Subject: Re: [v4 PATCH 1/2] NETFILTER module xt_hmark, new target for HASH based fwmark

From: Pablo Neira Ayuso <[email-masked]>
Date: Fri, 25 Nov 2011 18:36:49 +0100

On Fri, Nov 25, 2011 at 10:36:26AM +0100, Hans Schillstrom wrote:
> diff --git a/include/net/ipv6.h b/include/net/ipv6.h
> index 3f0258d..9e4d4f9 100644
> --- a/include/net/ipv6.h
> +++ b/include/net/ipv6.h
> @@ -39,6 +39,7 @@
>  #define NEXTHDR_ICMP		58	/* ICMP for IPv6. */
>  #define NEXTHDR_NONE		59	/* No next header */
>  #define NEXTHDR_DEST		60	/* Destination options header. */
> +#define NEXTHDR_SCTP		132	/* Stream Control Transport Protocol */
>  #define NEXTHDR_MOBILITY	135	/* Mobility header. */
>  
>  #define NEXTHDR_MAX		255

This has to go in a separated patch. Please, send it to netdev. I
think davem can pick that for 3.2-rc

> diff --git a/net/netfilter/Kconfig b/net/netfilter/Kconfig
> index 8260b13..41bee43 100644
> --- a/net/netfilter/Kconfig
> +++ b/net/netfilter/Kconfig
> @@ -471,6 +471,23 @@ config NETFILTER_XT_TARGET_HL
>  	since you can easily create immortal packets that loop
>  	forever on the network.
>  
> +config NETFILTER_XT_TARGET_HMARK
> +	tristate '"HMARK" target support'
> +	depends on NETFILTER_ADVANCED
> +	---help---
> +	This option adds the "HMARK" target.
> +
> +	The target allows you to create rules in the "raw" and "mangle" tables
> +	which alter the netfilter mark (nfmark) field within a given range.
> +	First a 32 bit hash value is generated then modulus by <limit> and
> +	finally an offset is added before it's written to nfmark.
> +
> +	Prior to routing, the nfmark can influence the routing method (see
> +	"Use netfilter MARK value as routing key") and can also be used by
> +	other subsystems to change their behavior.
> +
> +	The mark match can also be used to match nfmark produced by this module.
> +
>  config NETFILTER_XT_TARGET_IDLETIMER
>  	tristate  "IDLETIMER target support"
>  	depends on NETFILTER_ADVANCED
> diff --git a/net/netfilter/Makefile b/net/netfilter/Makefile
> index 1a02853..359eeb6 100644
> --- a/net/netfilter/Makefile
> +++ b/net/netfilter/Makefile
> @@ -56,6 +56,7 @@ obj-$(CONFIG_NETFILTER_XT_TARGET_CONNSECMARK) += xt_CONNSECMARK.o
>  obj-$(CONFIG_NETFILTER_XT_TARGET_CT) += xt_CT.o
>  obj-$(CONFIG_NETFILTER_XT_TARGET_DSCP) += xt_DSCP.o
>  obj-$(CONFIG_NETFILTER_XT_TARGET_HL) += xt_HL.o
> +obj-$(CONFIG_NETFILTER_XT_TARGET_HMARK) += xt_hmark.o
>  obj-$(CONFIG_NETFILTER_XT_TARGET_LED) += xt_LED.o
>  obj-$(CONFIG_NETFILTER_XT_TARGET_NFLOG) += xt_NFLOG.o
>  obj-$(CONFIG_NETFILTER_XT_TARGET_NFQUEUE) += xt_NFQUEUE.o
> diff --git a/net/netfilter/xt_hmark.c b/net/netfilter/xt_hmark.c
> new file mode 100644
> index 0000000..ae33293
> --- /dev/null
> +++ b/net/netfilter/xt_hmark.c
> @@ -0,0 +1,327 @@
> +/*
> + *	xt_hmark - Netfilter module to set mark as hash value
> + *
> + *	(C) 2011 Hans Schillstrom <[email-masked]>
> + *
> + *	Description:
> + *	This module calculates a hash value that can be modified by modulus
> + *	and an offset. The hash value is based on a direction independent
> + *	five tuple: src & dst addr src & dst ports and protocol.
> + *	However src & dst port can be masked and are not used for fragmented
> + *	packets, ESP and AH don't have ports so SPI will be used instead.
> + *	For ICMP error messages the hash mark values will be calculated on
> + *	the source packet i.e. the packet caused the error (If sufficient
> + *	amount of data exists).
> + *
> + *	This program is free software; you can redistribute it and/or modify
> + *	it under the terms of the GNU General Public License version 2 as
> + *	published by the Free Software Foundation.
> + */
> +
> +#include <linux/module.h>
> +#include <linux/skbuff.h>
> +#include <net/ip.h>
> +#include <linux/icmp.h>
> +
> +#include <linux/netfilter/xt_hmark.h>
> +#include <linux/netfilter/x_tables.h>
> +#include <net/netfilter/nf_nat.h>
> +
> +#if defined(CONFIG_IPV6) || defined(CONFIG_IPV6_MODULE)
> +#	define WITH_IPV6 1
> +#include <net/ipv6.h>
> +#include <linux/netfilter_ipv6/ip6_tables.h>
> +#endif
> +
> +

