![Florian Westphal](/assets/img/avatar_default.png)
Luca Moro says: ------ The issue lies in the filtering of ICMP and ICMPv6 errors that include an inner IP datagram. For these packets, icmp_error_message() extract the ICMP error and inner layer to search of a known state. If a state is found the packet is tagged as related (IP_CT_RELATED). The problem is that there is no correlation check between the inner and outer layer of the packet. So one can encapsulate an error with an inner layer matching a known state, while its outer layer is directed to a filtered host. In this case the whole packet will be tagged as related. This has various implications from a rule bypass (if a rule to related trafic is allow), to a known state oracle. Unfortunately, we could not find a real statement in a RFC on how this case should be filtered. The closest we found is RFC5927 (Section 4.3) but it is not very clear. A possible fix would be to check that the inner IP source is the same than the outer destination. We believed this kind of attack was not documented yet, so we started to write a blog post about it. You can find it attached to this mail (sorry for the extract quality). It contains more technical details, PoC and discussion about the identified behavior. We discovered later that https://www.gont.com.ar/papers/filtering-of-icmp-error-messages.pdf described a similar attack concept in 2004 but without the stateful filtering in mind. ----- This implements above suggested fix: In icmp(v6) error handler, take outer destination address, then pass that into the common function that does the "related" association. After obtaining the nf_conn of the matching inner-headers connection, check that the destination address of the opposite direction tuple is the same as the outer address and only set RELATED if thats the case. Reported-by: Luca Moro <luca.moro@synacktiv.com> Signed-off-by: Florian Westphal <fw@strlen.de> Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
378 lines
11 KiB
C
378 lines
11 KiB
C
/* (C) 1999-2001 Paul `Rusty' Russell
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* (C) 2002-2004 Netfilter Core Team <coreteam@netfilter.org>
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* (C) 2006-2010 Patrick McHardy <kaber@trash.net>
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License version 2 as
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* published by the Free Software Foundation.
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*/
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#include <linux/types.h>
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#include <linux/timer.h>
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#include <linux/netfilter.h>
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#include <linux/in.h>
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#include <linux/icmp.h>
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#include <linux/seq_file.h>
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#include <net/ip.h>
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#include <net/checksum.h>
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#include <linux/netfilter_ipv4.h>
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#include <net/netfilter/nf_conntrack_tuple.h>
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#include <net/netfilter/nf_conntrack_l4proto.h>
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#include <net/netfilter/nf_conntrack_core.h>
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#include <net/netfilter/nf_conntrack_timeout.h>
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#include <net/netfilter/nf_conntrack_zones.h>
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#include <net/netfilter/nf_log.h>
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static const unsigned int nf_ct_icmp_timeout = 30*HZ;
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bool icmp_pkt_to_tuple(const struct sk_buff *skb, unsigned int dataoff,
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struct net *net, struct nf_conntrack_tuple *tuple)
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{
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const struct icmphdr *hp;
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struct icmphdr _hdr;
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hp = skb_header_pointer(skb, dataoff, sizeof(_hdr), &_hdr);
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if (hp == NULL)
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return false;
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tuple->dst.u.icmp.type = hp->type;
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tuple->src.u.icmp.id = hp->un.echo.id;
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tuple->dst.u.icmp.code = hp->code;
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return true;
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}
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/* Add 1; spaces filled with 0. */
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static const u_int8_t invmap[] = {
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[ICMP_ECHO] = ICMP_ECHOREPLY + 1,
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[ICMP_ECHOREPLY] = ICMP_ECHO + 1,
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[ICMP_TIMESTAMP] = ICMP_TIMESTAMPREPLY + 1,
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[ICMP_TIMESTAMPREPLY] = ICMP_TIMESTAMP + 1,
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[ICMP_INFO_REQUEST] = ICMP_INFO_REPLY + 1,
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[ICMP_INFO_REPLY] = ICMP_INFO_REQUEST + 1,
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[ICMP_ADDRESS] = ICMP_ADDRESSREPLY + 1,
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[ICMP_ADDRESSREPLY] = ICMP_ADDRESS + 1
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};
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bool nf_conntrack_invert_icmp_tuple(struct nf_conntrack_tuple *tuple,
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const struct nf_conntrack_tuple *orig)
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{
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if (orig->dst.u.icmp.type >= sizeof(invmap) ||
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!invmap[orig->dst.u.icmp.type])
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return false;
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tuple->src.u.icmp.id = orig->src.u.icmp.id;
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tuple->dst.u.icmp.type = invmap[orig->dst.u.icmp.type] - 1;
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tuple->dst.u.icmp.code = orig->dst.u.icmp.code;
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return true;
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}
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/* Returns verdict for packet, or -1 for invalid. */
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int nf_conntrack_icmp_packet(struct nf_conn *ct,
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struct sk_buff *skb,
