audit: Limit audit requests to processes in the initial pid and user namespaces.

This allows the code to safely make the assumption that all of the
uids gids and pids that need to be send in audit messages are in the
initial namespaces.

If someone cares we may lift this restriction someday but start with
limiting access so at least the code is always correct.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Eric Paris <eparis@redhat.com>
Signed-off-by: N"Eric W. Biederman" <ebiederm@xmission.com>

kernel/audit.c

@@ -61,6 +61,7 @@
 #include <linux/netlink.h>
 #include <linux/freezer.h>
 #include <linux/tty.h>
+#include <linux/pid_namespace.h>
 
 #include "audit.h"
 
@@ -588,6 +589,11 @@ static int audit_netlink_ok(struct sk_buff *skb, u16 msg_type)
 {
 	int err = 0;
 
+	/* Only support the initial namespaces for now. */
+	if ((current_user_ns() != &init_user_ns) ||
+	    (task_active_pid_ns(current) != &init_pid_ns))
+		return -EPERM;
+
 	switch (msg_type) {
 	case AUDIT_GET:
 	case AUDIT_LIST: