Add a new sysctl/tunable to mac_portacl:

    security.mac.portacl.autoport_exempt

This sysctl exempts to bind port '0' as long as IP_PORTRANGELOW hasn't
been set on the socket.  This is quite useful as it allows applications
to use automatic binding without adding overly broad rules for the
binding of port 0.  This sysctl defaults to enabled.

This is a slight variation on the patch submitted by the contributor.

MFC after:	2 weeks
Submitted by:	Michal Mertl <mime at traveller dot cz>

1 files changed, 23 insertions, 0 deletions

diff --git a/sys/security/mac_portacl/mac_portacl.c b/sys/security/mac_portacl/mac_portacl.c
index c7551bb6a5fb..07dd147e1ab5 100644
--- a/sys/security/mac_portacl/mac_portacl.c
+++ b/sys/security/mac_portacl/mac_portacl.c
@@ -79,6 +79,7 @@
 #include <sys/sysctl.h>
 
 #include <netinet/in.h>
+#include <netinet/in_pcb.h>
 
 #include <vm/vm.h>
 
@@ -100,6 +101,13 @@ SYSCTL_INT(_security_mac_portacl, OID_AUTO, suser_exempt, CTLFLAG_RW,
 TUNABLE_INT("security.mac.portacl.suser_exempt",
     &mac_portacl_suser_exempt);
 
+static int	mac_portacl_autoport_exempt = 1;
+SYSCTL_INT(_security_mac_portacl, OID_AUTO, autoport_exempt, CTLFLAG_RW,
+    &mac_portacl_autoport_exempt, 0, "Allow automatic allocation through "
+    "binding port 0 if not IP_PORTRANGELOW");
+TUNABLE_INT("security.mac.portacl.autoport_exempt",
+    &mac_portacl_autoport_exempt);
+
 static int	mac_portacl_port_high = 1023;
 SYSCTL_INT(_security_mac_portacl, OID_AUTO, port_high, CTLFLAG_RW,
     &mac_portacl_port_high, 0, "Highest port to enforce for");
@@ -434,6 +442,7 @@ check_socket_bind(struct ucred *cred, struct socket *so,
     struct label *socketlabel, struct sockaddr *sockaddr)
 {
 	struct sockaddr_in *sin;
+	struct inpcb *inp;
 	int family, type;
 	u_int16_t port;
 
@@ -461,6 +470,20 @@ check_socket_bind(struct ucred *cred, struct socket *so,
 	sin = (struct sockaddr_in *) sockaddr;
 	port = ntohs(sin->sin_port);
 
+	/*
+	 * Sockets are frequently bound with a specific IP address but a port
+	 * number of '0' to request automatic port allocation.  This is often
+	 * desirable as long as IP_PORTRANGELOW isn't set, which might permit
+	 * automatic allocation of a "privileged" port.  The autoport exempt
+	 * flag exempts port 0 allocation from rule checking as long as a low
+	 * port isn't required.
+	 */
+	if (mac_portacl_autoport_exempt && port == 0) {
+		inp = sotoinpcb(so);
+		if ((inp->inp_flags & INP_LOWPORT) == 0)
+			return (0);
+	}
+
 	return (rules_check(cred, family, type, port));
 }