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

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Date:	Tue, 28 Jul 2015 14:50:50 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Oleg Nesterov <oleg@...hat.com>,
	David Howells <dhowells@...hat.com>,
	LKML <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, Jul 28, 2015 at 2:35 PM, Andrew Morton
<akpm@...ux-foundation.org> 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 _can_ be expensive, but if mm->mm_users == 1 it immediately returns
true, so it's only the cases where there is a race (like what's solved
here), or when it's a legit failure. This doesn't feel to me like it
should hit a real user very hard, since "real" callers of unshare will
normally have mm_users == 1.

-Kees

-- 
Kees Cook
Chrome OS Security
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