* Copyright (c) 2010-2012 The NetBSD Foundation, Inc.

/*-
* Copyright (c) 2010-2012 The NetBSD Foundation, Inc.
* All rights reserved.
*
* This material is based upon work partially supported by The
* NetBSD Foundation under a contract with Mindaugas Rasiukevicius.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
* ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
* TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
* BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
*/

/*
* NPF TCP state engine for connection tracking.
*/

#ifdef _KERNEL
#include <sys/cdefs.h>
__KERNEL_RCSID(0, "$NetBSD: npf_state_tcp.c,v 1.21 2020/05/30 14:16:56 rmind Exp $");

#include <sys/param.h>
#include <sys/types.h>

#include <netinet/in.h>
#include <netinet/tcp.h>
#endif

#include "npf_impl.h"

/*
* NPF TCP states. Note: these states are different from the TCP FSM
* states of RFC 793. The packet filter is a man-in-the-middle.
*/
#define NPF_TCPS_OK 255
#define NPF_TCPS_CLOSED 0
#define NPF_TCPS_SYN_SENT 1
#define NPF_TCPS_SIMSYN_SENT 2
#define NPF_TCPS_SYN_RECEIVED 3
#define NPF_TCPS_ESTABLISHED 4
#define NPF_TCPS_FIN_SENT 5
#define NPF_TCPS_FIN_RECEIVED 6
#define NPF_TCPS_CLOSE_WAIT 7
#define NPF_TCPS_FIN_WAIT 8
#define NPF_TCPS_CLOSING 9
#define NPF_TCPS_LAST_ACK 10
#define NPF_TCPS_TIME_WAIT 11

#define NPF_TCP_NSTATES 12

/* Timeouts */
#define NPF_TCPT_NEW 0
#define NPF_TCPT_ESTABLISHED 1
#define NPF_TCPT_HALFCLOSE 2
#define NPF_TCPT_CLOSE 3
#define NPF_TCPT_TIMEWAIT 4
#define NPF_TCPT_COUNT 5

/*
* Parameters.
*/
typedef struct {
int max_ack_win;
int strict_order_rst;
int timeouts[NPF_TCPT_COUNT];
} npf_state_tcp_params_t;

/*
* Helpers.
*/
#define SEQ_LT(a,b) ((int)((a)-(b)) < 0)
#define SEQ_LEQ(a,b) ((int)((a)-(b)) <= 0)
#define SEQ_GT(a,b) ((int)((a)-(b)) > 0)
#define SEQ_GEQ(a,b) ((int)((a)-(b)) >= 0)

/*
* List of TCP flag cases and conversion of flags to a case (index).
*/

#define TCPFC_INVALID 0
#define TCPFC_SYN 1
#define TCPFC_SYNACK 2
#define TCPFC_ACK 3
#define TCPFC_FIN 4
#define TCPFC_COUNT 5

static inline unsigned
npf_tcpfl2case(const unsigned tcpfl)
{
unsigned i, c;

CTASSERT(TH_FIN == 0x01);
CTASSERT(TH_SYN == 0x02);
CTASSERT(TH_ACK == 0x10);

/*
* Flags are shifted to use three least significant bits, thus each
* flag combination has a unique number ranging from 0 to 7, e.g.
* TH_SYN | TH_ACK has number 6, since (0x02 | (0x10 >> 2)) == 6.
* However, the requirement is to have number 0 for invalid cases,
* such as TH_SYN | TH_FIN, and to have the same number for TH_FIN
* and TH_FIN|TH_ACK cases. Thus, we generate a mask assigning 3
* bits for each number, which contains the actual case numbers:
*
* TCPFC_SYNACK << (6 << 2) == 0x2000000 (6 - SYN,ACK)
* TCPFC_FIN << (5 << 2) == 0x0400000 (5 - FIN,ACK)
* ...
*
* Hence, OR'ed mask value is 0x2430140.
*/
i = (tcpfl & (TH_SYN | TH_FIN)) | ((tcpfl & TH_ACK) >> 2);
c = (0x2430140 >> (i << 2)) & 7;

