apparmor: don't try to replace stale label in ptraceme check

begin_current_label_crit_section() must run in sleepable context because
when label_is_stale() is true, aa_replace_current_label() runs, which uses
prepare_creds(), which can sleep.

Until now, the ptraceme access check (which runs with tasklist_lock held)
violated this rule.

Fixes: b2d09ae4 ("apparmor: move ptrace checks to using labels")
Reported-by: NCyrill Gorcunov <gorcunov@gmail.com>
Reported-by: Nkernel test robot <rong.a.chen@intel.com>
Signed-off-by: NJann Horn <jannh@google.com>
Signed-off-by: NJohn Johansen <john.johansen@canonical.com>

security/apparmor/lsm.c

@@ -132,11 +132,11 @@ static int apparmor_ptrace_traceme(struct task_struct *parent)
 	struct aa_label *tracer, *tracee;
 	int error;
 
-	tracee = begin_current_label_crit_section();
+	tracee = __begin_current_label_crit_section();
 	tracer = aa_get_task_label(parent);
 	error = aa_may_ptrace(tracer, tracee, AA_PTRACE_TRACE);
 	aa_put_label(tracer);
-	end_current_label_crit_section(tracee);
+	__end_current_label_crit_section(tracee);
 
 	return error;
 }