diff options
Diffstat (limited to 'net/ipv4/tcp_minisocks.c')
| -rw-r--r-- | net/ipv4/tcp_minisocks.c | 794 |
1 files changed, 794 insertions, 0 deletions
diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c new file mode 100644 index 00000000..3cabafb5 --- /dev/null +++ b/net/ipv4/tcp_minisocks.c @@ -0,0 +1,794 @@ +/* + * INET An implementation of the TCP/IP protocol suite for the LINUX + * operating system. INET is implemented using the BSD Socket + * interface as the means of communication with the user level. + * + * Implementation of the Transmission Control Protocol(TCP). + * + * Authors: Ross Biro + * Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG> + * Mark Evans, <evansmp@uhura.aston.ac.uk> + * Corey Minyard <wf-rch!minyard@relay.EU.net> + * Florian La Roche, <flla@stud.uni-sb.de> + * Charles Hedrick, <hedrick@klinzhai.rutgers.edu> + * Linus Torvalds, <torvalds@cs.helsinki.fi> + * Alan Cox, <gw4pts@gw4pts.ampr.org> + * Matthew Dillon, <dillon@apollo.west.oic.com> + * Arnt Gulbrandsen, <agulbra@nvg.unit.no> + * Jorge Cwik, <jorge@laser.satlink.net> + */ + +#include <linux/mm.h> +#include <linux/module.h> +#include <linux/slab.h> +#include <linux/sysctl.h> +#include <linux/workqueue.h> +#include <net/tcp.h> +#include <net/inet_common.h> +#include <net/xfrm.h> + +int sysctl_tcp_syncookies __read_mostly = 1; +EXPORT_SYMBOL(sysctl_tcp_syncookies); + +int sysctl_tcp_abort_on_overflow __read_mostly; + +struct inet_timewait_death_row tcp_death_row = { + .sysctl_max_tw_buckets = NR_FILE * 2, + .period = TCP_TIMEWAIT_LEN / INET_TWDR_TWKILL_SLOTS, + .death_lock = __SPIN_LOCK_UNLOCKED(tcp_death_row.death_lock), + .hashinfo = &tcp_hashinfo, + .tw_timer = TIMER_INITIALIZER(inet_twdr_hangman, 0, + (unsigned long)&tcp_death_row), + .twkill_work = __WORK_INITIALIZER(tcp_death_row.twkill_work, + inet_twdr_twkill_work), +/* Short-time timewait calendar */ + + .twcal_hand = -1, + .twcal_timer = TIMER_INITIALIZER(inet_twdr_twcal_tick, 0, + (unsigned long)&tcp_death_row), +}; +EXPORT_SYMBOL_GPL(tcp_death_row); + +/* VJ's idea. Save last timestamp seen from this destination + * and hold it at least for normal timewait interval to use for duplicate + * segment detection in subsequent connections, before they enter synchronized + * state. + */ + +static int tcp_remember_stamp(struct sock *sk) +{ + const struct inet_connection_sock *icsk = inet_csk(sk); + struct tcp_sock *tp = tcp_sk(sk); + struct inet_peer *peer; + bool release_it; + + peer = icsk->icsk_af_ops->get_peer(sk, &release_it); + if (peer) { + if ((s32)(peer->tcp_ts - tp->rx_opt.ts_recent) <= 0 || + ((u32)get_seconds() - peer->tcp_ts_stamp > TCP_PAWS_MSL && + peer->tcp_ts_stamp <= (u32)tp->rx_opt.ts_recent_stamp)) { + peer->tcp_ts_stamp = (u32)tp->rx_opt.ts_recent_stamp; + peer->tcp_ts = tp->rx_opt.ts_recent; + } + if (release_it) + inet_putpeer(peer); + return 1; + } + + return 0; +} + +static int tcp_tw_remember_stamp(struct inet_timewait_sock *tw) +{ + struct sock *sk = (struct sock *) tw; + struct inet_peer *peer; + + peer = twsk_getpeer(sk); + if (peer) { + const struct tcp_timewait_sock *tcptw = tcp_twsk(sk); + + if ((s32)(peer->tcp_ts - tcptw->tw_ts_recent) <= 0 || + ((u32)get_seconds() - peer->tcp_ts_stamp > TCP_PAWS_MSL && + peer->tcp_ts_stamp <= (u32)tcptw->tw_ts_recent_stamp)) { + peer->tcp_ts_stamp = (u32)tcptw->tw_ts_recent_stamp; + peer->tcp_ts = tcptw->tw_ts_recent; + } + inet_putpeer(peer); + return 1; + } + return 0; +} + +static __inline__ int tcp_in_window(u32 seq, u32 end_seq, u32 s_win, u32 e_win) +{ + if (seq == s_win) + return 1; + if (after(end_seq, s_win) && before(seq, e_win)) + return 1; + return seq == e_win && seq == end_seq; +} + +/* + * * Main purpose of TIME-WAIT state is to close connection gracefully, + * when one of ends sits in LAST-ACK or CLOSING retransmitting FIN + * (and, probably, tail of data) and one or more our ACKs are lost. + * * What is TIME-WAIT timeout? It is associated with maximal packet + * lifetime in the internet, which results in wrong conclusion, that + * it is set to catch "old duplicate segments" wandering out of their path. + * It is not quite correct. This timeout is calculated so that it exceeds + * maximal retransmission timeout enough to allow to lose one (or more) + * segments sent by peer and our ACKs. This time may be calculated from RTO. + * * When TIME-WAIT socket receives RST, it means that another end + * finally closed and we are allowed to kill TIME-WAIT too. + * * Second purpose of TIME-WAIT is catching old duplicate segments. + * Well, certainly it is pure paranoia, but if we load TIME-WAIT + * with this semantics, we MUST NOT kill TIME-WAIT state with RSTs. + * * If we invented some more clever way to catch duplicates + * (f.e. based on PAWS), we could truncate TIME-WAIT to several RTOs. + * + * The algorithm below is based on FORMAL INTERPRETATION of RFCs. + * When you compare it to RFCs, please, read section SEGMENT ARRIVES + * from the very beginning. + * + * NOTE. With recycling (and later with fin-wait-2) TW bucket + * is _not_ stateless. It means, that strictly speaking we must + * spinlock it. I do not want! Well, probability of misbehaviour + * is ridiculously low and, seems, we could use some mb() tricks + * to avoid misread sequence numbers, states etc. --ANK + */ +enum tcp_tw_status +tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb, + const struct tcphdr *th) +{ + struct tcp_options_received tmp_opt; + const u8 *hash_location; + struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); + int paws_reject = 0; + + tmp_opt.saw_tstamp = 0; + if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) { + tcp_parse_options(skb, &tmp_opt, &hash_location, 0); + + if (tmp_opt.saw_tstamp) { + tmp_opt.ts_recent = tcptw->tw_ts_recent; + tmp_opt.ts_recent_stamp = tcptw->tw_ts_recent_stamp; + paws_reject = tcp_paws_reject(&tmp_opt, th->rst); + } + } + + if (tw->tw_substate == TCP_FIN_WAIT2) { + /* Just repeat all the checks of tcp_rcv_state_process() */ + + /* Out of window, send ACK */ + if (paws_reject || + !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq, + tcptw->tw_rcv_nxt, + tcptw->tw_rcv_nxt + tcptw->tw_rcv_wnd)) + return TCP_TW_ACK; + + if (th->rst) + goto kill; + + if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt)) + goto kill_with_rst; + + /* Dup ACK? */ + if (!th->ack || + !after(TCP_SKB_CB(skb)->end_seq, tcptw->tw_rcv_nxt) || + TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq) { + inet_twsk_put(tw); + return TCP_TW_SUCCESS; + } + + /* New data or FIN. If new data arrive after half-duplex close, + * reset. + */ + if (!th->fin || + TCP_SKB_CB(skb)->end_seq != tcptw->tw_rcv_nxt + 1) { +kill_with_rst: + inet_twsk_deschedule(tw, &tcp_death_row); + inet_twsk_put(tw); + return TCP_TW_RST; + } + + /* FIN arrived, enter true time-wait state. */ + tw->tw_substate = TCP_TIME_WAIT; + tcptw->tw_rcv_nxt = TCP_SKB_CB(skb)->end_seq; + if (tmp_opt.saw_tstamp) { + tcptw->tw_ts_recent_stamp = get_seconds(); + tcptw->tw_ts_recent = tmp_opt.rcv_tsval; + } + + if (tcp_death_row.sysctl_tw_recycle && + tcptw->tw_ts_recent_stamp && + tcp_tw_remember_stamp(tw)) + inet_twsk_schedule(tw, &tcp_death_row, tw->tw_timeout, + TCP_TIMEWAIT_LEN); + else + inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, + TCP_TIMEWAIT_LEN); + return TCP_TW_ACK; + } + + /* + * Now real TIME-WAIT state. + * + * RFC 1122: + * "When a connection is [...] on TIME-WAIT state [...] + * [a TCP] MAY accept a new SYN from the remote TCP to + * reopen the connection directly, if it: + * + * (1) assigns its initial sequence number for the new + * connection to be larger than the largest sequence + * number it used on the previous connection incarnation, + * and + * + * (2) returns to TIME-WAIT state if the SYN turns out + * to be an old duplicate". + */ + + if (!paws_reject && + (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt && + (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) { + /* In window segment, it may be only reset or bare ack. */ + + if (th->rst) { + /* This is TIME_WAIT assassination, in two flavors. + * Oh well... nobody has a sufficient solution to this + * protocol bug yet. + */ + if (sysctl_tcp_rfc1337 == 0) { +kill: + inet_twsk_deschedule(tw, &tcp_death_row); + inet_twsk_put(tw); + return TCP_TW_SUCCESS; + } + } + inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, + TCP_TIMEWAIT_LEN); + + if (tmp_opt.saw_tstamp) { + tcptw->tw_ts_recent = tmp_opt.rcv_tsval; + tcptw->tw_ts_recent_stamp = get_seconds(); + } + + inet_twsk_put(tw); + return TCP_TW_SUCCESS; + } + + /* Out of window segment. + + All the segments are ACKed immediately. + + The only exception is new SYN. We accept it, if it is + not old duplicate and we are not in danger to be killed + by delayed old duplicates. RFC check is that it has + newer sequence number works at rates <40Mbit/sec. + However, if paws works, it is reliable AND even more, + we even may relax silly seq space cutoff. + + RED-PEN: we violate main RFC requirement, if this SYN will appear + old duplicate (i.e. we receive RST in reply to SYN-ACK), + we must return socket to time-wait state. It is not good, + but not fatal yet. + */ + + if (th->syn && !th->rst && !th->ack && !paws_reject && + (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) || + (tmp_opt.saw_tstamp && + (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) { + u32 isn = tcptw->tw_snd_nxt + 65535 + 2; + if (isn == 0) + isn++; + TCP_SKB_CB(skb)->when = isn; + return TCP_TW_SYN; + } + + if (paws_reject) + NET_INC_STATS_BH(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED); + + if (!th->rst) { + /* In this case we must reset the TIMEWAIT timer. + * + * If it is ACKless SYN it may be both old duplicate + * and new good SYN with random sequence number <rcv_nxt. + * Do not reschedule in the last case. + */ + if (paws_reject || th->ack) + inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, + TCP_TIMEWAIT_LEN); + + /* Send ACK. Note, we do not put the bucket, + * it will be released by caller. + */ + return TCP_TW_ACK; + } + inet_twsk_put(tw); + return TCP_TW_SUCCESS; +} +EXPORT_SYMBOL(tcp_timewait_state_process); + +/* + * Move a socket to time-wait or dead fin-wait-2 state. + */ +void tcp_time_wait(struct sock *sk, int state, int timeo) +{ + struct inet_timewait_sock *tw = NULL; + const struct inet_connection_sock *icsk = inet_csk(sk); + const struct tcp_sock *tp = tcp_sk(sk); + int recycle_ok = 0; + + if (tcp_death_row.sysctl_tw_recycle && tp->rx_opt.ts_recent_stamp) + recycle_ok = tcp_remember_stamp(sk); + + if (tcp_death_row.tw_count < tcp_death_row.sysctl_max_tw_buckets) + tw = inet_twsk_alloc(sk, state); + + if (tw != NULL) { + struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); + const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1); + + tw->tw_transparent = inet_sk(sk)->transparent; + tw->tw_rcv_wscale = tp->rx_opt.rcv_wscale; + tcptw->tw_rcv_nxt = tp->rcv_nxt; + tcptw->tw_snd_nxt = tp->snd_nxt; + tcptw->tw_rcv_wnd = tcp_receive_window(tp); + tcptw->tw_ts_recent = tp->rx_opt.ts_recent; + tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp; + +#if IS_ENABLED(CONFIG_IPV6) + if (tw->tw_family == PF_INET6) { + struct ipv6_pinfo *np = inet6_sk(sk); + struct inet6_timewait_sock *tw6; + + tw->tw_ipv6_offset = inet6_tw_offset(sk->sk_prot); + tw6 = inet6_twsk((struct sock *)tw); + tw6->tw_v6_daddr = np->daddr; + tw6->tw_v6_rcv_saddr = np->rcv_saddr; + tw->tw_tclass = np->tclass; + tw->tw_ipv6only = np->ipv6only; + } +#endif + +#ifdef CONFIG_TCP_MD5SIG + /* + * The timewait bucket does not have the key DB from the + * sock structure. We just make a quick copy of the + * md5 key being used (if indeed we are using one) + * so the timewait ack generating code has the key. + */ + do { + struct tcp_md5sig_key *key; + tcptw->tw_md5_key = NULL; + key = tp->af_specific->md5_lookup(sk, sk); + if (key != NULL) { + tcptw->tw_md5_key = kmemdup(key, sizeof(*key), GFP_ATOMIC); + if (tcptw->tw_md5_key && tcp_alloc_md5sig_pool(sk) == NULL) + BUG(); + } + } while (0); +#endif + + /* Linkage updates. */ + __inet_twsk_hashdance(tw, sk, &tcp_hashinfo); + + /* Get the TIME_WAIT timeout firing. */ + if (timeo < rto) + timeo = rto; + + if (recycle_ok) { + tw->tw_timeout = rto; + } else { + tw->tw_timeout = TCP_TIMEWAIT_LEN; + if (state == TCP_TIME_WAIT) + timeo = TCP_TIMEWAIT_LEN; + } + + inet_twsk_schedule(tw, &tcp_death_row, timeo, + TCP_TIMEWAIT_LEN); + inet_twsk_put(tw); + } else { + /* Sorry, if we're out of memory, just CLOSE this + * socket up. We've got bigger problems than + * non-graceful socket closings. + */ + NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPTIMEWAITOVERFLOW); + } + + tcp_update_metrics(sk); + tcp_done(sk); +} + +void tcp_twsk_destructor(struct sock *sk) +{ +#ifdef CONFIG_TCP_MD5SIG + struct tcp_timewait_sock *twsk = tcp_twsk(sk); + if (twsk->tw_md5_key) { + tcp_free_md5sig_pool(); + kfree_rcu(twsk->tw_md5_key, rcu); + } +#endif +} +EXPORT_SYMBOL_GPL(tcp_twsk_destructor); + +static inline void TCP_ECN_openreq_child(struct tcp_sock *tp, + struct request_sock *req) +{ + tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0; +} + +/* This is not only more efficient than what we used to do, it eliminates + * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM + * + * Actually, we could lots of memory writes here. tp of listening + * socket contains all necessary default parameters. + */ +struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req, struct sk_buff *skb) +{ + struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC); + + if (newsk != NULL) { + const struct inet_request_sock *ireq = inet_rsk(req); + struct tcp_request_sock *treq = tcp_rsk(req); + struct inet_connection_sock *newicsk = inet_csk(newsk); + struct tcp_sock *newtp = tcp_sk(newsk); + struct tcp_sock *oldtp = tcp_sk(sk); + struct tcp_cookie_values *oldcvp = oldtp->cookie_values; + + /* TCP Cookie Transactions require space for the cookie pair, + * as it differs for each connection. There is no need to + * copy any s_data_payload stored at the original socket. + * Failure will prevent resuming the connection. + * + * Presumed copied, in order of appearance: + * cookie_in_always, cookie_out_never + */ + if (oldcvp != NULL) { + struct tcp_cookie_values *newcvp = + kzalloc(sizeof(*newtp->cookie_values), + GFP_ATOMIC); + + if (newcvp != NULL) { + kref_init(&newcvp->kref); + newcvp->cookie_desired = + oldcvp->cookie_desired; + newtp->cookie_values = newcvp; + } else { + /* Not Yet Implemented */ + newtp->cookie_values = NULL; + } + } + + /* Now setup tcp_sock */ + newtp->pred_flags = 0; + + newtp->rcv_wup = newtp->copied_seq = + newtp->rcv_nxt = treq->rcv_isn + 1; + + newtp->snd_sml = newtp->snd_una = + newtp->snd_nxt = newtp->snd_up = + treq->snt_isn + 1 + tcp_s_data_size(oldtp); + + tcp_prequeue_init(newtp); + + tcp_init_wl(newtp, treq->rcv_isn); + + newtp->srtt = 0; + newtp->mdev = TCP_TIMEOUT_INIT; + newicsk->icsk_rto = TCP_TIMEOUT_INIT; + + newtp->packets_out = 0; + newtp->retrans_out = 0; + newtp->sacked_out = 0; + newtp->fackets_out = 0; + newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH; + + /* So many TCP implementations out there (incorrectly) count the + * initial SYN frame in their delayed-ACK and congestion control + * algorithms that we must have the following bandaid to talk + * efficiently to them. -DaveM + */ + newtp->snd_cwnd = TCP_INIT_CWND; + newtp->snd_cwnd_cnt = 0; + newtp->bytes_acked = 0; + + newtp->frto_counter = 0; + newtp->frto_highmark = 0; + + if (newicsk->icsk_ca_ops != &tcp_init_congestion_ops && + !try_module_get(newicsk->icsk_ca_ops->owner)) + newicsk->icsk_ca_ops = &tcp_init_congestion_ops; + + tcp_set_ca_state(newsk, TCP_CA_Open); + tcp_init_xmit_timers(newsk); + skb_queue_head_init(&newtp->out_of_order_queue); + newtp->write_seq = newtp->pushed_seq = + treq->snt_isn + 1 + tcp_s_data_size(oldtp); + + newtp->rx_opt.saw_tstamp = 0; + + newtp->rx_opt.dsack = 0; + newtp->rx_opt.num_sacks = 0; + + newtp->urg_data = 0; + + if (sock_flag(newsk, SOCK_KEEPOPEN)) + inet_csk_reset_keepalive_timer(newsk, + keepalive_time_when(newtp)); + + newtp->rx_opt.tstamp_ok = ireq->tstamp_ok; + if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) { + if (sysctl_tcp_fack) + tcp_enable_fack(newtp); + } + newtp->window_clamp = req->window_clamp; + newtp->rcv_ssthresh = req->rcv_wnd; + newtp->rcv_wnd = req->rcv_wnd; + newtp->rx_opt.wscale_ok = ireq->wscale_ok; + if (newtp->rx_opt.wscale_ok) { + newtp->rx_opt.snd_wscale = ireq->snd_wscale; + newtp->rx_opt.rcv_wscale = ireq->rcv_wscale; + } else { + newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0; + newtp->window_clamp = min(newtp->window_clamp, 65535U); + } + newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) << + newtp->rx_opt.snd_wscale); + newtp->max_window = newtp->snd_wnd; + + if (newtp->rx_opt.tstamp_ok) { + newtp->rx_opt.ts_recent = req->ts_recent; + newtp->rx_opt.ts_recent_stamp = get_seconds(); + newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED; + } else { + newtp->rx_opt.ts_recent_stamp = 0; + newtp->tcp_header_len = sizeof(struct tcphdr); + } +#ifdef CONFIG_TCP_MD5SIG + newtp->md5sig_info = NULL; /*XXX*/ + if (newtp->af_specific->md5_lookup(sk, newsk)) + newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED; +#endif + if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len) + newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len; + newtp->rx_opt.mss_clamp = req->mss; + TCP_ECN_openreq_child(newtp, req); + + TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_PASSIVEOPENS); + } + return newsk; +} +EXPORT_SYMBOL(tcp_create_openreq_child); + +/* + * Process an incoming packet for SYN_RECV sockets represented + * as a request_sock. + */ + +struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb, + struct request_sock *req, + struct request_sock **prev) +{ + struct tcp_options_received tmp_opt; + const u8 *hash_location; + struct sock *child; + const struct tcphdr *th = tcp_hdr(skb); + __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK); + int paws_reject = 0; + + tmp_opt.saw_tstamp = 0; + if (th->doff > (sizeof(struct tcphdr)>>2)) { + tcp_parse_options(skb, &tmp_opt, &hash_location, 0); + + if (tmp_opt.saw_tstamp) { + tmp_opt.ts_recent = req->ts_recent; + /* We do not store true stamp, but it is not required, + * it can be estimated (approximately) + * from another data. + */ + tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->retrans); + paws_reject = tcp_paws_reject(&tmp_opt, th->rst); + } + } + + /* Check for pure retransmitted SYN. */ + if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn && + flg == TCP_FLAG_SYN && + !paws_reject) { + /* + * RFC793 draws (Incorrectly! It was fixed in RFC1122) + * this case on figure 6 and figure 8, but formal + * protocol description says NOTHING. + * To be more exact, it says that we should send ACK, + * because this segment (at least, if it has no data) + * is out of window. + * + * CONCLUSION: RFC793 (even with RFC1122) DOES NOT + * describe SYN-RECV state. All the description + * is wrong, we cannot believe to it and should + * rely only on common sense and implementation + * experience. + * + * Enforce "SYN-ACK" according to figure 8, figure 6 + * of RFC793, fixed by RFC1122. + */ + req->rsk_ops->rtx_syn_ack(sk, req, NULL); + return NULL; + } + + /* Further reproduces section "SEGMENT ARRIVES" + for state SYN-RECEIVED of RFC793. + It is broken, however, it does not work only + when SYNs are crossed. + + You would think that SYN crossing is impossible here, since + we should have a SYN_SENT socket (from connect()) on our end, + but this is not true if the crossed SYNs were sent to both + ends by a malicious third party. We must defend against this, + and to do that we first verify the ACK (as per RFC793, page + 36) and reset if it is invalid. Is this a true full defense? + To convince ourselves, let us consider a way in which the ACK + test can still pass in this 'malicious crossed SYNs' case. + Malicious sender sends identical SYNs (and thus identical sequence + numbers) to both A and B: + + A: gets SYN, seq=7 + B: gets SYN, seq=7 + + By our good fortune, both A and B select the same initial + send sequence number of seven :-) + + A: sends SYN|ACK, seq=7, ack_seq=8 + B: sends SYN|ACK, seq=7, ack_seq=8 + + So we are now A eating this SYN|ACK, ACK test passes. So + does sequence test, SYN is truncated, and thus we consider + it a bare ACK. + + If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this + bare ACK. Otherwise, we create an established connection. Both + ends (listening sockets) accept the new incoming connection and try + to talk to each other. 8-) + + Note: This case is both harmless, and rare. Possibility is about the + same as us discovering intelligent life on another plant tomorrow. + + But generally, we should (RFC lies!) to accept ACK + from SYNACK both here and in tcp_rcv_state_process(). + tcp_rcv_state_process() does not, hence, we do not too. + + Note that the case is absolutely generic: + we cannot optimize anything here without + violating protocol. All the checks must be made + before attempt to create socket. + */ + + /* RFC793 page 36: "If the connection is in any non-synchronized state ... + * and the incoming segment acknowledges something not yet + * sent (the segment carries an unacceptable ACK) ... + * a reset is sent." + * + * Invalid ACK: reset will be sent by listening socket + */ + if ((flg & TCP_FLAG_ACK) && + (TCP_SKB_CB(skb)->ack_seq != + tcp_rsk(req)->snt_isn + 1 + tcp_s_data_size(tcp_sk(sk)))) + return sk; + + /* Also, it would be not so bad idea to check rcv_tsecr, which + * is essentially ACK extension and too early or too late values + * should cause reset in unsynchronized states. + */ + + /* RFC793: "first check sequence number". */ + + if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq, + tcp_rsk(req)->rcv_isn + 1, tcp_rsk(req)->rcv_isn + 1 + req->rcv_wnd)) { + /* Out of window: send ACK and drop. */ + if (!