audit: only allow tasks to set their loginuid if it is -1

At the moment we allow tasks to set their loginuid if they have
CAP_AUDIT_CONTROL.  In reality we want tasks to set the loginuid when they
log in and it be impossible to ever reset.  We had to make it mutable even
after it was once set (with the CAP) because on update and admin might have
to restart sshd.  Now sshd would get his loginuid and the next user which
logged in using ssh would not be able to set his loginuid.

Systemd has changed how userspace works and allowed us to make the kernel
work the way it should.  With systemd users (even admins) are not supposed
to restart services directly.  The system will restart the service for
them.  Thus since systemd is going to loginuid==-1, sshd would get -1, and
sshd would be allowed to set a new loginuid without special permissions.

If an admin in this system were to manually start an sshd he is inserting
himself into the system chain of trust and thus, logically, it's his
loginuid that should be used!  Since we have old systems I make this a
Kconfig option.
Signed-off-by: NEric Paris <eparis@redhat.com>

fs/proc/base.c

@@ -1197,9 +1197,6 @@ static ssize_t proc_loginuid_write(struct file * file, const char __user * buf,
 	ssize_t length;
 	uid_t loginuid;
 
-	if (!capable(CAP_AUDIT_CONTROL))
-		return -EPERM;
-
 	rcu_read_lock();
 	if (current != pid_task(proc_pid(inode), PIDTYPE_PID)) {
 		rcu_read_unlock();

init/Kconfig

@@ -372,6 +372,20 @@ config AUDIT_TREE
 	depends on AUDITSYSCALL
 	select FSNOTIFY
 
+config AUDIT_LOGINUID_IMMUTABLE
+	bool "Make audit loginuid immutable"
+	depends on AUDIT
+	help
+	  The config option toggles if a task setting it's loginuid requires
+	  CAP_SYS_AUDITCONTROL or if that task should require no special permissions
+	  but should instead only allow setting its loginuid if it was never
+	  previously set.  On systems which use systemd or a similar central
+	  process to restart login services this should be set to true.  On older
+	  systems in which an admin would typically have to directly stop and
+	  start processes this should be set to false.  Setting this to true allows
+	  one to drop potentially dangerous capabilites from the login tasks,
+	  but may not be backwards compatible with older init systems.
+
 source "kernel/irq/Kconfig"
 
 menu "RCU Subsystem"

kernel/auditsc.c

@@ -2173,9 +2173,18 @@ static atomic_t session_id = ATOMIC_INIT(0);
 int audit_set_loginuid(uid_t loginuid)
 {
 	struct task_struct *task = current;
-	unsigned int sessionid = atomic_inc_return(&session_id);
 	struct audit_context *context = task->audit_context;
+	unsigned int sessionid;
+
+#ifdef CONFIG_AUDIT_LOGINUID_IMMUTABLE
+	if (task->loginuid != -1)
+		return -EPERM;
+#else /* CONFIG_AUDIT_LOGINUID_IMMUTABLE */
+	if (!capable(CAP_AUDIT_CONTROL))
+		return -EPERM;
+#endif  /* CONFIG_AUDIT_LOGINUID_IMMUTABLE */
 
+	sessionid = atomic_inc_return(&session_id);
 	if (context && context->in_syscall) {
 		struct audit_buffer *ab;