Commit 8ed1dc44 authored by Hannes Frederic Sowa's avatar Hannes Frederic Sowa Committed by David S. Miller
Browse files

ipv4: introduce hardened ip_no_pmtu_disc mode 



This new ip_no_pmtu_disc mode only allowes fragmentation-needed errors
to be honored by protocols which do more stringent validation on the
ICMP's packet payload. This knob is useful for people who e.g. want to
run an unmodified DNS server in a namespace where they need to use pmtu
for TCP connections (as they are used for zone transfers or fallback
for requests) but don't want to use possibly spoofed UDP pmtu information.

Currently the whitelisted protocols are TCP, SCTP and DCCP as they check
if the returned packet is in the window or if the association is valid.

Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: David Miller <davem@davemloft.net>
Cc: John Heffner <johnwheffner@gmail.com>
Suggested-by: default avatarFlorian Weimer <fweimer@redhat.com>
Signed-off-by: default avatarHannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 0954cf9c

Documentation/networking/ip-sysctl.txt

+12 −1
Original line number Diff line number Diff line
@@ -26,7 +26,18 @@ ip_no_pmtu_disc - INTEGER 
	discarded. Outgoing frames are handled the same as in mode 1,

	implicitly setting IP_PMTUDISC_DONT on every created socket.



	Possible values: 0-2

	Mode 3 is a hardend pmtu discover mode. The kernel will only

	accept fragmentation-needed errors if the underlying protocol

	can verify them besides a plain socket lookup. Current

	protocols for which pmtu events will be honored are TCP, SCTP

	and DCCP as they verify e.g. the sequence number or the

	association. This mode should not be enabled globally but is

	only intended to secure e.g. name servers in namespaces where

	TCP path mtu must still work but path MTU information of other

	protocols should be discarded. If enabled globally this mode

	could break other protocols.



	Possible values: 0-3

	Default: FALSE



min_pmtu - INTEGER

include/net/protocol.h

+6 −1
Original line number Diff line number Diff line
@@ -43,7 +43,12 @@ struct net_protocol { 
	int			(*handler)(struct sk_buff *skb);

	void			(*err_handler)(struct sk_buff *skb, u32 info);

	unsigned int		no_policy:1,

				netns_ok:1;

				netns_ok:1,

				/* does the protocol do more stringent

				 * icmp tag validation than simple

				 * socket lookup?

				 */

				icmp_strict_tag_validation:1;

};



#if IS_ENABLED(CONFIG_IPV6)

net/dccp/ipv4.c

+1 −0
Original line number Diff line number Diff line
@@ -989,6 +989,7 @@ static const struct net_protocol dccp_v4_protocol = { 
	.err_handler	= dccp_v4_err,

	.no_policy	= 1,

	.netns_ok	= 1,

	.icmp_strict_tag_validation = 1,

};



static const struct proto_ops inet_dccp_ops = {

net/ipv4/af_inet.c

+1 −0
Original line number Diff line number Diff line
@@ -1545,6 +1545,7 @@ static const struct net_protocol tcp_protocol = { 
	.err_handler	=	tcp_v4_err,

	.no_policy	=	1,

	.netns_ok	=	1,

	.icmp_strict_tag_validation = 1,

};



static const struct net_protocol udp_protocol = {

net/ipv4/icmp.c

+24 −4
Original line number Diff line number Diff line
@@ -668,6 +668,16 @@ static void icmp_socket_deliver(struct sk_buff *skb, u32 info) 
	rcu_read_unlock();

}



static bool icmp_tag_validation(int proto)

{

	bool ok;



	rcu_read_lock();

	ok = rcu_dereference(inet_protos[proto])->icmp_strict_tag_validation;

	rcu_read_unlock();

	return ok;

}



/*

 *	Handle ICMP_DEST_UNREACH, ICMP_TIME_EXCEED, ICMP_QUENCH, and

 *	ICMP_PARAMETERPROB.
@@ -705,12 +715,22 @@ static void icmp_unreach(struct sk_buff *skb) 
		case ICMP_PORT_UNREACH:

			break;

		case ICMP_FRAG_NEEDED:

			if (net->ipv4.sysctl_ip_no_pmtu_disc == 2) {

				goto out;

			} else if (net->ipv4.sysctl_ip_no_pmtu_disc) {

			/* for documentation of the ip_no_pmtu_disc

			 * values please see

			 * Documentation/networking/ip-sysctl.txt

			 */

			switch (net->ipv4.sysctl_ip_no_pmtu_disc) {

			default:

				LIMIT_NETDEBUG(KERN_INFO pr_fmt("%pI4: fragmentation needed and DF set\n"),

					       &iph->daddr);

			} else {

				break;

			case 2:

				goto out;

			case 3:

				if (!icmp_tag_validation(iph->protocol))

					goto out;

				/* fall through */

			case 0:

				info = ntohs(icmph->un.frag.mtu);

				if (!info)

					goto out;

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