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Date: Tue, 28 Jul 2015 14:02:18 -0400
From: Rik van Riel <riel@redhat.com>
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To: Kees Cook <keescook@chromium.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>
CC: Oleg Nesterov <oleg@redhat.com>, David Howells <dhowells@redhat.com>,
        linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Vladimir Davydov <vdavydov@parallels.com>,
        Ricky Zhou <rickyz@chromium.org>, Julien Tinnes <jln@google.com>
Subject: Re: [PATCH] user_ns: use correct check for single-threadedness
References: <20150728171500.GA2871@www.outflux.net>
In-Reply-To: <20150728171500.GA2871@www.outflux.net>
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On 07/28/2015 01:15 PM, Kees Cook 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.
> 
> Signed-off-by: Ricky Zhou <rickyz@chromium.org>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> Cc: stable@vger.kernel.org

Reviewed-by: Rik van Riel <riel@redhat.com>