526 lines
14 KiB
C
526 lines
14 KiB
C
/* $OpenBSD: pf_syncookies.c,v 1.7 2018/09/10 15:54:28 henning Exp $ */
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/* Copyright (c) 2016,2017 Henning Brauer <henning@openbsd.org>
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* Copyright (c) 2016 Alexandr Nedvedicky <sashan@openbsd.org>
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*
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* syncookie parts based on FreeBSD sys/netinet/tcp_syncache.c
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*
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* Copyright (c) 2001 McAfee, Inc.
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* Copyright (c) 2006,2013 Andre Oppermann, Internet Business Solutions AG
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* All rights reserved.
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*
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* This software was developed for the FreeBSD Project by Jonathan Lemon
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* and McAfee Research, the Security Research Division of McAfee, Inc. under
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* DARPA/SPAWAR contract N66001-01-C-8035 ("CBOSS"), as part of the
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* DARPA CHATS research program. [2001 McAfee, Inc.]
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions
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* are met:
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* 1. Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* 2. Redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in the
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* documentation and/or other materials provided with the distribution.
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*
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* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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* ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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* SUCH DAMAGE.
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*/
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/*
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* when we're under synflood, we use syncookies to prevent state table
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* exhaustion. Trigger for the synflood mode is the number of half-open
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* connections in the state table.
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* We leave synflood mode when the number of half-open states - including
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* in-flight syncookies - drops far enough again
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*/
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/*
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* syncookie enabled Initial Sequence Number:
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* 24 bit MAC
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* 3 bit WSCALE index
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* 3 bit MSS index
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* 1 bit SACK permitted
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* 1 bit odd/even secret
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*
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* References:
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* RFC4987 TCP SYN Flooding Attacks and Common Mitigations
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* http://cr.yp.to/syncookies.html (overview)
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* http://cr.yp.to/syncookies/archive (details)
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*/
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//#include "pflog.h"
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#include <sys/param.h>
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#include <sys/systm.h>
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#include <sys/mbuf.h>
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#include <sys/filio.h>
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#include <sys/socket.h>
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#include <sys/socketvar.h>
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#include <sys/kernel.h>
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#include <sys/time.h>
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#include <sys/proc.h>
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#include <sys/rwlock.h>
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#include <sys/syslog.h>
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#include <crypto/siphash/siphash.h>
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#include <net/if.h>
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#include <net/if_var.h>
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#include <net/if_types.h>
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#include <net/route.h>
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#include <netinet/in.h>
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#include <netinet/in_pcb.h>
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#include <netinet/ip.h>
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#include <netinet/tcp.h>
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#include <netinet/tcp_var.h>
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#include <net/pfvar.h>
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#include <netpfil/pf/pf_nv.h>
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#define DPFPRINTF(n, x) if (V_pf_status.debug >= (n)) printf x
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union pf_syncookie {
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uint8_t cookie;
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struct {
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uint8_t oddeven:1,
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sack_ok:1,
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wscale_idx:3,
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mss_idx:3;
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} flags;
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};
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#define PF_SYNCOOKIE_SECRET_SIZE SIPHASH_KEY_LENGTH
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#define PF_SYNCOOKIE_SECRET_LIFETIME 15 /* seconds */
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/* Protected by PF_RULES_xLOCK. */
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struct pf_syncookie_status {
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struct callout keytimeout;
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uint8_t oddeven;
