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

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Message-Id: <20150728143504.5aa996ba5955522a19c2d5f1@linux-foundation.org>
Date:	Tue, 28 Jul 2015 14:35:04 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Kees Cook <keescook@...omium.org>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Oleg Nesterov <oleg@...hat.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 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?

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