Introduce a sysctl that modifies the value of PROT_SOCK.

Add net.ipv4.ip_unprivileged_port_start, which is a per namespace sysctl
that denotes the first unprivileged inet port in the namespace.  To
disable all privileged ports set this to zero.  It also checks for
overlap with the local port range.  The privileged and local range may
not overlap.

The use case for this change is to allow containerized processes to bind
to priviliged ports, but prevent them from ever being allowed to modify
their container's network configuration.  The latter is accomplished by
ensuring that the network namespace is not a child of the user
namespace.  This modification was needed to allow the container manager
to disable a namespace's priviliged port restrictions without exposing
control of the network namespace to processes in the user namespace.

Signed-off-by: Krister Johansen <kjlx@templeofstupid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 53 insertions, 2 deletions

diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
index aae410bb655a..28fe8da4e1ac 100644
--- a/net/ipv4/af_inet.c
+++ b/net/ipv4/af_inet.c
@@ -479,7 +479,7 @@ int inet_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
 
 	snum = ntohs(addr->sin_port);
 	err = -EACCES;
-	if (snum && snum < PROT_SOCK &&
+	if (snum && snum < inet_prot_sock(net) &&
 	    !ns_capable(net->user_ns, CAP_NET_BIND_SERVICE))
 		goto out;
 
@@ -1700,6 +1700,9 @@ static __net_init int inet_init_net(struct net *net)
 	net->ipv4.sysctl_ip_default_ttl = IPDEFTTL;
 	net->ipv4.sysctl_ip_dynaddr = 0;
 	net->ipv4.sysctl_ip_early_demux = 1;
+#ifdef CONFIG_SYSCTL
+	net->ipv4.sysctl_ip_prot_sock = PROT_SOCK;
+#endif
 
 	return 0;
 }
diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index c8d283615c6f..1b861997fdc5 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -35,6 +35,8 @@ static int ip_local_port_range_min[] = { 1, 1 };
 static int ip_local_port_range_max[] = { 65535, 65535 };
 static int tcp_adv_win_scale_min = -31;
 static int tcp_adv_win_scale_max = 31;
+static int ip_privileged_port_min;
+static int ip_privileged_port_max = 65535;
 static int ip_ttl_min = 1;
 static int ip_ttl_max = 255;
 static int tcp_syn_retries_min = 1;
@@ -79,7 +81,12 @@ static int ipv4_local_port_range(struct ctl_table *table, int write,
 	ret = proc_dointvec_minmax(&tmp, write, buffer, lenp, ppos);
 
 	if (write && ret == 0) {
-		if (range[1] < range[0])
+		/* Ensure that the upper limit is not smaller than the lower,
+		 * and that the lower does not encroach upon the privileged
+		 * port limit.
+		 */
+		if ((range[1] < range[0]) ||
+		    (range[0] < net->ipv4.sysctl_ip_prot_sock))
 			ret = -EINVAL;
 		else
 			set_local_port_range(net, range);
@@ -88,6 +95,40 @@ static int ipv4_local_port_range(struct ctl_table *table, int write,
 	return ret;
 }
 
+/* Validate changes from /proc interface. */
+static int ipv4_privileged_ports(struct ctl_table *table, int write,
+				void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+	struct net *net = container_of(table->data, struct net,
+	    ipv4.sysctl_ip_prot_sock);
+	int ret;
+	int pports;
+	int range[2];
+	struct ctl_table tmp = {
+		.data = &pports,
+		.maxlen = sizeof(pports),
+		.mode = table->mode,
+		.extra1 = &ip_privileged_port_min,
+		.extra2 = &ip_privileged_port_max,
+	};
+
+	pports = net->ipv4.sysctl_ip_prot_sock;
+
+	ret = proc_dointvec_minmax(&tmp, write, buffer, lenp, ppos);
+
+	if (write && ret == 0) {
+		inet_get_local_port_range(net, &range[0], &range[1]);
+		/* Ensure that the local port range doesn't overlap with the
+		 * privileged port range.
+		 */
+		if (range[0] < pports)
+			ret = -EINVAL;
+		else
+			net->ipv4.sysctl_ip_prot_sock = pports;
+	}
+
+	return ret;
+}
 
 static void inet_get_ping_group_range_table(struct ctl_table *table, kgid_t *low, kgid_t *high)
 {
@@ -964,6 +1005,13 @@ static struct ctl_table ipv4_net_table[] = {
 		.extra2		= &one,
 	},
 #endif
+	{
+		.procname	= "ip_unprivileged_port_start",
+		.maxlen		= sizeof(int),
+		.data		= &init_net.ipv4.sysctl_ip_prot_sock,
+		.mode		= 0644,
+		.proc_handler	= ipv4_privileged_ports,
+	},
 	{ }
 };