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

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Message-ID: <20171214222439.rovm3t7iaakefati@hirez.programming.kicks-ass.net>
Date:   Thu, 14 Dec 2017 23:24:39 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.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 02:14:00PM -0800, Linus Torvalds wrote:
> On Thu, Dec 14, 2017 at 2:02 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > _Should_ being the operative word, because I cannot currently see it
> > DTRT. But maybe I'm missing the obvious -- I tend to do that at times.
> 
> At least the old get_user_pages_fast() code used to check the USER bit:
> 
>         unsigned long need_pte_bits = _PAGE_PRESENT|_PAGE_USER;
> 
>         if (write)
>                 need_pte_bits |= _PAGE_RW;
> 
> but that may have been lost when we converted over to the generic code.

The generic gup_pte_range() has pte_access_permitted() (which has the
above test) in the right place.

> It shouldn't actually _matter_, since we'd need to change access_ok()
> anyway (and gup had better check that!)

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.

And its the regular old get_user_pages() that's all sorts of broken wrt
!PAGE_USER too.