linux-kernel - Re: [PATCH v2 06/17] x86/ldt: Do not install LDT for kernel threads

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Message-ID: <20171214194334.GD3326@worktop>
Date:   Thu, 14 Dec 2017 20:43:34 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     linux-kernel@...r.kernel.org, tglx@...utronix.de
Cc:     x86@...nel.org, Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirsky <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Borislav Petkov <bpetkov@...e.de>,
        Greg KH <gregkh@...uxfoundation.org>, keescook@...gle.com,
        hughd@...gle.com, Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        David Laight <David.Laight@...lab.com>,
        Eduardo Valentin <eduval@...zon.com>, aliguori@...zon.com,
        Will Deacon <will.deacon@....com>, linux-mm@...ck.org,
        kirill.shutemov@...ux.intel.com, dan.j.williams@...el.com
Subject: Re: [PATCH v2 06/17] x86/ldt: Do not install LDT for kernel threads

On Thu, Dec 14, 2017 at 12:27:32PM +0100, Peter Zijlstra wrote:
> From: Thomas Gleixner <tglx@...utronix.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?