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Date: Tue, 28 Jul 2015 14:50:50 -0700
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Subject: Re: [PATCH] user_ns: use correct check for single-threadedness
From: Kees Cook <keescook@chromium.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
        Oleg Nesterov <oleg@redhat.com>, David Howells <dhowells@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Rik van Riel <riel@redhat.com>,
        Vladimir Davydov <vdavydov@parallels.com>,
        Ricky Zhou <rickyz@chromium.org>, Julien Tinnes <jln@google.com>
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On Tue, Jul 28, 2015 at 2:35 PM, Andrew Morton
<akpm@linux-foundation.org> wrote:
> On Tue, 28 Jul 2015 10:15:00 -0700 Kees Cook <keescook@chromium.org> wrote:
>
>> From: Ricky Zhou <rickyz@chromium.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