security,selinux,smack: kill security_task_wait hook

As reported by yangshukui, a permission denial from security_task_wait()
can lead to a soft lockup in zap_pid_ns_processes() since it only expects
sys_wait4() to return 0 or -ECHILD. Further, security_task_wait() can
in general lead to zombies; in the absence of some way to automatically
reparent a child process upon a denial, the hook is not useful.  Remove
the security hook and its implementations in SELinux and Smack.  Smack
already removed its check from its hook.

Reported-by: yangshukui <[email protected]>
Signed-off-by: Stephen Smalley <[email protected]>
Acked-by: Casey Schaufler <[email protected]>
Acked-by: Oleg Nesterov <[email protected]>
Signed-off-by: Paul Moore <[email protected]>

security/smack/smack_lsm.c

@@ -2271,25 +2271,6 @@ static int smack_task_kill(struct task_struct *p, struct siginfo *info,
 	return rc;
 }
 
-/**
- * smack_task_wait - Smack access check for waiting
- * @p: task to wait for
- *
- * Returns 0
- */
-static int smack_task_wait(struct task_struct *p)
-{
-	/*
-	 * Allow the operation to succeed.
-	 * Zombies are bad.
-	 * In userless environments (e.g. phones) programs
-	 * get marked with SMACK64EXEC and even if the parent
-	 * and child shouldn't be talking the parent still
-	 * may expect to know when the child exits.
-	 */
-	return 0;
-}
-
 /**
  * smack_task_to_inode - copy task smack into the inode blob
  * @p: task to copy from
@@ -4658,7 +4639,6 @@ static struct security_hook_list smack_hooks[] = {
 	LSM_HOOK_INIT(task_getscheduler, smack_task_getscheduler),
 	LSM_HOOK_INIT(task_movememory, smack_task_movememory),
 	LSM_HOOK_INIT(task_kill, smack_task_kill),
-	LSM_HOOK_INIT(task_wait, smack_task_wait),
 	LSM_HOOK_INIT(task_to_inode, smack_task_to_inode),
 
 	LSM_HOOK_INIT(ipc_permission, smack_ipc_permission),