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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Wed, 13 Dec 2017 10:35:00 -0800
Message-ID: <CA+55aFzNhHZaAFJZv5=2t8dnUt9mMaZVp9_5XvayN6gkfEvtrA@mail.gmail.com>
Subject: Re: [patch 05/16] mm: Allow special mappings with user access cleared
To: Peter Zijlstra <peterz@infradead.org>
Cc: Dave Hansen <dave.hansen@intel.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Borislav Petkov <bpetkov@suse.de>,
        Greg KH <gregkh@linuxfoundation.org>, Kees Cook <keescook@google.com>,
        Hugh Dickins <hughd@google.com>, Brian Gerst <brgerst@gmail.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <jgross@suse.com>,
        David Laight <David.Laight@aculab.com>,
        Eduardo Valentin <eduval@amazon.com>,
        "Liguori, Anthony" <aliguori@amazon.com>,
        Will Deacon <will.deacon@arm.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        "Aneesh Kumar K. V" <aneesh.kumar@linux.vnet.ibm.com>
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On Wed, Dec 13, 2017 at 10:32 AM, Peter Zijlstra <peterz@infradead.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