Comestic: unnecessary extra line.

> +MODULE_LICENSE("GPL");
> +MODULE_AUTHOR("Hans Schillstrom <[email-masked]>");
> +MODULE_DESCRIPTION("Xtables: packet range mark operations by hash value");
> +MODULE_ALIAS("ipt_HMARK");
> +MODULE_ALIAS("ip6t_HMARK");
> +
> +/*
> + * ICMP, get inner header so calc can be made on the source message
> + *       not the icmp header, i.e. same hash mark must be produced
> + *       on an icmp error message.
> + */
> +static int get_inner_hdr(struct sk_buff *skb, int iphsz, int nhoff)
> +{
> +	const struct icmphdr *icmph;
> +	struct icmphdr _ih;
> +	struct iphdr *iph = NULL;
> +
> +	/* Not enough header? */
> +	icmph = skb_header_pointer(skb, nhoff + iphsz, sizeof(_ih), &_ih);
> +	if (icmph == NULL)
> +		return nhoff;
> +
> +	if (icmph->type > NR_ICMP_TYPES)
> +		return nhoff;
> +
> +	/* Error message? */
> +	if (icmph->type != ICMP_DEST_UNREACH &&
> +	    icmph->type != ICMP_SOURCE_QUENCH &&
> +	    icmph->type != ICMP_TIME_EXCEEDED &&
> +	    icmph->type != ICMP_PARAMETERPROB &&
> +	    icmph->type != ICMP_REDIRECT)
> +		return nhoff;
> +	/* Checkin full IP header plus 8 bytes of protocol to
> +	 * avoid additional coding at protocol handlers.
> +	 */
> +	if (!pskb_may_pull(skb, nhoff + iphsz + sizeof(_ih) + 8))
> +		return nhoff;

skb_header_pointer again here, if conntrack is enabled, we can benefit
from handling fragments.

> +	iph = (struct iphdr *)(skb->data + nhoff + iphsz + sizeof(_ih));
> +	return nhoff + iphsz + sizeof(_ih);
> +}
> +/*
> + * ICMPv6
> + * Input nhoff Offset into network header
> + *       offset where ICMPv6 header starts
> + * Returns true if it's a icmp error and updates nhoff
> + */
> +#ifdef WITH_IPV6
> +static int get_inner6_hdr(struct sk_buff *skb, int *offset, int hdrlen)
> +{
> +	struct icmp6hdr *icmp6h;
> +	struct icmp6hdr _ih6;
> +
> +	icmp6h = skb_header_pointer(skb, *offset + hdrlen, sizeof(_ih6), &_ih6);
> +	if (icmp6h == NULL)
> +		return 0;
> +
> +	if (icmp6h->icmp6_type && icmp6h->icmp6_type < 128) {
> +		*offset += hdrlen + sizeof(_ih6);
> +		return 1;
> +	}
> +	return 0;
> +}
> +/*
> + * Calc hash value, special casre is taken on icmp and fragmented messages
> + * i.e. fragmented messages don't use ports.
> + */
> +__u32 hmark_v6(struct sk_buff *skb, const struct xt_action_param *par)
> +{
> +	struct xt_hmark_info *info = (struct xt_hmark_info *)par->targinfo;
> +	int nhoff, poff, hdrlen;
> +	u32 addr1, addr2, hash;
> +	struct ipv6hdr *ip6;
> +	u8 nexthdr;
> +	int frag = 0, ip6hdrlvl = 0;	/* Header level */
> +	struct ipv6_opt_hdr _hdr, *hp;
> +	union {
> +		u32 v32;
> +		u16 v16[2];
> +	} ports;
> +
> +	ports.v32 = 0;
> +	nhoff = skb_network_offset(skb);
> +
> +hdr_new:
> +	/* Get header info */
> +	ip6 = (struct ipv6hdr *) (skb->data + nhoff);
> +	nexthdr = ip6->nexthdr;
> +	hdrlen = sizeof(struct ipv6hdr);
> +	hp = skb_header_pointer(skb, nhoff + hdrlen, sizeof(_hdr), &_hdr);

you have to check return value of skb_header_pointer here.

