AUDIT: Allow login in non-init namespaces

It its possible to configure your PAM stack to refuse login if audit
messages (about the login) were unable to be sent.  This is common in
many distros and thus normal configuration of many containers.  The PAM
modules determine if audit is enabled/disabled in the kernel based on
the return value from sending an audit message on the netlink socket.
If userspace gets back ECONNREFUSED it believes audit is disabled in the
kernel.  If it gets any other error else it refuses to let the login
proceed.

Just about ever since the introduction of namespaces the kernel audit
subsystem has returned EPERM if the task sending a message was not in
the init user or pid namespace.  So many forms of containers have never
worked if audit was enabled in the kernel.

BUT if the container was not in net_init then the kernel network code
would send ECONNREFUSED (instead of the audit code sending EPERM).  Thus
by pure accident/dumb luck/bug if an admin configured the PAM stack to
reject all logins that didn't talk to audit, but then ran the login
untility in the non-init_net namespace, it would work!! Clearly this was
a bug, but it is a bug some people expected.

With the introduction of network namespace support in 3.14-rc1 the two
bugs stopped cancelling each other out.  Now, containers in the
non-init_net namespace refused to let users log in (just like PAM was
configfured!) Obviously some people were not happy that what used to let
users log in, now didn't!

This fix is kinda hacky.  We return ECONNREFUSED for all non-init
relevant namespaces.  That means that not only will the old broken
non-init_net setups continue to work, now the broken non-init_pid or
non-init_user setups will 'work'.  They don't really work, since audit
isn't logging things.  But it's what most users want.

In 3.15 we should have patches to support not only the non-init_net
(3.14) namespace but also the non-init_pid and non-init_user namespace.
So all will be right in the world.  This just opens the doors wide open
on 3.14 and hopefully makes users happy, if not the audit system...

Reported-by: Andre Tomt <andre@tomt.net>
Reported-by: Adam Richter <adam_richter2004@yahoo.com>
Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/audit.c

@@ -608,9 +608,19 @@ static int audit_netlink_ok(struct sk_buff *skb, u16 msg_type)
 	int err = 0;
 
 	/* Only support the initial namespaces for now. */
+	/*
+	 * We return ECONNREFUSED because it tricks userspace into thinking
+	 * that audit was not configured into the kernel.  Lots of users
+	 * configure their PAM stack (because that's what the distro does)
+	 * to reject login if unable to send messages to audit.  If we return
+	 * ECONNREFUSED the PAM stack thinks the kernel does not have audit
+	 * configured in and will let login proceed.  If we return EPERM
+	 * userspace will reject all logins.  This should be removed when we
+	 * support non init namespaces!!
+	 */
 	if ((current_user_ns() != &init_user_ns) ||
 	    (task_active_pid_ns(current) != &init_pid_ns))
-		return -EPERM;
+		return -ECONNREFUSED;
 
 	switch (msg_type) {
 	case AUDIT_LIST: