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

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Message-Id: <03293FB5-DC9B-47BB-8C3A-7827CD82FECB@amacapital.net>
Date:   Thu, 14 Dec 2017 14:30:34 -0800
From:   Andy Lutomirski <luto@...capital.net>
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 Dec 14, 2017, at 2:15 PM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
>> On Thu, Dec 14, 2017 at 2:11 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> 
>> That seems to rather defeat the point of using a VMA, though.
> 
> There never was any point in using a VMA per se.
> 
> The point was always to just map the damn thing in the user page
> tables, wasn't it?
> 
> The vma bit was just an implementation detail.

And all this is why I dislike using a VMA.  My patch puts it at a negative address. We could just as easily put it just above TASK_SIZE_MAX, but I'm a bit nervous about bugs that overrun an access_ok check by a small amount.  IIRC I found one of those in the net code once, and I didn't look very hard.