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uint8_t key[2][SIPHASH_KEY_LENGTH];
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uint32_t hiwat; /* absolute; # of states */
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uint32_t lowat;
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};
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VNET_DEFINE_STATIC(struct pf_syncookie_status, pf_syncookie_status);
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#define V_pf_syncookie_status VNET(pf_syncookie_status)
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static int pf_syncookies_setmode(u_int8_t);
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void pf_syncookie_rotate(void *);
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void pf_syncookie_newkey(void);
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uint32_t pf_syncookie_mac(struct pf_pdesc *, union pf_syncookie,
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uint32_t);
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uint32_t pf_syncookie_generate(struct mbuf *m, int off, struct pf_pdesc *,
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uint16_t);
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void
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pf_syncookies_init(void)
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{
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callout_init(&V_pf_syncookie_status.keytimeout, 1);
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PF_RULES_WLOCK();
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V_pf_syncookie_status.hiwat = PF_SYNCOOKIES_HIWATPCT *
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V_pf_limits[PF_LIMIT_STATES].limit / 100;
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V_pf_syncookie_status.lowat = PF_SYNCOOKIES_LOWATPCT *
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V_pf_limits[PF_LIMIT_STATES].limit / 100;
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pf_syncookies_setmode(PF_SYNCOOKIES_ADAPTIVE);
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PF_RULES_WUNLOCK();
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}
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void
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pf_syncookies_cleanup(void)
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{
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callout_stop(&V_pf_syncookie_status.keytimeout);
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}
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int
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pf_get_syncookies(struct pfioc_nv *nv)
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{
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nvlist_t *nvl = NULL;
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void *nvlpacked = NULL;
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int error;
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#define ERROUT(x) ERROUT_FUNCTION(errout, x)
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nvl = nvlist_create(0);
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if (nvl == NULL)
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ERROUT(ENOMEM);
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nvlist_add_bool(nvl, "enabled",
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V_pf_status.syncookies_mode != PF_SYNCOOKIES_NEVER);
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nvlist_add_bool(nvl, "adaptive",
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V_pf_status.syncookies_mode == PF_SYNCOOKIES_ADAPTIVE);
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nvlist_add_number(nvl, "highwater", V_pf_syncookie_status.hiwat);
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nvlist_add_number(nvl, "lowwater", V_pf_syncookie_status.lowat);
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nvlist_add_number(nvl, "halfopen_states",
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atomic_load_32(&V_pf_status.states_halfopen));
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nvlpacked = nvlist_pack(nvl, &nv->len);
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if (nvlpacked == NULL)
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ERROUT(ENOMEM);
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if (nv->size == 0) {
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ERROUT(0);
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} else if (nv->size < nv->len) {
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ERROUT(ENOSPC);
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}
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error = copyout(nvlpacked, nv->data, nv->len);
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#undef ERROUT
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errout:
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nvlist_destroy(nvl);
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free(nvlpacked, M_NVLIST);
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return (error);
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}
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int
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pf_set_syncookies(struct pfioc_nv *nv)
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{
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nvlist_t *nvl = NULL;
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void *nvlpacked = NULL;
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int error;
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bool enabled, adaptive;
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uint32_t hiwat, lowat;
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uint8_t newmode;
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#define ERROUT(x) ERROUT_FUNCTION(errout, x)
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if (nv->len > pf_ioctl_maxcount)
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return (ENOMEM);
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nvlpacked = malloc(nv->len, M_NVLIST, M_WAITOK);
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if (nvlpacked == NULL)
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return (ENOMEM);
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error = copyin(nv->data, nvlpacked, nv->len);
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if (error)
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ERROUT(error);
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nvl = nvlist_unpack(nvlpacked, nv->len, 0);
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if (nvl == NULL)
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ERROUT(EBADMSG);
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if (! nvlist_exists_bool(nvl, "enabled")
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|| ! nvlist_exists_bool(nvl, "adaptive"))
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ERROUT(EBADMSG);