KASSERT(c < TCPFC_COUNT);
return c;
}

/*
* NPF transition table of a tracked TCP connection.
*
* There is a single state, which is changed in the following way:
*
* new_state = npf_tcp_fsm[old_state][direction][npf_tcpfl2case(tcp_flags)];
*
* Note that this state is different from the state in each end (host).
*/

static const uint8_t npf_tcp_fsm[NPF_TCP_NSTATES][2][TCPFC_COUNT] = {
[NPF_TCPS_CLOSED] = {
[NPF_FLOW_FORW] = {
/* Handshake (1): initial SYN. */
[TCPFC_SYN] = NPF_TCPS_SYN_SENT,
},
},
[NPF_TCPS_SYN_SENT] = {
[NPF_FLOW_FORW] = {
/* SYN may be retransmitted. */
[TCPFC_SYN] = NPF_TCPS_OK,
},
[NPF_FLOW_BACK] = {
/* Handshake (2): SYN-ACK is expected. */
[TCPFC_SYNACK] = NPF_TCPS_SYN_RECEIVED,
/* Simultaneous initiation - SYN. */
[TCPFC_SYN] = NPF_TCPS_SIMSYN_SENT,
},
},
[NPF_TCPS_SIMSYN_SENT] = {
[NPF_FLOW_FORW] = {
/* Original SYN re-transmission. */
[TCPFC_SYN] = NPF_TCPS_OK,
/* SYN-ACK response to simultaneous SYN. */
[TCPFC_SYNACK] = NPF_TCPS_SYN_RECEIVED,
},
[NPF_FLOW_BACK] = {
/* Simultaneous SYN re-transmission.*/
[TCPFC_SYN] = NPF_TCPS_OK,
/* SYN-ACK response to original SYN. */
[TCPFC_SYNACK] = NPF_TCPS_SYN_RECEIVED,
/* FIN may occur early. */
[TCPFC_FIN] = NPF_TCPS_FIN_RECEIVED,
},
},
[NPF_TCPS_SYN_RECEIVED] = {
[NPF_FLOW_FORW] = {
/* Handshake (3): ACK is expected. */
[TCPFC_ACK] = NPF_TCPS_ESTABLISHED,
/* FIN may be sent early. */
[TCPFC_FIN] = NPF_TCPS_FIN_SENT,
/* Late SYN re-transmission. */
[TCPFC_SYN] = NPF_TCPS_OK,
},
[NPF_FLOW_BACK] = {
/* SYN-ACK may be retransmitted. */
[TCPFC_SYNACK] = NPF_TCPS_OK,
/* XXX: ACK of late SYN in simultaneous case? */
[TCPFC_ACK] = NPF_TCPS_OK,
/* FIN may occur early. */
[TCPFC_FIN] = NPF_TCPS_FIN_RECEIVED,
},
},
[NPF_TCPS_ESTABLISHED] = {
/*
* Regular ACKs (data exchange) or FIN.
* FIN packets may have ACK set.
*/
[NPF_FLOW_FORW] = {
[TCPFC_ACK] = NPF_TCPS_OK,
/* FIN by the sender. */
[TCPFC_FIN] = NPF_TCPS_FIN_SENT,
},
[NPF_FLOW_BACK] = {
[TCPFC_ACK] = NPF_TCPS_OK,
/* FIN by the receiver. */
[TCPFC_FIN] = NPF_TCPS_FIN_RECEIVED,
},
},
[NPF_TCPS_FIN_SENT] = {
[NPF_FLOW_FORW] = {
/* FIN may be re-transmitted. Late ACK as well. */
[TCPFC_ACK] = NPF_TCPS_OK,
[TCPFC_FIN] = NPF_TCPS_OK,
},
[NPF_FLOW_BACK] = {
/* If ACK, connection is half-closed now. */
[TCPFC_ACK] = NPF_TCPS_FIN_WAIT,
/* FIN or FIN-ACK race - immediate closing. */
[TCPFC_FIN] = NPF_TCPS_CLOSING,
},
},
[NPF_TCPS_FIN_RECEIVED] = {
/*
* FIN was received. Equivalent scenario to sent FIN.
*/
[NPF_FLOW_FORW] = {
[TCPFC_ACK] = NPF_TCPS_CLOSE_WAIT,
[TCPFC_FIN] = NPF_TCPS_CLOSING,
},
[NPF_FLOW_BACK] = {
[TCPFC_ACK] = NPF_TCPS_OK,
[TCPFC_FIN] = NPF_TCPS_OK,
},
},
[NPF_TCPS_CLOSE_WAIT] = {
/* Sender has sent the FIN and closed its end. */
[NPF_FLOW_FORW] = {
[TCPFC_ACK] = NPF_TCPS_OK,
[TCPFC_FIN] = NPF_TCPS_LAST_ACK,
},
[NPF_FLOW_BACK] = {
[TCPFC_ACK] = NPF_TCPS_OK,
[TCPFC_FIN] = NPF_TCPS_LAST_ACK,
},
},
[NPF_TCPS_FIN_WAIT] = {
/* Receiver has closed its end. */
[NPF_FLOW_FORW] = {
[TCPFC_ACK] = NPF_TCPS_OK,
[TCPFC_FIN] = NPF_TCPS_LAST_ACK,
},
[NPF_FLOW_BACK] = {
[TCPFC_ACK] = NPF_TCPS_OK,
[TCPFC_FIN] = NPF_TCPS_LAST_ACK,
},
},
[NPF_TCPS_CLOSING] = {
/* Race of FINs - expecting ACK. */
[NPF_FLOW_FORW] = {
[TCPFC_ACK] = NPF_TCPS_LAST_ACK,
},
[NPF_FLOW_BACK] = {
[TCPFC_ACK] = NPF_TCPS_LAST_ACK,
},
},
[NPF_TCPS_LAST_ACK] = {
/* FINs exchanged - expecting last ACK. */
[NPF_FLOW_FORW] = {
[TCPFC_ACK] = NPF_TCPS_TIME_WAIT,
},
[NPF_FLOW_BACK] = {
[TCPFC_ACK] = NPF_TCPS_TIME_WAIT,
},
},
[NPF_TCPS_TIME_WAIT] = {
/* May re-open the connection as per RFC 1122. */
[NPF_FLOW_FORW] = {
[TCPFC_SYN] = NPF_TCPS_SYN_SENT,
},
},
};

