Re: [net-next] netfilter: add segment routing header 'srh' match

[Pablo Neira Ayuso]

Hi Ahmed,

On Fri, Dec 29, 2017 at 12:07:52PM +0100, Ahmed Abdelsalam wrote:
> It allows matching packets based on Segment Routing Header
> (SRH) information.
> The implementation considers revision 7 of the SRH draft.
> https://tools.ietf.org/html/draft-ietf-6man-segment-routing-header-07
> 
> Currently supported match options include:
> (1) Next Header
> (2) Hdr Ext Len
> (3) Segments Left
> (4) Last Entry
> (5) Tag value of SRH
> 
> Signed-off-by: Ahmed Abdelsalam <amsalam20@xxxxxxxxx>
> ---
>  include/uapi/linux/netfilter_ipv6/ip6t_srh.h |  63 ++++++++++
>  net/ipv6/netfilter/Kconfig                   |   9 ++
>  net/ipv6/netfilter/Makefile                  |   1 +
>  net/ipv6/netfilter/ip6t_srh.c                | 165 +++++++++++++++++++++++++++
>  4 files changed, 238 insertions(+)
>  create mode 100644 include/uapi/linux/netfilter_ipv6/ip6t_srh.h
>  create mode 100644 net/ipv6/netfilter/ip6t_srh.c
> 
> diff --git a/include/uapi/linux/netfilter_ipv6/ip6t_srh.h b/include/uapi/linux/netfilter_ipv6/ip6t_srh.h
> new file mode 100644
> index 0000000..1b5dbd8
> --- /dev/null
> +++ b/include/uapi/linux/netfilter_ipv6/ip6t_srh.h
> @@ -0,0 +1,63 @@
> +/**
> + * Definitions for Segment Routing Header 'srh' match
> + *
> + * Author:
> + *       Ahmed Abdelsalam       <amsalam20@xxxxxxxxx>
> + */

Please, add this in SPDX format instead.

See include/uapi/linux/netfilter/xt_owner.h for instance.

> +#ifndef _IP6T_SRH_H
> +#define _IP6T_SRH_H
> +
> +#include <linux/types.h>
> +#include <linux/netfilter.h>
> +
> +/* Values for "mt_flags" field in struct ip6t_srh */
> +#define IP6T_SRH_NEXTHDR        0x0001
> +#define IP6T_SRH_LEN_EQ         0x0002
> +#define IP6T_SRH_LEN_GT         0x0004
> +#define IP6T_SRH_LEN_LT         0x0008
> +#define IP6T_SRH_SEGS_EQ        0x0010
> +#define IP6T_SRH_SEGS_GT        0x0020
> +#define IP6T_SRH_SEGS_LT        0x0040
> +#define IP6T_SRH_LAST_EQ        0x0080
> +#define IP6T_SRH_LAST_GT        0x0100
> +#define IP6T_SRH_LAST_LT        0x0200
> +#define IP6T_SRH_TAG            0x0400
> +#define IP6T_SRH_MASK           0x07FF
> +
> +/* Values for "mt_invflags" field in struct ip6t_srh */
> +#define IP6T_SRH_INV_NEXTHDR    0x0001
> +#define IP6T_SRH_INV_LEN_EQ     0x0002
> +#define IP6T_SRH_INV_LEN_GT     0x0004
> +#define IP6T_SRH_INV_LEN_LT     0x0008
> +#define IP6T_SRH_INV_SEGS_EQ    0x0010
> +#define IP6T_SRH_INV_SEGS_GT    0x0020
> +#define IP6T_SRH_INV_SEGS_LT    0x0040
> +#define IP6T_SRH_INV_LAST_EQ    0x0080
> +#define IP6T_SRH_INV_LAST_GT    0x0100
> +#define IP6T_SRH_INV_LAST_LT    0x0200
> +#define IP6T_SRH_INV_TAG        0x0400
> +#define IP6T_SRH_INV_MASK       0x07FF

Looking at all these EQ, GT, LT... I think this should be very easy to
implement in nf_tables with no kernel changes.

You only need to add the protocol definition to:

nftables/src/exthdr.c

Would you have a look into this? This would be very much appreciated
to we keep nftables in sync with what we have in iptables.