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enum ip_conntrack_info ctinfo,
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const struct nf_hook_state *state)
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{
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/* Do not immediately delete the connection after the first
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successful reply to avoid excessive conntrackd traffic
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and also to handle correctly ICMP echo reply duplicates. */
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unsigned int *timeout = nf_ct_timeout_lookup(ct);
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static const u_int8_t valid_new[] = {
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[ICMP_ECHO] = 1,
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[ICMP_TIMESTAMP] = 1,
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[ICMP_INFO_REQUEST] = 1,
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[ICMP_ADDRESS] = 1
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};
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if (state->pf != NFPROTO_IPV4)
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return -NF_ACCEPT;
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if (ct->tuplehash[0].tuple.dst.u.icmp.type >= sizeof(valid_new) ||
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!valid_new[ct->tuplehash[0].tuple.dst.u.icmp.type]) {
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/* Can't create a new ICMP `conn' with this. */
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pr_debug("icmp: can't create new conn with type %u\n",
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ct->tuplehash[0].tuple.dst.u.icmp.type);
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nf_ct_dump_tuple_ip(&ct->tuplehash[0].tuple);
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return -NF_ACCEPT;
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}
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if (!timeout)
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timeout = &nf_icmp_pernet(nf_ct_net(ct))->timeout;
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nf_ct_refresh_acct(ct, ctinfo, skb, *timeout);
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return NF_ACCEPT;
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}
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/* Check inner header is related to any of the existing connections */
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int nf_conntrack_inet_error(struct nf_conn *tmpl, struct sk_buff *skb,
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unsigned int dataoff,
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const struct nf_hook_state *state,
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u8 l4proto, union nf_inet_addr *outer_daddr)
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{
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struct nf_conntrack_tuple innertuple, origtuple;
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const struct nf_conntrack_tuple_hash *h;
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const struct nf_conntrack_zone *zone;
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enum ip_conntrack_info ctinfo;
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struct nf_conntrack_zone tmp;
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union nf_inet_addr *ct_daddr;
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enum ip_conntrack_dir dir;
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struct nf_conn *ct;
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WARN_ON(skb_nfct(skb));
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zone = nf_ct_zone_tmpl(tmpl, skb, &tmp);
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/* Are they talking about one of our connections? */
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if (!nf_ct_get_tuplepr(skb, dataoff,
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state->pf, state->net, &origtuple))
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return -NF_ACCEPT;
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/* Ordinarily, we'd expect the inverted tupleproto, but it's
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been preserved inside the ICMP. */
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if (!nf_ct_invert_tuple(&innertuple, &origtuple))
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return -NF_ACCEPT;
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h = nf_conntrack_find_get(state->net, zone, &innertuple);
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if (!h)
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return -NF_ACCEPT;
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/* Consider: A -> T (=This machine) -> B
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* Conntrack entry will look like this:
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* Original: A->B
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* Reply: B->T (SNAT case) OR A
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*
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* When this function runs, we got packet that looks like this:
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* iphdr|icmphdr|inner_iphdr|l4header (tcp, udp, ..).
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*
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* Above nf_conntrack_find_get() makes lookup based on inner_hdr,
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* so we should expect that destination of the found connection
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* matches outer header destination address.
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*
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* In above example, we can consider these two cases:
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* 1. Error coming in reply direction from B or M (middle box) to
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* T (SNAT case) or A.
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* Inner saddr will be B, dst will be T or A.
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* The found conntrack will be reply tuple (B->T/A).
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* 2. Error coming in original direction from A or M to B.
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* Inner saddr will be A, inner daddr will be B.
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* The found conntrack will be original tuple (A->B).
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*
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* In both cases, conntrack[dir].dst == inner.dst.
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*
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* A bogus packet could look like this:
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* Inner: B->T
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* Outer: B->X (other machine reachable by T).