/*
* npf_tcp_inwindow: determine whether the packet is in the TCP window
* and thus part of the connection we are tracking.
*/
static bool
npf_tcp_inwindow(npf_cache_t *npc, npf_state_t *nst, const npf_flow_t flow)
{
const npf_state_tcp_params_t *params;
const struct tcphdr * const th = npc->npc_l4.tcp;
const int tcpfl = th->th_flags;
npf_tcpstate_t *fstate, *tstate;
int tcpdlen, ackskew;
tcp_seq seq, ack, end;
uint32_t win;

params = npc->npc_ctx->params[NPF_PARAMS_TCP_STATE];
KASSERT(npf_iscached(npc, NPC_TCP));

/*
* Perform SEQ/ACK numbers check against boundaries. Reference:
*
* Rooij G., "Real stateful TCP packet filtering in IP Filter",
* 10th USENIX Security Symposium invited talk, Aug. 2001.
*
* There are four boundaries defined as following:
* I) SEQ + LEN <= MAX { SND.ACK + MAX(SND.WIN, 1) }
* II) SEQ >= MAX { SND.SEQ + SND.LEN - MAX(RCV.WIN, 1) }
* III) ACK <= MAX { RCV.SEQ + RCV.LEN }
* IV) ACK >= MAX { RCV.SEQ + RCV.LEN } - MAXACKWIN
*
* Let these members of npf_tcpstate_t be the maximum seen values of:
* nst_end - SEQ + LEN
* nst_maxend - ACK + MAX(WIN, 1)
* nst_maxwin - MAX(WIN, 1)
*/

tcpdlen = npf_tcpsaw(__UNCONST(npc), &seq, &ack, &win);
end = seq + tcpdlen;
if (tcpfl & TH_SYN) {
end++;
}
if (tcpfl & TH_FIN) {
end++;
}

fstate = &nst->nst_tcpst[flow];
tstate = &nst->nst_tcpst[!flow];
win = win ? (win << fstate->nst_wscale) : 1;

/*
* Initialise if the first packet.
* Note: only case when nst_maxwin is zero.
*/
if (__predict_false(fstate->nst_maxwin == 0)) {
/*
* Normally, it should be the first SYN or a re-transmission
* of SYN. The state of the other side will get set with a
* SYN-ACK reply (see below).
*/
fstate->nst_end = end;
fstate->nst_maxend = end;
fstate->nst_maxwin = win;
tstate->nst_end = 0;
tstate->nst_maxend = 0;
tstate->nst_maxwin = 1;

