linux/net/ipv4/syncookies.c
Florian Westphal 4dfc281702 [Syncookies]: Add support for TCP options via timestamps.
Allow the use of SACK and window scaling when syncookies are used
and the client supports tcp timestamps. Options are encoded into
the timestamp sent in the syn-ack and restored from the timestamp
echo when the ack is received.

Based on earlier work by Glenn Griffin.
This patch avoids increasing the size of structs by encoding TCP
options into the least significant bits of the timestamp and
by not using any 'timestamp offset'.

The downside is that the timestamp sent in the packet after the synack
will increase by several seconds.

changes since v1:
 don't duplicate timestamp echo decoding function, put it into ipv4/syncookie.c
 and have ipv6/syncookies.c use it.
 Feedback from Glenn Griffin: fix line indented with spaces, kill redundant if ()

Reviewed-by: Hagen Paul Pfeifer <hagen@jauu.net>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>
2008-04-10 03:12:40 -07:00

364 lines
9.9 KiB
C

/*
* 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 $
*/
#include <linux/tcp.h>
#include <linux/slab.h>
#include <linux/random.h>
#include <linux/cryptohash.h>
#include <linux/kernel.h>
#include <net/tcp.h>
/* Timestamps: lowest 9 bits store TCP options */
#define TSBITS 9
#define TSMASK (((__u32)1 << TSBITS) - 1)
extern int sysctl_tcp_syncookies;
__u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS];
EXPORT_SYMBOL(syncookie_secret);
static __init int init_syncookies(void)
{
get_random_bytes(syncookie_secret, sizeof(syncookie_secret));
return 0;
}
__initcall(init_syncookies);
#define COOKIEBITS 24 /* Upper bits store count */
#define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
static DEFINE_PER_CPU(__u32, cookie_scratch)[16 + 5 + SHA_WORKSPACE_WORDS];
static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
u32 count, int c)
{
__u32 *tmp = __get_cpu_var(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 lowest 9 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);
if (ireq->wscale_ok) {
options = ireq->snd_wscale;
options |= ireq->rcv_wscale << 4;
}
options |= ireq->sack_ok << 8;
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 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, __be32 saddr, __be32 daddr,
__be16 sport, __be16 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);
const struct iphdr *iph = ip_hdr(skb);
const struct tcphdr *th = tcp_hdr(skb);
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(iph->saddr, iph->daddr,
th->source, th->dest, ntohl(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)
{
const struct iphdr *iph = ip_hdr(skb);
const struct tcphdr *th = tcp_hdr(skb);
__u32 seq = ntohl(th->seq) - 1;
__u32 mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr,
th->source, th->dest, seq,
jiffies / (HZ * 60),
COUNTER_TRIES);
return mssind < NUM_MSS ? msstab[mssind] + 1 : 0;
}
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.
*
* The lowest 4 bits are for snd_wscale
* The next 4 lsb are for rcv_wscale
* The next lsb is for sack_ok
*/
void cookie_check_timestamp(struct tcp_options_received *tcp_opt)
{
/* echoed timestamp, 9 lowest bits contain options */
u32 options = tcp_opt->rcv_tsecr & TSMASK;
tcp_opt->snd_wscale = options & 0xf;
options >>= 4;
tcp_opt->rcv_wscale = options & 0xf;
tcp_opt->sack_ok = (options >> 4) & 0x1;
if (tcp_opt->sack_ok)
tcp_sack_reset(tcp_opt);
if (tcp_opt->snd_wscale || tcp_opt->rcv_wscale)
tcp_opt->wscale_ok = 1;
}
EXPORT_SYMBOL(cookie_check_timestamp);
struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
struct ip_options *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 tcp_options_received tcp_opt;
if (!sysctl_tcp_syncookies || !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);
/* check for timestamp cookie support */
memset(&tcp_opt, 0, sizeof(tcp_opt));
tcp_parse_options(skb, &tcp_opt, 0);
if (tcp_opt.saw_tstamp)
cookie_check_timestamp(&tcp_opt);
ret = NULL;
req = reqsk_alloc(&tcp_request_sock_ops); /* for safety */
if (!req)
goto out;
if (security_inet_conn_request(sk, skb, req)) {
reqsk_free(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->rmt_port = th->source;
ireq->loc_addr = ip_hdr(skb)->daddr;
ireq->rmt_addr = ip_hdr(skb)->saddr;
ireq->opt = NULL;
ireq->snd_wscale = tcp_opt.snd_wscale;
ireq->rcv_wscale = tcp_opt.rcv_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;
/* 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;
ireq->opt = kmalloc(opt_size, GFP_ATOMIC);
if (ireq->opt != NULL && ip_options_echo(ireq->opt, skb)) {
kfree(ireq->opt);
ireq->opt = NULL;
}
}
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 :
ireq->rmt_addr),
.saddr = ireq->loc_addr,
.tos = RT_CONN_FLAGS(sk) } },
.proto = IPPROTO_TCP,
.uli_u = { .ports =
{ .sport = th->dest,
.dport = th->source } } };
security_req_classify_flow(req, &fl);
if (ip_route_output_key(&init_net, &rt, &fl)) {
reqsk_free(req);
goto out;
}
}
/* Try to redo what tcp_v4_send_synack did. */
req->window_clamp = tp->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,
ireq->wscale_ok, &rcv_wscale);
ireq->rcv_wscale = rcv_wscale;
ret = get_cookie_sock(sk, skb, req, &rt->u.dst);
out: return ret;
}