From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1752573AbdLNToL (ORCPT <rfc822;w@1wt.eu>);
        Thu, 14 Dec 2017 14:44:11 -0500
Received: from merlin.infradead.org ([205.233.59.134]:35424 "EHLO
        merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1752023AbdLNToK (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Thu, 14 Dec 2017 14:44:10 -0500
Date: Thu, 14 Dec 2017 20:43:34 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: linux-kernel@vger.kernel.org, tglx@linutronix.de
Cc: x86@kernel.org, Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirsky <luto@kernel.org>, Dave Hansen <dave.hansen@intel.com>,
        Borislav Petkov <bpetkov@suse.de>,
        Greg KH <gregkh@linuxfoundation.org>, keescook@google.com,
        hughd@google.com, Brian Gerst <brgerst@gmail.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <jgross@suse.com>,
        David Laight <David.Laight@aculab.com>,
        Eduardo Valentin <eduval@amazon.com>, aliguori@amazon.com,
        Will Deacon <will.deacon@arm.com>, linux-mm@kvack.org,
        kirill.shutemov@linux.intel.com, dan.j.williams@intel.com
Subject: Re: [PATCH v2 06/17] x86/ldt: Do not install LDT for kernel threads
Message-ID: <20171214194334.GD3326@worktop>
References: <20171214112726.742649793@infradead.org>
 <20171214113851.398563731@infradead.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20171214113851.398563731@infradead.org>
User-Agent: Mutt/1.5.22.1 (2013-10-16)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Thu, Dec 14, 2017 at 12:27:32PM +0100, Peter Zijlstra wrote:
> From: Thomas Gleixner <tglx@linutronix.de>
> 
> Kernel threads can use the mm of a user process temporarily via use_mm(),
> but there is no point in installing the LDT which is associated to that mm
> for the kernel thread.

So thinking about this a bit more; I fear its not correct.

Suppose a kthread does use_mm() and we then schedule to a task of that
process, we'll not pass through switch_mm() and we'll not install the
LDT and bad things happen.

Or am I missing something?