PATCH? fix unshare(NEWPID) && vfork()

[Oleg Nesterov <oleg@redhat.com>]

Hello.

Colin reports that vfork() doesn't work after unshare(PIDNS). The
reason is trivial, copy_process() does:

	/*
	 * If the new process will be in a different pid namespace
	 * don't allow the creation of threads.
	 */
	if ((clone_flags & (CLONE_VM|CLONE_NEWPID)) &&
	    (task_active_pid_ns(current) != current->nsproxy->pid_ns))
		return ERR_PTR(-EINVAL);

and CLONE_VM obviously nacks vfork(). So perhaps we can relax
this check to CLONE_THREAD? Or should we really nack CLONE_VM
by security reasons?

OTOH. Perhaps we should also deny CLONE_PARENT in this case?

In short. So far I am thinking about the patch below but I got
lost and totally confused. Will try to think more tomorrow, but
I would like to see the fix from someone who still understands
this all.

Oleg.

--- x/kernel/fork.c	2013-08-14 18:34:06.000000000 +0200
+++ x/kernel/fork.c	2013-08-19 19:03:43.848823039 +0200
@@ -1172,14 +1172,6 @@ static struct task_struct *copy_process(
 				current->signal->flags & SIGNAL_UNKILLABLE)
 		return ERR_PTR(-EINVAL);
 
-	/*
-	 * If the new process will be in a different pid namespace
-	 * don't allow the creation of threads.
-	 */
-	if ((clone_flags & (CLONE_VM|CLONE_NEWPID)) &&
-	    (task_active_pid_ns(current) != current->nsproxy->pid_ns))
-		return ERR_PTR(-EINVAL);
-
 	retval = security_task_create(clone_flags);
 	if (retval)
 		goto fork_out;
@@ -1578,8 +1570,9 @@ long do_fork(unsigned long clone_flags,
 	 * Do some preliminary argument and permissions checking before we
 	 * actually start allocating stuff
 	 */
-	if (clone_flags & (CLONE_NEWUSER | CLONE_NEWPID)) {
-		if (clone_flags & (CLONE_THREAD|CLONE_PARENT))
+	if ((clone_flags & (CLONE_NEWUSER | CLONE_NEWPID)) ||
+	    (task_active_pid_ns(current) != current->nsproxy->pid_ns)) {
+		if (clone_flags & (CLONE_THREAD | CLONE_PARENT | CLONE_NEWPID))
 			return -EINVAL;
 	}