// 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(false, 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);
if (tcp_rsk(req)->req_usec_ts)
return ts * NSEC_PER_USEC;
return ts * NSEC_PER_MSEC;
}
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 = ntohl(th->ack_seq) - 1;
__u32 seq = ntohl(th->seq) - 1;
__u32 mssind;
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)
{
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);
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);
static int cookie_tcp_reqsk_init(struct sock *sk, struct sk_buff *skb,
struct request_sock *req)
{
struct inet_request_sock *ireq = inet_rsk(req);
struct tcp_request_sock *treq = tcp_rsk(req);
const struct tcphdr *th = tcp_hdr(skb);
req->num_retrans = 0;
ireq->ir_num = ntohs(th->dest);
ireq->ir_rmt_port = th->source;
ireq->ir_iif = inet_request_bound_dev_if(sk, skb);
ireq->ir_mark = inet_request_mark(sk, skb);
if (IS_ENABLED(CONFIG_SMC))
ireq->smc_ok = 0;
treq->snt_synack = 0;
treq->tfo_listener = false;
treq->txhash = net_tx_rndhash();
treq->rcv_isn = ntohl(th->seq) - 1;
treq->snt_isn = ntohl(th->ack_seq) - 1;
treq->syn_tos = TCP_SKB_CB(skb)->ip_dsfield;
treq->req_usec_ts = false;
#if IS_ENABLED(CONFIG_MPTCP)
treq->is_mptcp = sk_is_mptcp(sk);
if (treq->is_mptcp)
return mptcp_subflow_init_cookie_req(req, sk, skb);
#endif
return 0;
}
#if IS_ENABLED(CONFIG_BPF)
struct request_sock *cookie_bpf_check(struct sock *sk, struct sk_buff *skb)
{
struct request_sock *req = inet_reqsk(skb->sk);
skb->sk = NULL;
skb->destructor = NULL;
if (cookie_tcp_reqsk_init(sk, skb, req)) {
reqsk_free(req);
req = NULL;
}
return req;
}
EXPORT_SYMBOL_GPL(cookie_bpf_check);
#endif
struct request_sock *cookie_tcp_reqsk_alloc(const struct request_sock_ops *ops,
struct sock *sk, struct sk_buff *skb,
struct tcp_options_received *tcp_opt,
int mss, u32 tsoff)
{
struct inet_request_sock *ireq;
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;
if (cookie_tcp_reqsk_init(sk, skb, req)) {
reqsk_free(req);
return NULL;
}
ireq = inet_rsk(req);
treq = tcp_rsk(req);
req->mss = mss;
req->ts_recent = tcp_opt->saw_tstamp ? tcp_opt->rcv_tsval : 0;
ireq->snd_wscale = tcp_opt->snd_wscale;
ireq->tstamp_ok = tcp_opt->saw_tstamp;
ireq->sack_ok = tcp_opt->sack_ok;
ireq->wscale_ok = tcp_opt->wscale_ok;
ireq->ecn_ok = !!(tcp_opt->rcv_tsecr & TS_OPT_ECN);
treq->ts_off = tsoff;
return req;
}
EXPORT_SYMBOL_GPL(cookie_tcp_reqsk_alloc);
static struct request_sock *cookie_tcp_check(struct net *net, struct sock *sk,
struct sk_buff *skb)
{
struct tcp_options_received tcp_opt;
u32 tsoff = 0;
int mss;
if (tcp_synq_no_recent_overflow(sk))
goto out;
mss = __cookie_v4_check(ip_hdr(skb), tcp_hdr(skb));
if (!mss) {
__NET_INC_STATS(net, LINUX_MIB_SYNCOOKIESFAILED);
goto out;
}
__NET_INC_STATS(net, LINUX_MIB_SYNCOOKIESRECV);
/* check for timestamp cookie support */
memset(&tcp_opt, 0, sizeof(tcp_opt));
tcp_parse_options(net, skb, &tcp_opt, 0, NULL);
if (tcp_opt.saw_tstamp && tcp_opt.rcv_tsecr) {
tsoff = secure_tcp_ts_off(net,
ip_hdr(skb)->daddr,
ip_hdr(skb)->saddr);
tcp_opt.rcv_tsecr -= tsoff;
}
if (!cookie_timestamp_decode(net, &tcp_opt))
goto out;
return cookie_tcp_reqsk_alloc(&tcp_request_sock_ops, sk, skb,
&tcp_opt, mss, tsoff);
out:
return ERR_PTR(-EINVAL);
}
/* 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;
const struct tcphdr *th = tcp_hdr(skb);
struct tcp_sock *tp = tcp_sk(sk);
struct inet_request_sock *ireq;
struct net *net = sock_net(sk);
struct request_sock *req;
struct sock *ret = sk;
struct flowi4 fl4;
struct rtable *rt;
__u8 rcv_wscale;
int full_space;
SKB_DR(reason);
if (!READ_ONCE(net->ipv4.sysctl_tcp_syncookies) ||
!th->ack || th->rst)
goto out;
if (cookie_bpf_ok(skb)) {
req = cookie_bpf_check(sk, skb);
} else {
req = cookie_tcp_check(net, sk, skb);
if (IS_ERR(req))
goto out;
}
if (!req) {
SKB_DR_SET(reason, NO_SOCKET);
goto out_drop;
}
ireq = inet_rsk(req);
sk_rcv_saddr_set(req_to_sk(req), ip_hdr(skb)->daddr);
sk_daddr_set(req_to_sk(req), ip_hdr(skb)->saddr);
/* 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(net, skb));
if (security_inet_conn_request(sk, skb, req)) {
SKB_DR_SET(reason, SECURITY_HOOK);
goto out_free;
}
tcp_ao_syncookie(sk, skb, req, AF_INET);
/*
* 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,
ip_sock_rt_tos(sk), ip_sock_rt_scope(sk),
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(net, &fl4);
if (IS_ERR(rt)) {
SKB_DR_SET(reason, IP_OUTNOROUTES);
goto out_free;
}
/* Try to redo what tcp_v4_send_synack did. */
req->rsk_window_clamp = READ_ONCE(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));
/* req->syncookie is set true only if ACK is validated
* by BPF kfunc, then, rcv_wscale is already configured.
*/
if (!req->syncookie)
ireq->rcv_wscale = rcv_wscale;
ireq->ecn_ok &= cookie_ecn_ok(net, &rt->dst);
ret = tcp_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) {
SKB_DR_SET(reason, NO_SOCKET);
goto out_drop;
}
inet_sk(ret)->cork.fl.u.ip4 = fl4;
out:
return ret;
out_free:
reqsk_free(req);
out_drop:
sk_skb_reason_drop(sk, skb, reason);
return NULL;
}
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