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

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Date:   Thu, 14 Dec 2017 14:52:58 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andy Lutomirski <luto@...nel.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 2:24 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>
> get_user_pages_fast() (both of them) do indeed test access_ok(), but the
> regular get_user_pages() does not, I suspect because it can operate on a
> foreign mm.

That sounds wrong.

We actually had some very serious reasons why get_user_pages_fast()
needed to check access_ok().

I happen to forget what those reasons were, though.

My mind may be going.

But I think it was something like "you could walk off the page tables
because the undefined address range generates nonsensical values for
the pgd_offset() functions" etc.

But maybe the regular get_user_pages() has some other way to protect
against that.

          Linus