> +
> +/**
> + *      struct ip6t_srh - SRH match options
> + *      @ next_hdr: Next header field of SRH
> + *      @ hdr_len: Extension header length field of SRH
> + *      @ segs_left: Segments left field of SRH
> + *      @ last_entry: Last entry field of SRH
> + *      @ tag: Tag field of SRH
> + *      @ mt_flags: match options
> + *      @ mt_invflags: Invert the sense of match options
> + */
> +
> +struct ip6t_srh {
> +	__u8                    next_hdr;
> +	__u8                    hdr_len;
> +	__u8                    segs_left;
> +	__u8                    last_entry;
> +	__u16                   tag;
> +	__u16                   mt_flags;
> +	__u16                   mt_invflags;
> +};
> +
> +#endif /*_IP6T_SRH_H*/
> diff --git a/net/ipv6/netfilter/Kconfig b/net/ipv6/netfilter/Kconfig
> index 6acb2ee..e1818eb 100644
> --- a/net/ipv6/netfilter/Kconfig
> +++ b/net/ipv6/netfilter/Kconfig
> @@ -232,6 +232,15 @@ config IP6_NF_MATCH_RT
>  
>  	  To compile it as a module, choose M here.  If unsure, say N.
>  
> +config IP6_NF_MATCH_SRH
> +        tristate '"srh" Segment Routing header match support'
> +        depends on NETFILTER_ADVANCED
> +        help
> +          srh matching allows you to match packets based on the segment
> +	  routing header of the packet.
> +
> +          To compile it as a module, choose M here.  If unsure, say N.
> +
>  # The targets
>  config IP6_NF_TARGET_HL
>  	tristate '"HL" hoplimit target support'
> diff --git a/net/ipv6/netfilter/Makefile b/net/ipv6/netfilter/Makefile
> index c6ee0cd..e0d51a9 100644
> --- a/net/ipv6/netfilter/Makefile
> +++ b/net/ipv6/netfilter/Makefile
> @@ -54,6 +54,7 @@ obj-$(CONFIG_IP6_NF_MATCH_MH) += ip6t_mh.o
>  obj-$(CONFIG_IP6_NF_MATCH_OPTS) += ip6t_hbh.o
>  obj-$(CONFIG_IP6_NF_MATCH_RPFILTER) += ip6t_rpfilter.o
>  obj-$(CONFIG_IP6_NF_MATCH_RT) += ip6t_rt.o
> +obj-$(CONFIG_IP6_NF_MATCH_SRH) += ip6t_srh.o
>  
>  # targets
>  obj-$(CONFIG_IP6_NF_TARGET_MASQUERADE) += ip6t_MASQUERADE.o
> diff --git a/net/ipv6/netfilter/ip6t_srh.c b/net/ipv6/netfilter/ip6t_srh.c
> new file mode 100644
> index 0000000..75e41dc9
> --- /dev/null
> +++ b/net/ipv6/netfilter/ip6t_srh.c
> @@ -0,0 +1,165 @@
> +/*
> + * Kernel module to match Segment Routing Header (SRH) parameters.
> + *
> + * Author:
> + * Ahmed Abdelsalam <amsalam20@xxxxxxxxx>
> + *
> + *
> + *  This program is free software; you can redistribute it and/or
> + *	modify it under the terms of the GNU General Public License
> + *	as published by the Free Software Foundation; either version 2
> + *	of the License, or (at your option) any later version.
> + */
> +
> +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
> +#include <linux/module.h>
> +#include <linux/skbuff.h>
> +#include <linux/ipv6.h>
> +#include <linux/types.h>
> +#include <net/ipv6.h>
> +#include <net/seg6.h>
> +
> +#include <linux/netfilter/x_tables.h>
> +#include <linux/netfilter_ipv6/ip6t_srh.h>
> +#include <linux/netfilter_ipv6/ip6_tables.h>
> +
> +MODULE_LICENSE("GPL");
> +MODULE_DESCRIPTION("Xtables: IPv6 Segment Routing Header match");
> +MODULE_AUTHOR("Ahmed Abdelsalam  <amsalam20@xxxxxxxxx>");

Not a deal breaker, but modern code usually places these MODULE_*
lines at the end of the file.

> +
> +/* Test a struct->mt_invflags and a boolean for inequality */
> +#define NF_SRH_INVF(ptr, flag, boolean)	\
> +	((boolean) ^ !!((ptr)->mt_invflags & (flag)))
> +
> +static bool srh_mt6(const struct sk_buff *skb, struct xt_action_param *par)
> +{
> +	const struct ip6t_srh *srhinfo = par->matchinfo;
> +	struct ipv6_sr_hdr *srh;
> +	struct ipv6_sr_hdr _srh;
> +	int hdrlen, srhoff = 0;
> +
> +	if (ipv6_find_hdr(skb, &srhoff, IPPROTO_ROUTING, NULL, NULL) < 0)
> +		return false;
> +
> +	srh = skb_header_pointer(skb, srhoff, sizeof(_srh), &_srh);
> +

Remove unnecessary line break.