/*
* Handle TCP Window Scaling (RFC 1323). Both sides may
* send this option in their SYN packets.
*/
fstate->nst_wscale = 0;
(void)npf_fetch_tcpopts(npc, NULL, &fstate->nst_wscale);

tstate->nst_wscale = 0;

/* Done. */
return true;
}

if (fstate->nst_end == 0) {
/*
* Should be a SYN-ACK reply to SYN. If SYN is not set,
* then we are in the middle of connection and lost tracking.
*/
fstate->nst_end = end;
fstate->nst_maxend = end + 1;
fstate->nst_maxwin = win;
fstate->nst_wscale = 0;

/* Handle TCP Window Scaling (must be ignored if no SYN). */
if (tcpfl & TH_SYN) {
(void)npf_fetch_tcpopts(npc, NULL, &fstate->nst_wscale);
}
}

if ((tcpfl & TH_ACK) == 0) {
/* Pretend that an ACK was sent. */
ack = tstate->nst_end;
} else if ((tcpfl & (TH_ACK|TH_RST)) == (TH_ACK|TH_RST) && ack == 0) {
/* Workaround for some TCP stacks. */
ack = tstate->nst_end;
}

if (__predict_false(tcpfl & TH_RST)) {
/* RST to the initial SYN may have zero SEQ - fix it up. */
if (seq == 0 && nst->nst_state == NPF_TCPS_SYN_SENT) {
end = fstate->nst_end;
seq = end;
}

/* Strict in-order sequence for RST packets (RFC 5961). */
if (params->strict_order_rst && (fstate->nst_end - seq) > 1) {
return false;
}
}

/*
* Determine whether the data is within previously noted window,
* that is, upper boundary for valid data (I).
*/
if (!SEQ_LEQ(end, fstate->nst_maxend)) {
npf_stats_inc(npc->npc_ctx, NPF_STAT_INVALID_STATE_TCP1);
return false;
}

/* Lower boundary (II), which is no more than one window back. */
if (!SEQ_GEQ(seq, fstate->nst_end - tstate->nst_maxwin)) {
npf_stats_inc(npc->npc_ctx, NPF_STAT_INVALID_STATE_TCP2);
return false;
}

/*
* Boundaries for valid acknowledgments (III, IV) - one predicted
* window up or down, since packets may be fragmented.
*/
ackskew = tstate->nst_end - ack;
if (ackskew < -(int)params->max_ack_win ||
ackskew > ((int)params->max_ack_win << fstate->nst_wscale)) {
npf_stats_inc(npc->npc_ctx, NPF_STAT_INVALID_STATE_TCP3);
return false;
}

/*
* Packet has been passed.
*
* Negative ackskew might be due to fragmented packets. Since the
* total length of the packet is unknown - bump the boundary.
*/

if (ackskew < 0) {
tstate->nst_end = ack;
}
/* Keep track of the maximum window seen. */
if (fstate->nst_maxwin < win) {
fstate->nst_maxwin = win;
}
if (SEQ_GT(end, fstate->nst_end)) {
fstate->nst_end = end;
}
/* Note the window for upper boundary. */
if (SEQ_GEQ(ack + win, tstate->nst_maxend)) {
tstate->nst_maxend = ack + win;
}
return true;
}

/*
* npf_state_tcp: inspect TCP segment, determine whether it belongs to
* the connection and track its state.
*/
bool
npf_state_tcp(npf_cache_t *npc, npf_state_t *nst, npf_flow_t flow)
{
const struct tcphdr * const th = npc->npc_l4.tcp;
const unsigned tcpfl = th->th_flags, state = nst->nst_state;
unsigned nstate;

KASSERT(nst->nst_state < NPF_TCP_NSTATES);

/* Look for a transition to a new state. */
if (__predict_true((tcpfl & TH_RST) == 0)) {
const u_int flagcase = npf_tcpfl2case(tcpfl);
nstate = npf_tcp_fsm[state][flow][flagcase];
} else if (state == NPF_TCPS_TIME_WAIT) {
/* Prevent TIME-WAIT assassination (RFC 1337). */
nstate = NPF_TCPS_OK;
} else {
nstate = NPF_TCPS_CLOSED;
}

