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Message-ID: <20090529085918.GA11887@ioremap.net>
Date: Fri, 29 May 2009 12:59:18 +0400
From: Evgeniy Polyakov <zbr@...emap.net>
To: Patrick McHardy <kaber@...sh.net>
Cc: netdev@...r.kernel.org, David Miller <davem@...emloft.net>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Netfilter Development Mailinglist
<netfilter-devel@...r.kernel.org>,
Jan Engelhardt <jengelh@...ozas.de>
Subject: Re: [resend] Passive OS fingerprint xtables match.
Hi Patrick.
Thanks for your comments.
On Wed, May 27, 2009 at 06:28:16PM +0200, Patrick McHardy (kaber@...sh.net) wrote:
> >You will find something like this in the syslog:
> >Windows [2000:SP3:Windows XP Pro SP1, 2000 SP3]: 11.22.33.55:4024 ->
> >11.22.33.44:139 hops=4
> >Linux [2.5:]: 1.2.3.4:44448 -> 11.22.33.44:22 hops=4
>
> Please convert this to use nf_log_packet().
Ok, will use.
> >--- /dev/null
> >+++ b/include/linux/netfilter/xt_osf.h
> >+#ifndef _XT_OSF_H
> >+#define _XT_OSF_H
> >+
> >+#define MAXGENRELEN 32
> >+#define MAXDETLEN 64
>
> ^ Unused
>
> >+
> >+#define XT_OSF_GENRE (1<<0)
> >+#define XT_OSF_TTL (1<<1)
> >+#define XT_OSF_LOG (1<<2)
> >+#define XT_OSF_UNUSED (1<<3)
>
> ^ Unused? :)
I will drop those constants.
> >+#define XT_OSF_CONNECTOR (1<<4)
> >+#define XT_OSF_INVERT (1<<5)
> >+
> >+#define XT_OSF_LOGLEVEL_ALL 0
> >+#define XT_OSF_LOGLEVEL_FIRST 1
> >+#define XT_OSF_LOGLEVEL_ALL_KNOWN 2
>
> What does this do?
There are 3 log levels - dump all matched entries, only the first
matched and dump unknown packet data if needed.
> >+#define XT_OSF_TTL_TRUE 0 /* True ip and fingerprint
> >TTL comparison */
> >+#define XT_OSF_TTL_LESS 1 /* Check if ip TTL is less
> >than fingerprint one */
> >+#define XT_OSF_TTL_NOCHECK 2 /* Do not compare ip and fingerprint
> >TTL at all */
>
> These seem redundant - having neither of TRUE or LESS seems
> equivalent to NOCHECK. Perhaps thats the reason why its not
> used at all :) Looking at the code, "TRUE" would be better
> named as "EQUAL".
There are only three types of TTL check - equal (for true), less than
fingerprint one and when no check is performed at all. NOCHECK is
actually used, but LESS one does not have a special check, but a default
clause when neither TRUE or NOCHECK is specified.
> >+struct xt_osf_info {
> >+ char genre[MAXGENRELEN];
> >+ __u32 len;
> >+ __u32 flags;
> >+ __u32 loglevel;
> >+ __u32 ttl;
> >+};
>
> Unless you're really really sure that this is not going to
> change, please use netlink attributes. Similar for the other
> ABI structures.
It was not changed for the last 5 years, maybe even more.
There are no other fields in the tcp header to match against :)
> >+struct xt_osf_user_finger {
> >+ struct xt_osf_wc wss;
> >+
> >+ __u8 ttl, df;
> >+ __u16 ss, mss;
> >+ __u16 opt_num;
> >+
> >+ char genre[MAXGENRELEN];
> >+ char version[MAXGENRELEN];
> >+ char subtype[MAXGENRELEN];
> >+
> >+ /* MAX_IPOPTLEN is maximum if all options are NOPs or EOLs */
> >+ struct xt_osf_opt opt[MAX_IPOPTLEN];
>
> This really looks like you should use nested attributes.
