Commit 878aed8d authored by Florian Westphal's avatar Florian Westphal Committed by Pablo Neira Ayuso
Browse files

netfilter: nat: force port remap to prevent shadowing well-known ports 



If destination port is above 32k and source port below 16k
assume this might cause 'port shadowing' where a 'new' inbound
connection matches an existing one, e.g.

inbound X:41234 -> Y:53 matches existing conntrack entry
        Z:53 -> X:4123, where Z got natted to X.

In this case, new packet is natted to Z:53 which is likely
unwanted.

We avoid the rewrite for connections that originate from local host:
port-shadowing is only possible with forwarded connections.

Also adjust test case.

v3: no need to call tuple_force_port_remap if already in random mode (Phil)

Signed-off-by: default avatarFlorian Westphal <fw@strlen.de>
Acked-by: default avatarPhil Sutter <phil@nwl.cc>
Acked-by: default avatarEric Garver <eric@garver.life>
Signed-off-by: default avatarPablo Neira Ayuso <pablo@netfilter.org>
parent 4a6fbdd8

net/netfilter/nf_nat_core.c

+40 −3
Original line number Diff line number Diff line
@@ -494,6 +494,38 @@ static void nf_nat_l4proto_unique_tuple(struct nf_conntrack_tuple *tuple, 
	goto another_round;

}



static bool tuple_force_port_remap(const struct nf_conntrack_tuple *tuple)

{

	u16 sp, dp;



	switch (tuple->dst.protonum) {

	case IPPROTO_TCP:

		sp = ntohs(tuple->src.u.tcp.port);

		dp = ntohs(tuple->dst.u.tcp.port);

		break;

	case IPPROTO_UDP:

	case IPPROTO_UDPLITE:

		sp = ntohs(tuple->src.u.udp.port);

		dp = ntohs(tuple->dst.u.udp.port);

		break;

	default:

		return false;

	}



	/* IANA: System port range: 1-1023,

	 *         user port range: 1024-49151,

	 *      private port range: 49152-65535.

	 *

	 * Linux default ephemeral port range is 32768-60999.

	 *

	 * Enforce port remapping if sport is significantly lower

	 * than dport to prevent NAT port shadowing, i.e.

	 * accidental match of 'new' inbound connection vs.

	 * existing outbound one.

	 */

	return sp < 16384 && dp >= 32768;

}



/* Manipulate the tuple into the range given. For NF_INET_POST_ROUTING,

 * we change the source to map into the range. For NF_INET_PRE_ROUTING

 * and NF_INET_LOCAL_OUT, we change the destination to map into the
@@ -507,11 +539,17 @@ get_unique_tuple(struct nf_conntrack_tuple *tuple, 
		 struct nf_conn *ct,

		 enum nf_nat_manip_type maniptype)

{

	bool random_port = range->flags & NF_NAT_RANGE_PROTO_RANDOM_ALL;

	const struct nf_conntrack_zone *zone;

	struct net *net = nf_ct_net(ct);



	zone = nf_ct_zone(ct);



	if (maniptype == NF_NAT_MANIP_SRC &&

	    !random_port &&

	    !ct->local_origin)

		random_port = tuple_force_port_remap(orig_tuple);



	/* 1) If this srcip/proto/src-proto-part is currently mapped,

	 * and that same mapping gives a unique tuple within the given

	 * range, use that.
@@ -520,8 +558,7 @@ get_unique_tuple(struct nf_conntrack_tuple *tuple, 
	 * So far, we don't do local source mappings, so multiple

	 * manips not an issue.

	 */

	if (maniptype == NF_NAT_MANIP_SRC &&

	    !(range->flags & NF_NAT_RANGE_PROTO_RANDOM_ALL)) {

	if (maniptype == NF_NAT_MANIP_SRC && !random_port) {

		/* try the original tuple first */

		if (in_range(orig_tuple, range)) {

			if (!nf_nat_used_tuple(orig_tuple, ct)) {
@@ -545,7 +582,7 @@ get_unique_tuple(struct nf_conntrack_tuple *tuple, 
	 */



	/* Only bother mapping if it's not already in range and unique */

	if (!(range->flags & NF_NAT_RANGE_PROTO_RANDOM_ALL)) {

	if (!random_port) {

		if (range->flags & NF_NAT_RANGE_PROTO_SPECIFIED) {

			if (!(range->flags & NF_NAT_RANGE_PROTO_OFFSET) &&

			    l4proto_in_range(tuple, maniptype,

tools/testing/selftests/netfilter/nft_nat.sh

+3 −2
Original line number Diff line number Diff line
@@ -880,8 +880,9 @@ EOF 
		return $ksft_skip

	fi



	# test default behaviour. Packet from ns1 to ns0 is redirected to ns2.

	test_port_shadow "default" "CLIENT"

	# test default behaviour. Packet from ns1 to ns0 is not redirected

	# due to automatic port translation.

	test_port_shadow "default" "ROUTER"



	# test packet filter based mitigation: prevent forwarding of

	# packets claiming to come from the service port.