/* Determine whether TCP packet really belongs to this connection. */
if (!npf_tcp_inwindow(npc, nst, flow)) {
return false;
}
if (__predict_true(nstate == NPF_TCPS_OK)) {
return true;
}

nst->nst_state = nstate;
return true;
}

int
npf_state_tcp_timeout(npf_t *npf, const npf_state_t *nst)
{
static const uint8_t state_timeout_idx[NPF_TCP_NSTATES] = {
[NPF_TCPS_CLOSED] = NPF_TCPT_CLOSE,
/* Unsynchronised states. */
[NPF_TCPS_SYN_SENT] = NPF_TCPT_NEW,
[NPF_TCPS_SIMSYN_SENT] = NPF_TCPT_NEW,
[NPF_TCPS_SYN_RECEIVED] = NPF_TCPT_NEW,
/* Established (synchronised state). */
[NPF_TCPS_ESTABLISHED] = NPF_TCPT_ESTABLISHED,
/* Half-closed cases. */
[NPF_TCPS_FIN_SENT] = NPF_TCPT_HALFCLOSE,
[NPF_TCPS_FIN_RECEIVED] = NPF_TCPT_HALFCLOSE,
[NPF_TCPS_CLOSE_WAIT] = NPF_TCPT_HALFCLOSE,
[NPF_TCPS_FIN_WAIT] = NPF_TCPT_HALFCLOSE,
/* Full close cases. */
[NPF_TCPS_CLOSING] = NPF_TCPT_CLOSE,
[NPF_TCPS_LAST_ACK] = NPF_TCPT_CLOSE,
[NPF_TCPS_TIME_WAIT] = NPF_TCPT_TIMEWAIT,
};
const npf_state_tcp_params_t *params;
const unsigned state = nst->nst_state;

KASSERT(state < NPF_TCP_NSTATES);
params = npf->params[NPF_PARAMS_TCP_STATE];
return params->timeouts[state_timeout_idx[state]];
}

void
npf_state_tcp_sysinit(npf_t *npf)
{
npf_state_tcp_params_t *params = npf_param_allocgroup(npf,
NPF_PARAMS_TCP_STATE, sizeof(npf_state_tcp_params_t));
npf_param_t param_map[] = {
/*
* TCP connection timeout table (in seconds).
*/

/* Unsynchronised states. */
{
"state.tcp.timeout.new",
&params->timeouts[NPF_TCPT_NEW],
.default_val = 30,
.min = 0, .max = INT_MAX
},
/* Established. */
{
"state.tcp.timeout.established",
&params->timeouts[NPF_TCPT_ESTABLISHED],
.default_val = 60 * 60 * 24,
.min = 0, .max = INT_MAX
},
/* Half-closed cases. */
{
"state.tcp.timeout.half_close",
&params->timeouts[NPF_TCPT_HALFCLOSE],
.default_val = 60 * 60 * 6,
.min = 0, .max = INT_MAX
},
/* Full close cases. */
{
"state.tcp.timeout.close",
&params->timeouts[NPF_TCPT_CLOSE],
.default_val = 10,
.min = 0, .max = INT_MAX
},
/* TCP time-wait (2 * MSL). */
{
"state.tcp.timeout.time_wait",
&params->timeouts[NPF_TCPT_TIMEWAIT],
.default_val = 60 * 2 * 2,
.min = 0, .max = INT_MAX
},

/*
* Enforce strict order RST.
*/
{
"state.tcp.strict_order_rst",
&params->strict_order_rst,
.default_val = 1, // true
.min = 0, .max = 1
},

/*
* TCP state tracking: maximum allowed ACK window.
*/
{
"state.tcp.max_ack_win",
&params->max_ack_win,
.default_val = 66000,
.min = 0, .max = INT_MAX
},
};
npf_param_register(npf, param_map, __arraycount(param_map));
}

void
npf_state_tcp_sysfini(npf_t *npf)
{
const size_t len = sizeof(npf_state_tcp_params_t);
npf_param_freegroup(npf, NPF_PARAMS_TCP_STATE, len);
}