> +	while (nexthdr) {
> +		switch (nexthdr) {
> +		case IPPROTO_ICMPV6:
> +			/* ICMP Error then move ptr to inner header */
> +			if (get_inner6_hdr(skb, &nhoff, hdrlen)) {
> +				ip6hdrlvl++;
> +				if (!pskb_may_pull(skb, sizeof(_hdr) + nhoff))
> +					return XT_CONTINUE;
> +				goto hdr_new;
> +			}
> +			nhoff += hdrlen;
> +			goto hdr_rdy;
> +
> +		case NEXTHDR_FRAGMENT:
> +			if (!ip6hdrlvl) /* Do not use ports if fragmented */
> +				frag = 1;
> +			break;
> +
> +		/* End of hdr traversing cont. with ports and hash calc. */
> +		case NEXTHDR_IPV6:	/* Do not process tunnels */
> +		case NEXTHDR_TCP:
> +		case NEXTHDR_UDP:
> +		case NEXTHDR_ESP:
> +		case NEXTHDR_AUTH:
> +		case NEXTHDR_SCTP:
> +		case NEXTHDR_NONE:	/* Last hdr of something unknown */
> +			nhoff += hdrlen;
> +			goto hdr_rdy;
> +		default:
> +			return XT_CONTINUE;
> +		}
> +		if (!hp)
> +			return XT_CONTINUE;
> +		nhoff += hdrlen;	/* eat current header */
> +		nexthdr =  hp->nexthdr;	/* Next header */
> +		hdrlen = ipv6_optlen(hp);
> +		hp = skb_header_pointer(skb, nhoff + hdrlen, sizeof(_hdr),
> +					&_hdr);

same here.

> +		if (!pskb_may_pull(skb, nhoff))

why this after skb_header_pointer?

[... trimmed off ...]
>       poff = proto_ports_offset(ip_proto);
>       nhoff += ip->ihl * 4 + poff;
>       if (frag || poff < 0 || !pskb_may_pull(skb, nhoff + 4))
>               goto noports;
>
>       ports.v32 = * (__force u32 *) (skb->data + nhoff);
>       if (ip_proto == IPPROTO_ESP || ip_proto == IPPROTO_AH) {
>               ports.v32 = (ports.v32 & info->spimask) | nfo->spiset;
>       } else {
>               if (snatport)   /* Replace nat'ed port(s) */
>                       ports.v16[1] = snatport;
>               if (dnatport)
>                       ports.v16[0] = dnatport;
>               ports.v32 = (ports.v32 & info->pmask.v32) |
>                               info->pset.v32;
>               if (ports.v16[1] < ports.v16[0])
>                       swap(ports.v16[0], ports.v16[1]);
>       }
>
>noports:
>       ip_proto &= info->prmask;
>       /* get a consistent hash (same value on both flow directions)/
>       if (addr2 < addr1)
>               swap(addr1, addr2);
>
>       hash = jhash_3words(addr1, addr2, ports.v32, info->hashrnd) ^ p_proto;
>       if (info->hmod)
>               skb->mark = (hash % info->hmod) + info->hoffs;
>       return XT_CONTINUE;
> }

Hm, I think the fragmentation handling is broken.

Say that the first fragment contains the transport header
header, then the mark is calculated based on the address and ports.
Then, later on fragments will receive the mark based on the network
header only. They may have different marks.

If you don't want to use conntrack in your setup and you want to handle
fragments, then you have to configure HMARK to calculate the hashing
based on the network addresses. If you want to fully support fragments,
then enable conntrack and you can configure HMARK to calculate the
hashing based on network address + transport bits.

Fix this by removing the fragmentation handling, then assume that
people can select between two hashing configuration for HMARK. One
based for network address which is fragment-safe, one that uses the
transport layer information, that requires conntrack. Otherwise, I
don't see a sane way to handle this situation.

I think this has to be documented in the iptables manpage for HMARK.