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

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Message-ID: <CA+55aFzNhHZaAFJZv5=2t8dnUt9mMaZVp9_5XvayN6gkfEvtrA@mail.gmail.com>
Date:   Wed, 13 Dec 2017 10:35:00 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     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>,
        "Aneesh Kumar K. V" <aneesh.kumar@...ux.vnet.ibm.com>
Subject: Re: [patch 05/16] mm: Allow special mappings with user access cleared

On Wed, Dec 13, 2017 at 10:32 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>
> Now, if VM_NOUSER were to live, the above change would ensure write(2)
> cannot read from such VMAs, where the existing test for FOLL_WRITE
> already disallows read(2) from writing to them.

So I don't mind at all the notion of disallowing access to some
special mappings at the vma level. So a VM_NOUSER flag that just
disallows get_user_pages entirely I'm ok with.

It's the protection keys in particular that I don't like having to
worry about. They are subtle and have odd architecture-specific
meaning, and needs to be checked at all levels in the page table tree.

               Linus