audit: log audit netlink multicast bind and unbind

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=PROCTITLE msg=audit(2020-04-22 10:10:21.787:10) : proctitle=/init
type=SYSCALL msg=audit(2020-04-22 10:10:21.787:10) : arch=x86_64 syscall=bind success=yes exit=0 a0=0x19 a1=0x555f4aac7e90 a2=0xc a3=0x7ffcb792ff44 items=0 ppid=0 pid=1 auid=unset uid=root gid=root euid=root suid=root fsuid=root egid=root sgid=root fsgid=root tty=(none) ses=unset comm=systemd exe=/usr/lib/systemd/systemd subj=kernel key=(null)
type=UNKNOWN[1335] msg=audit(2020-04-22 10:10:21.787:10) : 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 events from the test suite that just uses close():
type=PROCTITLE msg=audit(2020-04-22 11:47:08.501:442) : proctitle=/usr/bin/perl -w amcast_joinpart/test
type=SYSCALL msg=audit(2020-04-22 11:47:08.501:442) : arch=x86_64 syscall=bind success=yes exit=0 a0=0x7 a1=0x563004378760 a2=0xc a3=0x0 items=0 ppid=815 pid=818 auid=root uid=root gid=root euid=root suid=root fsuid=root egid=root sgid=root fsgid=root tty=ttyS0 ses=1 comm=perl exe=/usr/bin/perl subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 key=(null)
type=UNKNOWN[1335] msg=audit(2020-04-22 11:47:08.501:442) : pid=818 uid=root auid=root tty=ttyS0 ses=1 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(2020-04-22 11:47:08.501:443) : pid=818 uid=root auid=root tty=ttyS0 ses=1 subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 comm=perl exe=/usr/bin/perl nl-mcgrp=1 op=disconnect res=yes

And the events from the test suite using setsockopt with NETLINK_DROP_MEMBERSHIP:
type=PROCTITLE msg=audit(2020-04-22 11:39:53.291:439) : proctitle=/usr/bin/perl -w amcast_joinpart/test
type=SYSCALL msg=audit(2020-04-22 11:39:53.291:439) : arch=x86_64 syscall=bind success=yes exit=0 a0=0x7 a1=0x5560877c2d20 a2=0xc a3=0x0 items=0 ppid=772 pid=775 auid=root uid=root gid=root euid=root suid=root fsuid=root egid=root sgid=root fsgid=root tty=ttyS0 ses=1 comm=perl exe=/usr/bin/perl subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 key=(null)
type=UNKNOWN[1335] msg=audit(2020-04-22 11:39:53.291:439) : pid=775 uid=root auid=root tty=ttyS0 ses=1 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=PROCTITLE msg=audit(2020-04-22 11:39:53.292:440) : proctitle=/usr/bin/perl -w amcast_joinpart/test
type=SYSCALL msg=audit(2020-04-22 11:39:53.292:440) : arch=x86_64 syscall=setsockopt success=yes exit=0 a0=0x7 a1=SOL_NETLINK a2=0x2 a3=0x7ffc8366f000 items=0 ppid=772 pid=775 auid=root uid=root gid=root euid=root suid=root fsuid=root egid=root sgid=root fsgid=root tty=ttyS0 ses=1 comm=perl exe=/usr/bin/perl subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 key=(null)
type=UNKNOWN[1335] msg=audit(2020-04-22 11:39:53.292:440) : pid=775 uid=root auid=root tty=ttyS0 ses=1 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 at
  https://github.com/linux-audit/audit-kernel/issues/28
With the feature description at
  https://github.com/linux-audit/audit-kernel/wiki/RFE-Audit-Multicast-Socket-Join-Part
The testsuite support is at
  https://github.com/rgbriggs/audit-testsuite/compare/ghak28-mcast-part-join
  https://github.com/linux-audit/audit-testsuite/pull/93
And the userspace support patch is at
  https://github.com/linux-audit/audit-userspace/pull/114Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

include/uapi/linux/audit.h

@@ -117,6 +117,7 @@
 #define AUDIT_TIME_INJOFFSET	1332	/* Timekeeping offset injected */
 #define AUDIT_TIME_ADJNTPVAL	1333	/* NTP value adjustment */
 #define AUDIT_BPF		1334	/* BPF subsystem */
+#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

@@ -1530,20 +1530,60 @@ 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(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(group, "connect", err);
+	return err;
+}
 
-	return 0;
+static void audit_multicast_unbind(struct net *net, int group)
+{
+	audit_log_multicast(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,
 	};