linux-kernel - Re: [PATCH] user_ns: use correct check for single-threadedness

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Date:	Wed, 5 Aug 2015 19:23:56 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	David Howells <dhowells@...hat.com>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Ricky Zhou <rickyz@...omium.org>,
	Julien Tinnes <jln@...gle.com>
Subject: Re: [PATCH] user_ns: use correct check for single-threadedness

On 07/29, Kirill A. Shutemov wrote:
>
> On Tue, Jul 28, 2015 at 02:35:04PM -0700, Andrew Morton wrote:
> > On Tue, 28 Jul 2015 10:15:00 -0700 Kees Cook <keescook@...omium.org> wrote:
> >
> > > From: Ricky Zhou <rickyz@...omium.org>
> > >
> > > Checking mm_users > 1 does not mean a process is multithreaded. For
> > > example, reading /proc/PID/maps temporarily increments mm_users, allowing
> > > other processes to (accidentally) interfere with unshare() calls.
> > >
> > > This fixes observed failures of unshare(CLONE_NEWUSER) incorrectly
> > > returning EINVAL if another processes happened to be simultaneously
> > > reading the maps file.
> >
> > Yikes.  current_is_single_threaded() is expensive.  Are we sure this
> > isn't going to kill someone's workload?
>
> It's expensive only if mm_users > 1. We will go to for_each_process() only
> if somebody outside of the process grabs mm_users references (like reading
> /proc/PID/maps).

Yes,

> Or if it called it from multithreaded application.

Not really, please note the "negative fast-path" check:

	if (atomic_read(&task->signal->live) != 1)	
		return false;

> Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

Same here, I think the patch is fine.

I don't think that sys_setns() is performance critical, and Eric seems
to agree with this change too.

Oleg.

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