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*
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* In this case, lookup yields connection A->B and will
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* set packet from B->X as *RELATED*, even though no connection
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* from X was ever seen.
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*/
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ct = nf_ct_tuplehash_to_ctrack(h);
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dir = NF_CT_DIRECTION(h);
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ct_daddr = &ct->tuplehash[dir].tuple.dst.u3;
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if (!nf_inet_addr_cmp(outer_daddr, ct_daddr)) {
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if (state->pf == AF_INET) {
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nf_l4proto_log_invalid(skb, state->net, state->pf,
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l4proto,
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"outer daddr %pI4 != inner %pI4",
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&outer_daddr->ip, &ct_daddr->ip);
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} else if (state->pf == AF_INET6) {
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nf_l4proto_log_invalid(skb, state->net, state->pf,
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l4proto,
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"outer daddr %pI6 != inner %pI6",
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&outer_daddr->ip6, &ct_daddr->ip6);
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}
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nf_ct_put(ct);
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return -NF_ACCEPT;
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}
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ctinfo = IP_CT_RELATED;
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if (dir == IP_CT_DIR_REPLY)
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ctinfo += IP_CT_IS_REPLY;
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/* Update skb to refer to this connection */
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nf_ct_set(skb, ct, ctinfo);
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return NF_ACCEPT;
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}
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static void icmp_error_log(const struct sk_buff *skb,
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const struct nf_hook_state *state,
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const char *msg)
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{
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nf_l4proto_log_invalid(skb, state->net, state->pf,
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IPPROTO_ICMP, "%s", msg);
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}
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/* Small and modified version of icmp_rcv */
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int nf_conntrack_icmpv4_error(struct nf_conn *tmpl,
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struct sk_buff *skb, unsigned int dataoff,
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const struct nf_hook_state *state)
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{
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union nf_inet_addr outer_daddr;
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const struct icmphdr *icmph;
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struct icmphdr _ih;
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/* Not enough header? */
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icmph = skb_header_pointer(skb, dataoff, sizeof(_ih), &_ih);
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if (icmph == NULL) {
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icmp_error_log(skb, state, "short packet");
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return -NF_ACCEPT;
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}
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/* See ip_conntrack_proto_tcp.c */
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if (state->net->ct.sysctl_checksum &&
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state->hook == NF_INET_PRE_ROUTING &&
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nf_ip_checksum(skb, state->hook, dataoff, 0)) {
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icmp_error_log(skb, state, "bad hw icmp checksum");
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return -NF_ACCEPT;
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}
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/*
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* 18 is the highest 'known' ICMP type. Anything else is a mystery
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*
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* RFC 1122: 3.2.2 Unknown ICMP messages types MUST be silently
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* discarded.
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*/
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if (icmph->type > NR_ICMP_TYPES) {
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icmp_error_log(skb, state, "invalid icmp type");
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return -NF_ACCEPT;
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}
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/* Need to track icmp error message? */
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if (icmph->type != ICMP_DEST_UNREACH &&
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icmph->type != ICMP_SOURCE_QUENCH &&
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icmph->type != ICMP_TIME_EXCEEDED &&
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icmph->type != ICMP_PARAMETERPROB &&
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icmph->type != ICMP_REDIRECT)
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return NF_ACCEPT;
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memset(&outer_daddr, 0, sizeof(outer_daddr));
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outer_daddr.ip = ip_hdr(skb)->daddr;
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dataoff += sizeof(*icmph);
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return nf_conntrack_inet_error(tmpl, skb, dataoff, state,
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IPPROTO_ICMP, &outer_daddr);
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}
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#if IS_ENABLED(CONFIG_NF_CT_NETLINK)
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#include <linux/netfilter/nfnetlink.h>
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#include <linux/netfilter/nfnetlink_conntrack.h>
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static int icmp_tuple_to_nlattr(struct sk_buff *skb,
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const struct nf_conntrack_tuple *t)
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{
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if (nla_put_be16(skb, CTA_PROTO_ICMP_ID, t->src.u.icmp.id) ||
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nla_put_u8(skb, CTA_PROTO_ICMP_TYPE, t->dst.u.icmp.type) ||
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nla_put_u8(skb, CTA_PROTO_ICMP_CODE, t->dst.u.icmp.code))