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enabled = nvlist_get_bool(nvl, "enabled");
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adaptive = nvlist_get_bool(nvl, "adaptive");
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PFNV_CHK(pf_nvuint32_opt(nvl, "highwater", &hiwat,
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V_pf_syncookie_status.hiwat));
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PFNV_CHK(pf_nvuint32_opt(nvl, "lowwater", &lowat,
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V_pf_syncookie_status.lowat));
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if (lowat >= hiwat)
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ERROUT(EINVAL);
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newmode = PF_SYNCOOKIES_NEVER;
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if (enabled)
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newmode = adaptive ? PF_SYNCOOKIES_ADAPTIVE : PF_SYNCOOKIES_ALWAYS;
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PF_RULES_WLOCK();
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error = pf_syncookies_setmode(newmode);
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V_pf_syncookie_status.lowat = lowat;
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V_pf_syncookie_status.hiwat = hiwat;
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PF_RULES_WUNLOCK();
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#undef ERROUT
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errout:
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nvlist_destroy(nvl);
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free(nvlpacked, M_NVLIST);
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return (error);
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}
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static int
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pf_syncookies_setmode(u_int8_t mode)
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{
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if (mode > PF_SYNCOOKIES_MODE_MAX)
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return (EINVAL);
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if (V_pf_status.syncookies_mode == mode)
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return (0);
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V_pf_status.syncookies_mode = mode;
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if (V_pf_status.syncookies_mode == PF_SYNCOOKIES_ALWAYS) {
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pf_syncookie_newkey();
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V_pf_status.syncookies_active = true;
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}
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return (0);
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}
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int
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pf_synflood_check(struct pf_pdesc *pd)
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{
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MPASS(pd->proto == IPPROTO_TCP);
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PF_RULES_RASSERT();
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if (pd->pf_mtag && (pd->pf_mtag->flags & PF_MTAG_FLAG_SYNCOOKIE_RECREATED))
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return (0);
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if (V_pf_status.syncookies_mode != PF_SYNCOOKIES_ADAPTIVE)
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return (V_pf_status.syncookies_mode);
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if (!V_pf_status.syncookies_active &&
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atomic_load_32(&V_pf_status.states_halfopen) >
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V_pf_syncookie_status.hiwat) {
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/* We'd want to 'pf_syncookie_newkey()' here, but that requires
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* the rules write lock, which we can't get with the read lock
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* held. */
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callout_reset(&V_pf_syncookie_status.keytimeout, 0,
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pf_syncookie_rotate, curvnet);
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V_pf_status.syncookies_active = true;
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DPFPRINTF(LOG_WARNING,
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("synflood detected, enabling syncookies\n"));
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// XXXTODO V_pf_status.lcounters[LCNT_SYNFLOODS]++;
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}
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return (V_pf_status.syncookies_active);
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}
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void
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pf_syncookie_send(struct mbuf *m, int off, struct pf_pdesc *pd)
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{
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uint16_t mss;
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uint32_t iss;
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mss = max(V_tcp_mssdflt, pf_get_mss(m, off, pd->hdr.tcp.th_off, pd->af));
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iss = pf_syncookie_generate(m, off, pd, mss);
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pf_send_tcp(NULL, pd->af, pd->dst, pd->src, *pd->dport, *pd->sport,
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iss, ntohl(pd->hdr.tcp.th_seq) + 1, TH_SYN|TH_ACK, 0, mss,
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0, true, 0, 0, pd->act.rtableid);
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counter_u64_add(V_pf_status.lcounters[KLCNT_SYNCOOKIES_SENT], 1);
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/* XXX Maybe only in adaptive mode? */
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atomic_add_64(&V_pf_status.syncookies_inflight[V_pf_syncookie_status.oddeven],
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1);
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}
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bool
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pf_syncookie_check(struct pf_pdesc *pd)
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{
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uint32_t hash, ack, seq;
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union pf_syncookie cookie;
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MPASS(pd->proto == IPPROTO_TCP);
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PF_RULES_RASSERT();
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seq = ntohl(pd->hdr.tcp.th_seq) - 1;
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ack = ntohl(pd->hdr.tcp.th_ack) - 1;
