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

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Date:	Mon, 19 Aug 2013 11:10:41 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Brad Spengler <spender@...ecurity.net>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Colin Walters <walters@...hat.com>,
	"linux-kernel@...r.kernel.org" <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:
> 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.

By way of (partial) explanation:

http://marc.info/?l=linux-kernel&m=135545831607095

(tl;dr: I think that CLONE_VM is irrelevant here, but there may be
other issues lurking around.)

--Andy

>
> --- 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;
>         }
>
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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