> +	if (!srh)
> +		return false;
> +
> +	hdrlen = ipv6_optlen(srh);
> +	if (skb->len - srhoff < hdrlen)
> +		return false;
> +
> +	if (srh->type != IPV6_SRCRT_TYPE_4)
> +		return false;
> +
> +	if (srh->segments_left > srh->first_segment)
> +		return false;
> +
> +	/* Next Header matching */
> +	if (srhinfo->mt_flags & IP6T_SRH_NEXTHDR)
> +		if (NF_SRH_INVF(srhinfo, IP6T_SRH_INV_NEXTHDR,
> +				!(srh->nexthdr == srhinfo->next_hdr)))
> +			return false;
> +
> +	/* Header Extension Length matching */
> +	if (srhinfo->mt_flags & IP6T_SRH_LEN_EQ)
> +		if (NF_SRH_INVF(srhinfo, IP6T_SRH_INV_LEN_EQ,
> +				!(srh->hdrlen == srhinfo->hdr_len)))
> +			return false;
> +
> +	if (srhinfo->mt_flags & IP6T_SRH_LEN_GT)
> +		if (NF_SRH_INVF(srhinfo, IP6T_SRH_INV_LEN_GT,
> +				!(srh->hdrlen > srhinfo->hdr_len)))
> +			return false;
> +
> +	if (srhinfo->mt_flags & IP6T_SRH_LEN_LT)
> +		if (NF_SRH_INVF(srhinfo, IP6T_SRH_INV_LEN_LT,
> +				!(srh->hdrlen < srhinfo->hdr_len)))
> +			return false;
> +
> +	/* Segments Left matching */
> +	if (srhinfo->mt_flags & IP6T_SRH_SEGS_EQ)
> +		if (NF_SRH_INVF(srhinfo, IP6T_SRH_INV_SEGS_EQ,
> +				!(srh->segments_left == srhinfo->segs_left)))
> +			return false;
> +
> +	if (srhinfo->mt_flags & IP6T_SRH_SEGS_GT)
> +		if (NF_SRH_INVF(srhinfo, IP6T_SRH_INV_SEGS_GT,
> +				!(srh->segments_left > srhinfo->segs_left)))
> +			return false;
> +
> +	if (srhinfo->mt_flags & IP6T_SRH_SEGS_LT)
> +		if (NF_SRH_INVF(srhinfo, IP6T_SRH_INV_SEGS_LT,
> +				!(srh->segments_left < srhinfo->segs_left)))
> +			return false;
> +
> +	/**
> +	 * Last Entry matching
> +	 * Last_Entry field was introduced in revision 6 of the SRH draft.
> +	 * It was called First_Segment in the previous revision
> +	 */
> +	if (srhinfo->mt_flags & IP6T_SRH_LAST_EQ)
> +		if (NF_SRH_INVF(srhinfo, IP6T_SRH_INV_LAST_EQ,
> +				!(srh->first_segment == srhinfo->last_entry)))
> +			return false;
> +
> +	if (srhinfo->mt_flags & IP6T_SRH_LAST_GT)
> +		if (NF_SRH_INVF(srhinfo, IP6T_SRH_INV_LAST_GT,
> +				!(srh->first_segment > srhinfo->last_entry)))
> +			return false;
> +
> +	if (srhinfo->mt_flags & IP6T_SRH_LAST_LT)
> +		if (NF_SRH_INVF(srhinfo, IP6T_SRH_INV_LAST_LT,
> +				!(srh->first_segment < srhinfo->last_entry)))
> +			return false;
> +
> +	/**
> +	 * Tag matchig
> +	 * Tag field was introduced in revision 6 of the SRH draft.
> +	 */
> +	if (srhinfo->mt_flags & IP6T_SRH_TAG)
> +		if (NF_SRH_INVF(srhinfo, IP6T_SRH_INV_TAG,
> +				!(srh->tag == srhinfo->tag)))
> +			return false;
> +	return true;
> +}
> +
> +static int srh_mt6_check(const struct xt_mtchk_param *par)
> +{
> +	const struct ip6t_srh *srhinfo = par->matchinfo;
> +
> +	if (srhinfo->mt_flags & ~IP6T_SRH_MASK) {
> +		pr_debug("unknown srh match flags  %X\n", srhinfo->mt_flags);

Better call pr_err() here. I know we don't do this in other
extensions, but we should not use pr_debug() for this. ip6tables
explicit refers to 'dmesg' when -EINVAL is returned.

Thanks.