h.th->source, skb->h.th->dest, + ntohl(skb->h.th->seq), + jiffies / (HZ * 60), mssind); +} + +/* + * This (misnamed) value is the age of syncookie which is permitted. + * Its ideal value should be dependent on TCP_TIMEOUT_INIT and + * sysctl_tcp_retries1. It's a rather complicated formula (exponential + * backoff) to compute at runtime so it's currently hardcoded here. + */ +#define COUNTER_TRIES 4 +/* + * Check if a ack sequence number is a valid syncookie. + * Return the decoded mss if it is, or 0 if not. + */ +static inline int cookie_check(struct sk_buff *skb, __u32 cookie) +{ + __u32 seq; + __u32 mssind; + + seq = ntohl(skb->h.th->seq)-1; + mssind = check_tcp_syn_cookie(cookie, + skb->nh.iph->saddr, skb->nh.iph->daddr, + skb->h.th->source, skb->h.th->dest, + seq, jiffies / (HZ * 60), COUNTER_TRIES); + + return mssind < NUM_MSS ? msstab[mssind] + 1 : 0; +} + +extern struct or_calltable or_ipv4; + +static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb, + struct open_request *req, + struct dst_entry *dst) +{ + struct tcp_sock *tp = tcp_sk(sk); + struct sock *child; + + child = tp->af_specific->syn_recv_sock(sk, skb, req, dst); + if (child) + tcp_acceptq_queue(sk, req, child); + else + tcp_openreq_free(req); + + return child; +} + +struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb, + struct ip_options *opt) +{ + struct tcp_sock *tp = tcp_sk(sk); + __u32 cookie = ntohl(skb->h.th->ack_seq) - 1; + struct sock *ret = sk; + struct open_request *req; + int mss; + struct rtable *rt; + __u8 rcv_wscale; + + if (!sysctl_tcp_syncookies || !skb->h.th->ack) + goto out; + + if (time_after(jiffies, tp->last_synq_overflow + TCP_TIMEOUT_INIT) || + (mss = cookie_check(skb, cookie)) == 0) { + NET_INC_STATS_BH(LINUX_MIB_SYNCOOKIESFAILED); + goto out; + } + + NET_INC_STATS_BH(LINUX_MIB_SYNCOOKIESRECV); + + req = tcp_openreq_alloc(); + ret = NULL; + if (!req) + goto out; + + req->rcv_isn = htonl(skb->h.th->seq) - 1; + req->snt_isn = cookie; + req->mss = mss; + req->rmt_port = skb->h.th->source; + req->af.v4_req.loc_addr = skb->nh.iph->daddr; + req->af.v4_req.rmt_addr = skb->nh.iph->saddr; + req->class = &or_ipv4; /* for savety */ + req->af.v4_req.opt = 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) + */ + if (opt && opt->optlen) { + int opt_size = sizeof(struct ip_options) + opt->optlen; + + req->af.v4_req.opt = kmalloc(opt_size, GFP_ATOMIC); + if (req->af.v4_req.opt) { + if (ip_options_echo(req->af.v4_req.opt, skb)) { + kfree(req->af.v4_req.opt); + req->af.v4_req.opt = NULL; + } + } + } + + req->snd_wscale = req->rcv_wscale = req->tstamp_ok = 0; + req->wscale_ok = req->sack_ok = 0; + req->expires = 0UL; + req->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. + */ + { + struct flowi fl = { .nl_u = { .ip4_u = + { .daddr = ((opt && opt->srr) ? + opt->faddr : + req->af.v4_req.rmt_addr), + .saddr = req->af.v4_req.loc_addr, + .tos = RT_CONN_FLAGS(sk) } }, + .proto = IPPROTO_TCP, + .uli_u = { .ports = + { .sport = skb->h.th->dest, + .dport = skb->h.th->source } } }; + if (ip_route_output_key(&rt, &fl)) { + tcp_openreq_free(req); + goto out; + } + } + + /* Try to redo what tcp_v4_send_synack did. */ + req->window_clamp = dst_metric(&rt->u.dst, RTAX_WINDOW); + tcp_select_initial_window(tcp_full_space(sk), req->mss, + &req->rcv_wnd, &req->window_clamp, + 0, &rcv_wscale); + /* BTW win scale with syncookies is 0 by definition */ + req->rcv_wscale = rcv_wscale; + + ret = get_cookie_sock(sk, skb, req, &rt->u.dst); +out: return ret; +} |