(flg & TCP_FLAG_RST)) + req->rsk_ops->send_ack(sk, skb, req); + if (paws_reject) + NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED); + return NULL; + } + + /* In sequence, PAWS is OK. */ + + if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_isn + 1)) + req->ts_recent = tmp_opt.rcv_tsval; + + if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) { + /* Truncate SYN, it is out of window starting + at tcp_rsk(req)->rcv_isn + 1. */ + flg &= ~TCP_FLAG_SYN; + } + + /* RFC793: "second check the RST bit" and + * "fourth, check the SYN bit" + */ + if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) { + TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_ATTEMPTFAILS); + goto embryonic_reset; + } + + /* ACK sequence verified above, just make sure ACK is + * set. If ACK not set, just silently drop the packet. + */ + if (!(flg & TCP_FLAG_ACK)) + return NULL; + + /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */ + if (req->retrans < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept && + TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) { + inet_rsk(req)->acked = 1; + NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP); + return NULL; + } + if (tmp_opt.saw_tstamp && tmp_opt.rcv_tsecr) + tcp_rsk(req)->snt_synack = tmp_opt.rcv_tsecr; + else if (req->retrans) /* don't take RTT sample if retrans && ~TS */ + tcp_rsk(req)->snt_synack = 0; + + /* OK, ACK is valid, create big socket and + * feed this segment to it. It will repeat all + * the tests. THIS SEGMENT MUST MOVE SOCKET TO + * ESTABLISHED STATE. If it will be dropped after + * socket is created, wait for troubles. + */ + child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL); + if (child == NULL) + goto listen_overflow; + + inet_csk_reqsk_queue_unlink(sk, req, prev); + inet_csk_reqsk_queue_removed(sk, req); + + inet_csk_reqsk_queue_add(sk, req, child); + return child; + +listen_overflow: + if (!sysctl_tcp_abort_on_overflow) { + inet_rsk(req)->acked = 1; + return NULL; + } + +embryonic_reset: + NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_EMBRYONICRSTS); + if (!(flg & TCP_FLAG_RST)) + req->rsk_ops->send_reset(sk, skb); + + inet_csk_reqsk_queue_drop(sk, req, prev); + return NULL; +} +EXPORT_SYMBOL(tcp_check_req); + +/* + * Queue segment on the new socket if the new socket is active, + * otherwise we just shortcircuit this and continue with + * the new socket. + */ + +int tcp_child_process(struct sock *parent, struct sock *child, + struct sk_buff *skb) +{ + int ret = 0; + int state = child->sk_state; + + if (!sock_owned_by_user(child)) { + ret = tcp_rcv_state_process(child, skb, tcp_hdr(skb), + skb->len); + /* Wakeup parent, send SIGIO */ + if (state == TCP_SYN_RECV && child->sk_state != state) + parent->sk_data_ready(parent, 0); + } else { + /* Alas, it is possible again, because we do lookup + * in main socket hash table and lock on listening + * socket does not protect us more. + */ + __sk_add_backlog(child, skb); + } + + bh_unlock_sock(child); + sock_put(child); + return ret; +} +EXPORT_SYMBOL(tcp_child_process); |