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

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Date:   Thu, 14 Dec 2017 13:44:03 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     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 1:22 PM, Andy Lutomirski <luto@...nel.org> wrote:
>
> 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.

Sure there is.

I really don't understand why you guys think it has to be RO.

All it has to be is not _user_ accessible. And that's a requirement
regardless, because no way in hell should users be able to read the
damn thing.

So it clearly needs to have the PAGE_USER bit clear (to avoid users
accessing it directly), and it needs to be marked somehow for
get_user_pages() to refuse it too, and access_ok() needs to fail it so
that we can't do get_user/put_user on it.

But the whole RO vs RW is not fundamentally critical.

Now, I do agree that RO is much much better in general, and it avoids
the requirement to play games with "access_ok()" and friends (assuming
we're just ok with users reading it), but I disagree with the whole
"this is fundamental".

                 Linus