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goto nla_put_failure;
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return 0;
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nla_put_failure:
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return -1;
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}
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static const struct nla_policy icmp_nla_policy[CTA_PROTO_MAX+1] = {
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[CTA_PROTO_ICMP_TYPE] = { .type = NLA_U8 },
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[CTA_PROTO_ICMP_CODE] = { .type = NLA_U8 },
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[CTA_PROTO_ICMP_ID] = { .type = NLA_U16 },
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};
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static int icmp_nlattr_to_tuple(struct nlattr *tb[],
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struct nf_conntrack_tuple *tuple)
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{
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if (!tb[CTA_PROTO_ICMP_TYPE] ||
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!tb[CTA_PROTO_ICMP_CODE] ||
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!tb[CTA_PROTO_ICMP_ID])
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return -EINVAL;
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tuple->dst.u.icmp.type = nla_get_u8(tb[CTA_PROTO_ICMP_TYPE]);
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tuple->dst.u.icmp.code = nla_get_u8(tb[CTA_PROTO_ICMP_CODE]);
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tuple->src.u.icmp.id = nla_get_be16(tb[CTA_PROTO_ICMP_ID]);
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if (tuple->dst.u.icmp.type >= sizeof(invmap) ||
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!invmap[tuple->dst.u.icmp.type])
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return -EINVAL;
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return 0;
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}
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static unsigned int icmp_nlattr_tuple_size(void)
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{
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static unsigned int size __read_mostly;
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if (!size)
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size = nla_policy_len(icmp_nla_policy, CTA_PROTO_MAX + 1);
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return size;
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}
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#endif
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#ifdef CONFIG_NF_CONNTRACK_TIMEOUT
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#include <linux/netfilter/nfnetlink.h>
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#include <linux/netfilter/nfnetlink_cttimeout.h>
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static int icmp_timeout_nlattr_to_obj(struct nlattr *tb[],
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struct net *net, void *data)
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{
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unsigned int *timeout = data;
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struct nf_icmp_net *in = nf_icmp_pernet(net);
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if (tb[CTA_TIMEOUT_ICMP_TIMEOUT]) {
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if (!timeout)
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timeout = &in->timeout;
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*timeout =
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ntohl(nla_get_be32(tb[CTA_TIMEOUT_ICMP_TIMEOUT])) * HZ;
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} else if (timeout) {
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/* Set default ICMP timeout. */
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*timeout = in->timeout;
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}
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return 0;
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}
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static int
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icmp_timeout_obj_to_nlattr(struct sk_buff *skb, const void *data)
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{
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const unsigned int *timeout = data;
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if (nla_put_be32(skb, CTA_TIMEOUT_ICMP_TIMEOUT, htonl(*timeout / HZ)))
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goto nla_put_failure;
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return 0;
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nla_put_failure:
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return -ENOSPC;
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}
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static const struct nla_policy
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icmp_timeout_nla_policy[CTA_TIMEOUT_ICMP_MAX+1] = {
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[CTA_TIMEOUT_ICMP_TIMEOUT] = { .type = NLA_U32 },
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};
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#endif /* CONFIG_NF_CONNTRACK_TIMEOUT */
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void nf_conntrack_icmp_init_net(struct net *net)
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{
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struct nf_icmp_net *in = nf_icmp_pernet(net);
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in->timeout = nf_ct_icmp_timeout;
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}
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const struct nf_conntrack_l4proto nf_conntrack_l4proto_icmp =
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{
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.l4proto = IPPROTO_ICMP,
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#if IS_ENABLED(CONFIG_NF_CT_NETLINK)
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.tuple_to_nlattr = icmp_tuple_to_nlattr,
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.nlattr_tuple_size = icmp_nlattr_tuple_size,
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.nlattr_to_tuple = icmp_nlattr_to_tuple,
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.nla_policy = icmp_nla_policy,
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#endif
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#ifdef CONFIG_NF_CONNTRACK_TIMEOUT
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.ctnl_timeout = {
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.nlattr_to_obj = icmp_timeout_nlattr_to_obj,
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.obj_to_nlattr = icmp_timeout_obj_to_nlattr,
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.nlattr_max = CTA_TIMEOUT_ICMP_MAX,
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.obj_size = sizeof(unsigned int),
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.nla_policy = icmp_timeout_nla_policy,
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},
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#endif /* CONFIG_NF_CONNTRACK_TIMEOUT */
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};
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