[PATCH 1/2] copy_process: fix CLONE_PARENT && ->exit_signal interaction

[Oleg Nesterov <oleg@redhat.com>]

clone(CLONE_PARENT | SIGXXX) blindly sets ->exit_signal = SIGXXX. It is not
clear to me what was the supposed behaviour but this does not look right.
The parent of the forking task will receive this signal which bypasses all
security checks.

With this patch CLONE_PARENT re-uses both ->real_parent and ->exit_signal,
this looks at least logical.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reported-by: Chris Evans <scarybeasts@gmail.com>

--- 6.29-rc3/kernel/fork.c~1_EXIT_SIGNAL	2009-02-09 01:03:48.000000000 +0100
+++ 6.29-rc3/kernel/fork.c	2009-02-25 18:09:56.000000000 +0100
@@ -1217,10 +1217,16 @@ static struct task_struct *copy_process(
 			!cpu_online(task_cpu(p))))
 		set_task_cpu(p, smp_processor_id());
 
-	/* CLONE_PARENT re-uses the old parent */
-	if (clone_flags & (CLONE_PARENT|CLONE_THREAD))
+	/* CLONE_PARENT re-uses the old parent and exit_signal */
+	if (clone_flags & (CLONE_PARENT|CLONE_THREAD)) {
+		/*
+		 * Do this under tasklist_lock to avoid the race with
+		 * re-parenting to init.
+		 */
+		if (!(clone_flags & CLONE_THREAD))
+			p->exit_signal = current->group_leader->exit_signal;
 		p->real_parent = current->real_parent;
-	else
+	} else
 		p->real_parent = current;
 
 	spin_lock(&current->sighand->siglock);