audit: log audit netlink multicast bind and unbind events

Log information about programs connecting to and disconnecting from the
audit netlink multicast socket. This is needed so that during
investigations a security officer can tell who or what had access to the
audit trail.  This helps to meet the FAU_SAR.2 requirement for Common
Criteria.  Here is the systemd startup event:

type=UNKNOWN[1335] msg=audit(2020-01-17 10:30:33.731:6) : pid=1 uid=root auid=unset tty=(none) ses=unset subj=kernel comm=systemd exe=/usr/lib/systemd/systemd nl-mcgrp=1 op=connect res=yes

And the events from the test suite:

type=PROCTITLE msg=audit(2020-01-17 10:36:24.050:294) : proctitle=/usr/bin/perl -w amcast_joinpart/test
type=SOCKADDR msg=audit(2020-01-17 10:36:24.050:294) : saddr={ saddr_fam=netlink nlnk-fam=16 nlnk-pid=0 }
type=SYSCALL msg=audit(2020-01-17 10:36:24.050:294) : arch=x86_64 syscall=bind success=yes exit=0 a0=0x7 a1=0x55d65cb79090 a2=0xc a3=0x0 items=0 ppid=671 pid=674 auid=root uid=root gid=root euid=root suid=root fsuid=root egid=root sgid=root fsgid=root tty=ttyS0 ses=3 comm=perl exe=/usr/bin/perl subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 key=(null)
type=UNKNOWN[1335] msg=audit(2020-01-17 10:36:24.050:294) : pid=674 uid=root auid=root tty=ttyS0 ses=3 subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 comm=perl exe=/usr/bin/perl nl-mcgrp=1 op=connect res=yes

type=UNKNOWN[1335] msg=audit(2020-01-17 10:36:24.051:295) : pid=674 uid=root auid=root tty=ttyS0 ses=3 subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 comm=perl exe=/usr/bin/perl nl-mcgrp=1 op=disconnect res=yes

Please see the upstream issue tracker:
  linux-audit/audit-kernel#28
  https://github.com/linux-audit/audit-kernel/wiki/RFE-Audit-Multicast-Socket-Join-Part
  https://github.com/rgbriggs/audit-testsuite/compare/ghak28-mcast-part-join

Signed-off-by: Richard Guy Briggs <[email protected]>

include/uapi/linux/audit.h

@@ -116,6 +116,7 @@
 #define AUDIT_FANOTIFY		1331	/* Fanotify access decision */
 #define AUDIT_TIME_INJOFFSET	1332	/* Timekeeping offset injected */
 #define AUDIT_TIME_ADJNTPVAL	1333	/* NTP value adjustment */
+#define AUDIT_EVENT_LISTENER	1335	/* Task joined multicast read socket */
 
 #define AUDIT_AVC		1400	/* SE Linux avc denial or grant */
 #define AUDIT_SELINUX_ERR	1401	/* Internal SE Linux Errors */

kernel/audit.c

@@ -1520,20 +1520,60 @@ static void audit_receive(struct sk_buff  *skb)
 	audit_ctl_unlock();
 }
 
+/* Log information about who is connecting to the audit multicast socket */
+static void audit_log_multicast_bind(int group, const char *op, int err)
+{
+	const struct cred *cred;
+	struct tty_struct *tty;
+	char comm[sizeof(current->comm)];
+	struct audit_buffer *ab;
+
+	if (!audit_enabled)
+		return;
+
+	ab = audit_log_start(audit_context(), GFP_KERNEL, AUDIT_EVENT_LISTENER);
+	if (!ab)
+		return;
+
+	cred = current_cred();
+	tty = audit_get_tty();
+	audit_log_format(ab, "pid=%u uid=%u auid=%u tty=%s ses=%u",
+			 task_pid_nr(current),
+			 from_kuid(&init_user_ns, cred->uid),
+			 from_kuid(&init_user_ns, audit_get_loginuid(current)),
+			 tty ? tty_name(tty) : "(none)",
+			 audit_get_sessionid(current));
+	audit_put_tty(tty);
+	audit_log_task_context(ab); /* subj= */
+	audit_log_format(ab, " comm=");
+	audit_log_untrustedstring(ab, get_task_comm(comm, current));
+	audit_log_d_path_exe(ab, current->mm); /* exe= */
+	audit_log_format(ab, " nl-mcgrp=%d op=%s res=%d", group, op, !err);
+	audit_log_end(ab);
+}
+
 /* Run custom bind function on netlink socket group connect or bind requests. */
-static int audit_bind(struct net *net, int group)
+static int audit_multicast_bind(struct net *net, int group)
 {
+	int err = 0;
+
 	if (!capable(CAP_AUDIT_READ))
-		return -EPERM;
+		err = -EPERM;
+	audit_log_multicast_bind(group, "connect", err);
+	return err;
+}
 
-	return 0;
+static void audit_multicast_unbind(struct net *net, int group)
+{
+	audit_log_multicast_bind(group, "disconnect", 0);
 }
 
 static int __net_init audit_net_init(struct net *net)
 {
 	struct netlink_kernel_cfg cfg = {
 		.input	= audit_receive,
-		.bind	= audit_bind,
+		.bind	= audit_multicast_bind,
+		.unbind	= audit_multicast_unbind,
 		.flags	= NL_CFG_F_NONROOT_RECV,
 		.groups	= AUDIT_NLGRP_MAX,
 	};