net: add sysctl for enabling RFC 8335 PROBE messages

Section 8 of RFC 8335 specifies potential security concerns of
responding to PROBE requests, and states that nodes that support PROBE
functionality MUST be able to enable/disable responses and that
responses MUST be disabled by default
Signed-off-by: Andreas Roeseler <andreas.a.roeseler@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.rst

@@ -1143,6 +1143,12 @@ icmp_echo_ignore_all - BOOLEAN
 
 	Default: 0
 
+icmp_echo_enable_probe - BOOLEAN
+        If set to one, then the kernel will respond to RFC 8335 PROBE
+        requests sent to it.
+
+        Default: 0
+
 icmp_echo_ignore_broadcasts - BOOLEAN
 	If set non-zero, then the kernel will ignore all ICMP ECHO and
 	TIMESTAMP requests sent to it via broadcast/multicast.

include/net/netns/ipv4.h

@@ -84,6 +84,7 @@ struct netns_ipv4 {
 #endif
 
 	u8 sysctl_icmp_echo_ignore_all;
+	u8 sysctl_icmp_echo_enable_probe;
 	u8 sysctl_icmp_echo_ignore_broadcasts;
 	u8 sysctl_icmp_ignore_bogus_error_responses;
 	u8 sysctl_icmp_errors_use_inbound_ifaddr;

net/ipv4/sysctl_net_ipv4.c

@@ -598,6 +598,15 @@ static struct ctl_table ipv4_net_table[] = {
 		.mode		= 0644,
 		.proc_handler	= proc_dou8vec_minmax,
 	},
+	{
+		.procname	= "icmp_echo_enable_probe",
+		.data		= &init_net.ipv4.sysctl_icmp_echo_enable_probe,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= SYSCTL_ONE
+	},
 	{
 		.procname	= "icmp_echo_ignore_broadcasts",
 		.data		= &init_net.ipv4.sysctl_icmp_echo_ignore_broadcasts,