This will be an unneded complication - we should provide an option
sequence, and maximum number of options is strickly determined by
the protocol specification. How does having separate attributes for each
option simplify the process?
> >+/* Defines for IANA option kinds */
> >+
> >+enum iana_options {
> >+ OSFOPT_EOL = 0, /* End of options */
> >+ OSFOPT_NOP, /* NOP */
> >+ OSFOPT_MSS, /* Maximum segment size */
> >+ OSFOPT_WSO, /* Window scale option */
> >+ OSFOPT_SACKP, /* SACK permitted */
> >+ OSFOPT_SACK, /* SACK */
> >+ OSFOPT_ECHO,
> >+ OSFOPT_ECHOREPLY,
> >+ OSFOPT_TS, /* Timestamp option */
> >+ OSFOPT_POCP, /* Partial Order Connection Permitted */
> >+ OSFOPT_POSP, /* Partial Order Service Profile */
> >+
> >+ /* Others are not used in the current OSF */
> >+ OSFOPT_EMPTY = 255,
> >+};
>
> Why do we need to duplicate these?
Why duplicate? It is the only place of the constants describing used
option numbers. include/net/tcp.h does not have 'partial order' options
in particular.
> >+config NETFILTER_XT_MATCH_OSF
> >+ tristate '"osf" Passive OS fingerprint match'
> >+ depends on NETFILTER_ADVANCED
>
> && NFNETLINK
Will add.
> >--- /dev/null
> >+++ b/net/netfilter/xt_osf.c
> >@@ -0,0 +1,469 @@
> >+/*
> >+ * Copyright (c) 2003+ Evgeniy Polyakov <zbr@...emap.net>
> >+ *
> >+ *
> >+ * 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.
> >+ *
> >+ * This program is distributed in the hope that it will be useful,
> >+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
> >+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
> >+ * GNU General Public License for more details.
> >+ *
> >+ * You should have received a copy of the GNU General Public License
> >+ * along with this program; if not, write to the Free Software
> >+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
> >+ */
> >+
> >+#include <linux/module.h>
> >+#include <linux/kernel.h>
> >+
> >+#include <linux/connector.h>
>
> Not needed. The remaining ones look like some (percpu?) could be
> removed as well.
Yup.
> >+struct xt_osf_finger_storage
> >+{
>
> Please place the opening bracket consistently with the other
> structure definitions.
I.e. always on the new line? :)
> >+ struct list_head finger_list;
> >+ spinlock_t finger_lock;
> >+};
> >+
> >+/*
> >+ * Indexed by dont-fragment bit.
> >+ * It is the only constant value in the fingerprint.
> >+ */
> >+struct xt_osf_finger_storage xt_osf_fingers[2];
>
> static
Ok.
> >+
> >+struct xt_osf_message {
> >+ struct cn_msg cmsg;
> >+ struct xt_osf_nlmsg nlmsg;
> >+};
>
> Unused.
Yes.
> >+static int xt_osf_setup_callback(struct sock *ctnl, struct sk_buff *skb,
> >+ struct nlmsghdr *nlh, struct nlattr *osf_attrs[])
> >+{
> >+ struct xt_osf_user_finger *f;
> >+ struct nfgenmsg *nfmsg = NLMSG_DATA(nlh);
> >+ u16 delete = ntohs(nfmsg->res_id);
>
> This looks like abuse, we use message types to distinguish between
> additions and deletions, alternative NLM_F_REPLACE.
Why to introduce the whole new callback function and attribute when the
only difference is to add or remove a processed entry?
