xref: /src/sys/net/npf/npf_ext_normalize.c

/*-
 * Copyright (c) 2009-2012 The NetBSD Foundation, Inc.
 * All rights reserved.
 *
 * 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.
 */

#ifdef _KERNEL
#include <sys/cdefs.h>
__KERNEL_RCSID(0, "$NetBSD: npf_ext_normalize.c,v 1.11 2021/03/08 20:01:54 christos Exp $");

#include <sys/types.h>
#include <sys/module.h>
#include <sys/kmem.h>

#include <net/if.h>
#include <netinet/in_systm.h>
#include <netinet/in.h>
#include <netinet/in_var.h>
#endif

#include "npf.h"
#include "npf_impl.h"

/*
 * NPF extension module definition and the identifier.
 */
NPF_EXT_MODULE(npf_ext_normalize, "");

#define	NPFEXT_NORMALIZE_VER	1

static void *		npf_ext_normalize_id;

/*
 * Normalisation parameters.
 */
typedef struct {
	unsigned	n_minttl;
	unsigned	n_maxmss;
	bool		n_random_id;
	bool		n_no_df;
} npf_normalize_t;

/*
 * npf_normalize_ctor: a constructor for the normalisation rule procedure
 * with the given parameters.
 */
static int
npf_normalize_ctor(npf_rproc_t *rp, const nvlist_t *params)
{
	npf_normalize_t *np;

	/* Create a structure for normalisation parameters. */
	np = kmem_zalloc(sizeof(npf_normalize_t), KM_SLEEP);

	/* IP ID randomisation and IP_DF flag cleansing. */
	np->n_random_id = dnvlist_get_bool(params, "random-id", false);
	np->n_no_df = dnvlist_get_bool(params, "no-df", false);

	/* Minimum IP TTL and maximum TCP MSS. */
	np->n_minttl = dnvlist_get_number(params, "min-ttl", 0);
	np->n_maxmss = dnvlist_get_number(params, "max-mss", 0);

	/* Assign the parameters for this rule procedure. */
	npf_rproc_assign(rp, np);
	return 0;
}

/*
 * npf_normalize_dtor: a destructor for a normalisation rule procedure.
 */
static void
npf_normalize_dtor(npf_rproc_t *rp, void *params)
{
	/* Free our meta-data, associated with the procedure. */
	kmem_free(params, sizeof(npf_normalize_t));
}

/*
 * npf_normalize_ip4: routine to normalize IPv4 header (randomize ID,
 * clear "don't fragment" and/or enforce minimum TTL).
 */
static inline void
npf_normalize_ip4(npf_cache_t *npc, npf_normalize_t *np)
{
	struct ip *ip = npc->npc_ip.v4;
	uint16_t cksum = ip->ip_sum;
	uint16_t ip_off = ip->ip_off;
	uint8_t ttl = ip->ip_ttl;
	unsigned minttl = np->n_minttl;

	KASSERT(np->n_random_id || np->n_no_df || minttl);

	/* Randomize IPv4 ID. */
	if (np->n_random_id) {
		uint16_t oid = ip->ip_id, nid;

		nid = ip_randomid();
		cksum = npf_fixup16_cksum(cksum, oid, nid);
		ip->ip_id = nid;
	}

	/* IP_DF flag cleansing. */
	if (np->n_no_df && (ip_off & htons(IP_DF)) != 0) {
		uint16_t nip_off = ip_off & ~htons(IP_DF);

		cksum = npf_fixup16_cksum(cksum, ip_off, nip_off);
		ip->ip_off = nip_off;
	}

	/* Enforce minimum TTL. */
	if (minttl && ttl < minttl) {
		cksum = npf_fixup16_cksum(cksum, ttl, minttl);
		ip->ip_ttl = minttl;
	}

	/* Update IPv4 checksum. */
	ip->ip_sum = cksum;
}

/*
 * npf_normalize: the main routine to normalize IPv4 and/or TCP headers.
 */
static bool
npf_normalize(npf_cache_t *npc, void *params, const npf_match_info_t *mi,
    int *decision)
{
	npf_normalize_t *np = params;
	uint16_t cksum, mss, maxmss = np->n_maxmss;
	uint16_t old[2], new[2];
	struct tcphdr *th;
	int wscale;
	bool mid;

	/* Skip, if already blocking. */
	if (*decision == NPF_DECISION_BLOCK) {
		return true;
	}

	/* Normalize IPv4.  Nothing to do for IPv6. */
	if (npf_iscached(npc, NPC_IP4) && (np->n_random_id || np->n_minttl)) {
		npf_normalize_ip4(npc, np);
	}
	th = npc->npc_l4.tcp;

	/*
	 * TCP Maximum Segment Size (MSS) "clamping".  Only if SYN packet.
	 * Fetch MSS and check whether rewrite to lower is needed.
	 */
	if (maxmss == 0 || !npf_iscached(npc, NPC_TCP) ||
	    (th->th_flags & TH_SYN) == 0) {
		/* Not required; done. */
		return true;
	}
	mss = 0;
	if (!npf_fetch_tcpopts(npc, &mss, &wscale)) {
		return true;
	}
	if (ntohs(mss) <= maxmss) {
		/* Nothing else to do. */
		return true;
	}
	maxmss = htons(maxmss);

	/*
	 * Store new MSS, calculate TCP checksum and update it. The MSS may
	 * not be aligned and fall in the middle of two uint16_t's, so we
	 * need to take care of that when calculating the checksum.
	 *
	 * WARNING: must re-fetch the TCP header after the modification.
	 */
	if (npf_set_mss(npc, maxmss, old, new, &mid) &&
	    !nbuf_cksum_barrier(npc->npc_nbuf, mi->mi_di)) {
		th = npc->npc_l4.tcp;
		if (mid) {
			cksum = th->th_sum;
			cksum = npf_fixup16_cksum(cksum, old[0], new[0]);
			cksum = npf_fixup16_cksum(cksum, old[1], new[1]);
		} else {
			cksum = npf_fixup16_cksum(th->th_sum, mss, maxmss);
		}
		th->th_sum = cksum;
	}

	return true;
}

__dso_public int
npf_ext_normalize_init(npf_t *npf)
{
	static const npf_ext_ops_t npf_normalize_ops = {
		.version	= NPFEXT_NORMALIZE_VER,
		.ctx		= NULL,
		.ctor		= npf_normalize_ctor,
		.dtor		= npf_normalize_dtor,
		.proc		= npf_normalize
	};
	npf_ext_normalize_id = npf_ext_register(npf,
	    "normalize", &npf_normalize_ops);
	return npf_ext_normalize_id ? 0 : EEXIST;
}

__dso_public int
npf_ext_normalize_fini(npf_t *npf)
{
	return npf_ext_unregister(npf, npf_ext_normalize_id);
}

#ifdef _KERNEL
static int
npf_ext_normalize_modcmd(modcmd_t cmd, void *arg)
{
	npf_t *npf = npf_getkernctx();

	switch (cmd) {
	case MODULE_CMD_INIT:
		return npf_ext_normalize_init(npf);
	case MODULE_CMD_FINI:
		return npf_ext_unregister(npf, npf_ext_normalize_id);
	case MODULE_CMD_AUTOUNLOAD:
		return npf_autounload_p() ? 0 : EBUSY;
	default:
		return ENOTTY;
	}
	return 0;
}
#endif