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cookie.cookie = (ack & 0xff) ^ (ack >> 24);
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/* we don't know oddeven before setting the cookie (union) */
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if (atomic_load_64(&V_pf_status.syncookies_inflight[cookie.flags.oddeven])
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== 0)
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return (0);
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hash = pf_syncookie_mac(pd, cookie, seq);
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if ((ack & ~0xff) != (hash & ~0xff))
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return (false);
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return (true);
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}
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uint8_t
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pf_syncookie_validate(struct pf_pdesc *pd)
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{
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uint32_t ack;
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union pf_syncookie cookie;
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if (! pf_syncookie_check(pd))
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return (0);
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ack = ntohl(pd->hdr.tcp.th_ack) - 1;
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cookie.cookie = (ack & 0xff) ^ (ack >> 24);
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counter_u64_add(V_pf_status.lcounters[KLCNT_SYNCOOKIES_VALID], 1);
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atomic_add_64(&V_pf_status.syncookies_inflight[cookie.flags.oddeven], -1);
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return (1);
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}
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/*
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* all following functions private
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*/
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void
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pf_syncookie_rotate(void *arg)
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{
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CURVNET_SET((struct vnet *)arg);
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/* do we want to disable syncookies? */
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if (V_pf_status.syncookies_active &&
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((V_pf_status.syncookies_mode == PF_SYNCOOKIES_ADAPTIVE &&
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(atomic_load_32(&V_pf_status.states_halfopen) +
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atomic_load_64(&V_pf_status.syncookies_inflight[0]) +
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atomic_load_64(&V_pf_status.syncookies_inflight[1])) <
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V_pf_syncookie_status.lowat) ||
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V_pf_status.syncookies_mode == PF_SYNCOOKIES_NEVER)
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) {
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V_pf_status.syncookies_active = false;
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DPFPRINTF(PF_DEBUG_MISC, ("syncookies disabled\n"));
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}
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/* nothing in flight any more? delete keys and return */
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if (!V_pf_status.syncookies_active &&
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atomic_load_64(&V_pf_status.syncookies_inflight[0]) == 0 &&
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atomic_load_64(&V_pf_status.syncookies_inflight[1]) == 0) {
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memset(V_pf_syncookie_status.key[0], 0,
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PF_SYNCOOKIE_SECRET_SIZE);
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memset(V_pf_syncookie_status.key[1], 0,
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PF_SYNCOOKIE_SECRET_SIZE);
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CURVNET_RESTORE();
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return;
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}
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PF_RULES_WLOCK();
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/* new key, including timeout */
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pf_syncookie_newkey();
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PF_RULES_WUNLOCK();
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CURVNET_RESTORE();
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}
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void
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pf_syncookie_newkey(void)
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{
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PF_RULES_WASSERT();
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MPASS(V_pf_syncookie_status.oddeven < 2);
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V_pf_syncookie_status.oddeven = (V_pf_syncookie_status.oddeven + 1) & 0x1;
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atomic_store_64(&V_pf_status.syncookies_inflight[V_pf_syncookie_status.oddeven], 0);
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arc4random_buf(V_pf_syncookie_status.key[V_pf_syncookie_status.oddeven],
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PF_SYNCOOKIE_SECRET_SIZE);
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callout_reset(&V_pf_syncookie_status.keytimeout,
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PF_SYNCOOKIE_SECRET_LIFETIME * hz, pf_syncookie_rotate, curvnet);
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}
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/*
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* Distribution and probability of certain MSS values. Those in between are
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* rounded down to the next lower one.
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* [An Analysis of TCP Maximum Segment Sizes, S. Alcock and R. Nelson, 2011]
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* .2% .3% 5% 7% 7% 20% 15% 45%
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*/
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static int pf_syncookie_msstab[] =
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{ 216, 536, 1200, 1360, 1400, 1440, 1452, 1460 };
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/*
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* Distribution and probability of certain WSCALE values.
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* The absence of the WSCALE option is encoded with index zero.
