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

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Date:   Wed, 13 Dec 2017 10:21:06 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     "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>,
        "Aneesh Kumar K. V" <aneesh.kumar@...ux.vnet.ibm.com>
Subject: Re: [patch 05/16] mm: Allow special mappings with user access cleared

On 12/13/2017 10:08 AM, Linus Torvalds wrote:
> On Wed, Dec 13, 2017 at 7:54 AM, Peter Zijlstr <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().

We did introduce some support for it here:

> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=33a709b25a760b91184bb335cf7d7c32b8123013

> They are not "security". They are a debug aid and safety against
> random mis-use.

Totally agree.  It's not about security.  As I mentioned in the commit,
the goal here was to try to make pkey-protected access behavior
consistent with mprotect().

I still think this was nice to do and probably surprises users less than
if we didn't have it.

> We already allow access to PROT_NONE for gdb and friends, very much on purpose.

Yup, exactly, and that's one of the reasons that I tried to call those
out as "remote" access that are specicifially no subject to protection keys.