/*-
* 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

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