> >+ struct xt_osf_finger *kf = NULL, *sf;
> >+ struct xt_osf_finger_storage *st;
> >+ int err;
> >+
> >+ if (!osf_attrs[OSF_ATTR_FINGER])
> >+ return -EINVAL;
> >+
> >+ f = nla_data(osf_attrs[OSF_ATTR_FINGER]);
> >+ st = &xt_osf_fingers[!!f->df];
> >+
> >+ /*
> >+ * If 'delete' is set to 0 then we add attached fingerprint,
> >+ * otherwise remove, and in this case we do not need to allocate
> >data.
> >+ */
> >+ if (!delete) {
> >+ kf = kmalloc(sizeof(struct xt_osf_finger), GFP_KERNEL);
> >+ if (!kf)
> >+ return -ENOMEM;
> >+
> >+ memcpy(&kf->finger, f, sizeof(struct xt_osf_user_finger));
> >+ }
> >+
> >+ err = -ENOENT;
> >+
> >+ rcu_read_lock();
> >+ list_for_each_entry_rcu(sf, &st->finger_list, finger_entry) {
> >+ if (memcmp(&sf->finger, f, sizeof(struct
> >xt_osf_user_finger)))
> >+ continue;
> >+
> >+ if (delete) {
> >+ spin_lock_bh(&st->finger_lock);
>
> This lock looks useless, all changes are done in netlink context under
> the nfnl mutex.
Agree.
> >+ list_del_rcu(&sf->finger_entry);
> >+ spin_unlock_bh(&st->finger_lock);
> >+ call_rcu(&sf->rcu_head, xt_osf_finger_free_rcu);
> >+ } else {
> >+ kfree(kf);
> >+ kf = NULL;
> >+ }
> >+
> >+ err = 0;
> >+ break;
> >+ }
> >+
> >+ if (kf) {
> >+ spin_lock_bh(&st->finger_lock);
Here too.
> >+ list_add_tail_rcu(&kf->finger_entry, &st->finger_list);
> >+ spin_unlock_bh(&st->finger_lock);
> >+#if 0
> >+ printk(KERN_INFO "Added rule for %s:%s:%s.\n",
> >+ kf->finger.genre, kf->finger.version,
> >kf->finger.subtype);
> >+#endif
> >+ }
> >+ rcu_read_unlock();
> >+
> >+ return 0;
> >+}
...
> >+static bool xt_osf_match_packet(const struct sk_buff *skb,
> >+ const struct xt_match_param *p)
> >+{
> >+ const struct xt_osf_info *info = p->matchinfo;
> >+ const struct iphdr *ip = ip_hdr(skb);
> >+ const struct tcphdr *tcp;
> >+ struct tcphdr _tcph;
> >+ int fmatch = FMATCH_WRONG, fcount = 0;
> >+ unsigned int optsize = 0, check_WSS = 0;
> >+ u16 window, totlen, mss = 0;
> >+ bool df;
> >+ const unsigned char *optp = NULL, *_optp = NULL;
> >+ unsigned char opts[MAX_IPOPTLEN];
> >+ const struct xt_osf_finger *kf;
> >+ const struct xt_osf_user_finger *f;
> >+ const struct xt_osf_finger_storage *st;
> >+
> >+ if (!info)
> >+ return false;
> >+
> >+ tcp = skb_header_pointer(skb, ip_hdrlen(skb), sizeof(struct tcphdr),
> >&_tcph);
> >+ if (!tcp)
> >+ return false;
> >+
> >+ if (!tcp->syn)
> >+ return false;
> >+
> >+ totlen = ntohs(ip->tot_len);
> >+ df = ntohs(ip->frag_off) & IP_DF;
> >+ window = ntohs(tcp->window);
> >+
> >+ if (tcp->doff * 4 > sizeof(struct tcphdr)) {
> >+ optsize = tcp->doff * 4 - sizeof(struct tcphdr);
> >+
> >+ if (optsize > sizeof(opts))
> >+ optsize = sizeof(opts);
> >+
> >+ _optp = optp = skb_header_pointer(skb, ip_hdrlen(skb) +
> >sizeof(struct tcphdr),
>
> Please break the line at 80 characters
Will do.
