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

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Date:   Thu, 14 Dec 2017 13:27:58 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>, X86 ML <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>,
        Kees Cook <keescook@...gle.com>,
        Hugh Dickins <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>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        Will Deacon <will.deacon@....com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dan Williams <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 11:43 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 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?
>

Nah, you're probably right.