linux-kernel - Re: [patch 05/16] mm: Allow special mappings with user access cleared

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Date:   Thu, 14 Dec 2017 10:23:21 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        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>
Subject: Re: [patch 05/16] mm: Allow special mappings with user access cleared

Linus Torvalds <torvalds@...ux-foundation.org> writes:

> On Wed, Dec 13, 2017 at 7:54 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>>
>> Which is why get_user_pages() _should_ enforce this.
>>
>> What use are protection keys if you can trivially circumvent them?
>
> No, we will *not* worry about protection keys in get_user_pages().
>
> They are not "security". They are a debug aid and safety against random mis-use.
>
> In particular, they are very much *NOT* about "trivially circumvent
> them". The user could just change their mapping thing, for chrissake!
>
> We already allow access to PROT_NONE for gdb and friends, very much on purpose.
>

Can you clarify this? We recently did fix read access on PROT_NONE via
gup here for ppc64 https://lkml.kernel.org/r/20171204021912.25974-2-aneesh.kumar@linux.vnet.ibm.com

What is the expected behaviour against gup and get_user_pages for
PROT_NONE. 

Another issue is we end up behaving differently with PROT_NONE mapping
based on whether autonuma is enabled or not. For a PROT_NONE mapping we
return true with pte_protnone().

-aneesh