> >+ optsize, opts);
> >+ }
> >+
> >+ st = &xt_osf_fingers[df];
> >+
> >+ rcu_read_lock();
> >+ list_for_each_entry_rcu(kf, &st->finger_list, finger_entry) {
> >+ f = &kf->finger;
> >+
> >+ if (!(info->flags & XT_OSF_LOG) && strcmp(info->genre,
> >f->genre))
> >+ continue;
> >+
> >+ optp = _optp;
> >+ fmatch = FMATCH_WRONG;
> >+
> >+ if (totlen == f->ss && xt_osf_ttl(skb, info, f->ttl)) {
> >+ int foptsize, optnum;
> >+
> >+ check_WSS = 0;
> >+
> >+ switch (f->wss.wc) {
> >+ case 0:
> >+ check_WSS = 0;
> >+ break;
> >+ case 'S':
> >+ check_WSS = 1;
> >+ break;
> >+ case 'T':
> >+ check_WSS = 2;
> >+ break;
> >+ case '%':
> >+ check_WSS = 3;
> >+ break;
> >+ default:
> >+ check_WSS = 4;
> >+ break;
> >+ }
>
> This is really pushing my taste-buds. Whatever this does, please at
> use symbolic constants so the reader at least has a chance to understand
> it.
That's a bit unlcear window size processing state machine.
It links together knowledge about window-size, mss, mtu dependancy on
plain size numbers and modulo operations (like window size is multiple
of x bytes).
> >+ if (check_WSS == 4)
> >+ continue;
> >+
> >+ /* Check options */
> >+
> >+ foptsize = 0;
> >+ for (optnum = 0; optnum < f->opt_num; ++optnum)
> >+ foptsize += f->opt[optnum].length;
> >+
> >+ if (foptsize > MAX_IPOPTLEN || optsize >
> >MAX_IPOPTLEN || optsize != foptsize)
> >+ continue;
> >+
> >+ for (optnum = 0; optnum < f->opt_num; ++optnum) {
> >+ if (f->opt[optnum].kind == (*optp)) {
> >+ __u32 len = f->opt[optnum].length;
> >+ const __u8 *optend = optp + len;
> >+ int loop_cont = 0;
> >+
> >+ fmatch = FMATCH_OK;
> >+
> >+ switch (*optp) {
> >+ case OSFOPT_MSS:
> >+ mss = optp[3];
> >+ mss <<= 8;
> >+ mss |= optp[2];
> >+
> >+ mss = ntohs(mss);
> >+ break;
> >+ case OSFOPT_TS:
> >+ loop_cont = 1;
> >+ break;
> >+ }
> >+
> >+ optp = optend;
> >+ } else
> >+ fmatch = FMATCH_OPT_WRONG;
> >+
> >+ if (fmatch != FMATCH_OK)
> >+ break;
> >+ }
> >+
> >+ if (fmatch != FMATCH_OPT_WRONG) {
> >+ fmatch = FMATCH_WRONG;
> >+
> >+ switch (check_WSS) {
> >+ case 0:
> >+ if (f->wss.val == 0 || window ==
> >f->wss.val)
> >+ fmatch = FMATCH_OK;
> >+ break;
> >+ case 1: /* MSS */
> >+#define SMART_MSS_1 1460
> >+#define SMART_MSS_2 1448
>
> Sigh. This entire function is completely unreadable and full of
> unexplained magic. I'll stop here, please clean this before
> resubmitting.
This is a special hack for special modems, which can decrease mss, and
since there is no common knowledge on how to differentiate them, there
is a check against different types.
This function is a core processing state machine, which checks received
packet parameters and compares them against prebuilt set of rules, which
include not only simple equal/not-equal, but also a bit more complex,
like multiple of (various parameter, mss, window-size, plain number).
--
Evgeniy Polyakov
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