linux-kernel - Re: PATCH? fix unshare(NEWPID) && vfork()

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Date:	Mon, 19 Aug 2013 10:46:34 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Brad Spengler <spender@...ecurity.net>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Colin Walters <walters@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: PATCH? fix unshare(NEWPID) && vfork()

On Mon, Aug 19, 2013 at 10:25 AM, Oleg Nesterov <oleg@...hat.com> wrote:
>
> 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?

I agree that we should probably deny CLONE_PARENT, which makes more
sense paired with CLONE_NEWPID. I think we should also disallow
CLONE_THREAD, which is the thread goup.

And I *think* we can drop CLONE_VM. I suspect that snuck in as a
(misguided) attempt at CLONE_THREAD, as implied by the comment.

In fact, if you go look at the history of that CLONE_VM test, it came from

    unshare(CLONE_NEWPID)
    clone(CLONE_THREAD|CLONE_SIGHAND|CLONE_VM)

and the commit message talks about not setting pid_ns->child_reaper.
Which is very much about the PID, not about the shared VM space.

So I think your patch is correct, although I'm not sure why you move
the test. The new test you have look complicated as hell, so I think
you're actually making things worse by making them unreadable.

                  Linus
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