linux-kernel - Re: [PATCH v2 11/17] selftests/x86/ldt

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Date:   Thu, 14 Dec 2017 13:22:23 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...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 11/17] selftests/x86/ldt_gdt: Prepare for access bit forced

On Thu, Dec 14, 2017 at 11:43 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Dec 14, 2017 at 8:20 AM, Andy Lutomirski <luto@...nel.org> wrote:
>>
>> If this turns out to need reverting because it breaks Wine or
>> something, we're really going to regret it.
>
> I really don't see that as very likely. We already play other (much
> more fundamental) games with segments.
>

I dunno.  Maybe Wine or DOSEMU apps expect to be able to create a
non-accessed segment and then read out the accessed bit using LAR or
modify_ldt() later.

> But I do agree that it would be good to consider this "turn LDT
> read-only" a separate series just in case.

Which kind of kills the whole thing.  There's no way the idea of
putting the LDT in a VMA is okay if it's RW.  You just get the kernel
to put_user() a call gate into it and it's game over.

I have a competing patch that just aliases the LDT high up in kernel
land and shares it in the user tables.  I like a lot of the cleanups
in this series, but I don't like the actual LDT-in-a-VMA part.