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* [WSCALE values histograms, Allman, 2012]
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* X 10 10 35 5 6 14 10% by host
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* X 11 4 5 5 18 49 3% by connections
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*/
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static int pf_syncookie_wstab[] = { 0, 0, 1, 2, 4, 6, 7, 8 };
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uint32_t
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pf_syncookie_mac(struct pf_pdesc *pd, union pf_syncookie cookie, uint32_t seq)
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{
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SIPHASH_CTX ctx;
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uint32_t siphash[2];
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PF_RULES_RASSERT();
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MPASS(pd->proto == IPPROTO_TCP);
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SipHash24_Init(&ctx);
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SipHash_SetKey(&ctx, V_pf_syncookie_status.key[cookie.flags.oddeven]);
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switch (pd->af) {
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case AF_INET:
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SipHash_Update(&ctx, pd->src, sizeof(pd->src->v4));
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SipHash_Update(&ctx, pd->dst, sizeof(pd->dst->v4));
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break;
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case AF_INET6:
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SipHash_Update(&ctx, pd->src, sizeof(pd->src->v6));
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SipHash_Update(&ctx, pd->dst, sizeof(pd->dst->v6));
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break;
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default:
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panic("unknown address family");
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}
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SipHash_Update(&ctx, pd->sport, sizeof(*pd->sport));
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SipHash_Update(&ctx, pd->dport, sizeof(*pd->dport));
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SipHash_Update(&ctx, &seq, sizeof(seq));
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SipHash_Update(&ctx, &cookie, sizeof(cookie));
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SipHash_Final((uint8_t *)&siphash, &ctx);
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return (siphash[0] ^ siphash[1]);
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}
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uint32_t
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pf_syncookie_generate(struct mbuf *m, int off, struct pf_pdesc *pd,
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uint16_t mss)
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{
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uint8_t i, wscale;
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uint32_t iss, hash;
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union pf_syncookie cookie;
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PF_RULES_RASSERT();
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cookie.cookie = 0;
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/* map MSS */
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for (i = nitems(pf_syncookie_msstab) - 1;
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|
pf_syncookie_msstab[i] > mss && i > 0; i--)
|
|
/* nada */;
|
|
cookie.flags.mss_idx = i;
|
|
|
|
/* map WSCALE */
|
|
wscale = pf_get_wscale(m, off, pd->hdr.tcp.th_off, pd->af);
|
|
for (i = nitems(pf_syncookie_wstab) - 1;
|
|
pf_syncookie_wstab[i] > wscale && i > 0; i--)
|
|
/* nada */;
|
|
cookie.flags.wscale_idx = i;
|
|
cookie.flags.sack_ok = 0; /* XXX */
|
|
|
|
cookie.flags.oddeven = V_pf_syncookie_status.oddeven;
|
|
hash = pf_syncookie_mac(pd, cookie, ntohl(pd->hdr.tcp.th_seq));
|
|
|
|
/*
|
|
* Put the flags into the hash and XOR them to get better ISS number
|
|
* variance. This doesn't enhance the cryptographic strength and is
|
|
* done to prevent the 8 cookie bits from showing up directly on the
|
|
* wire.
|
|
*/
|
|
iss = hash & ~0xff;
|
|
iss |= cookie.cookie ^ (hash >> 24);
|
|
|
|
return (iss);
|
|
}
|
|
|
|
struct mbuf *
|
|
pf_syncookie_recreate_syn(uint8_t ttl, int off, struct pf_pdesc *pd)
|
|
{
|
|
uint8_t wscale;
|
|
uint16_t mss;
|
|
uint32_t ack, seq;
|
|
union pf_syncookie cookie;
|
|
|
|
seq = ntohl(pd->hdr.tcp.th_seq) - 1;
|
|
ack = ntohl(pd->hdr.tcp.th_ack) - 1;
|
|
cookie.cookie = (ack & 0xff) ^ (ack >> 24);
|
|
|
|
if (cookie.flags.mss_idx >= nitems(pf_syncookie_msstab) ||
|
|
cookie.flags.wscale_idx >= nitems(pf_syncookie_wstab))
|
|
return (NULL);
|
|
|
|
mss = pf_syncookie_msstab[cookie.flags.mss_idx];
|
|
wscale = pf_syncookie_wstab[cookie.flags.wscale_idx];
|
|
|
|
return (pf_build_tcp(NULL, pd->af, pd->src, pd->dst, *pd->sport,
|
|
*pd->dport, seq, 0, TH_SYN, wscale, mss, ttl, false, 0,
|
|
PF_MTAG_FLAG_SYNCOOKIE_RECREATED, pd->act.rtableid));
|
|
}
|