yama: don't abuse rcu_read_lock/get_task_struct in yama_task_prctl()

current->group_leader is stable, no need to take rcu_read_lock() and do
get/put_task_struct().

Signed-off-by: Oleg Nesterov <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Kees Cook <[e-mail]>

security/yama/yama_lsm.c

···
 			   unsigned long arg4, unsigned long arg5)
 {
 	int rc = -ENOSYS;
-	struct task_struct *myself = current;
+	struct task_struct *myself;
 
 	switch (option) {
 	case PR_SET_PTRACER:
···
 		 * leader checking is handled later when walking the ancestry
 		 * at the time of PTRACE_ATTACH check.
 		 */
-		rcu_read_lock();
-		if (!thread_group_leader(myself))
-			myself = rcu_dereference(myself->group_leader);
-		get_task_struct(myself);
-		rcu_read_unlock();
+		myself = current->group_leader;
 
 		if (arg2 == 0) {
 			yama_ptracer_del(NULL, myself);
···
 			}
 		}
 
-		put_task_struct(myself);
 		break;
 	}