linux-kernel - Re: [PATCH 02/23] x86, kaiser: do not set _PAGE_USER for init

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Date:   Thu, 2 Nov 2017 00:07:26 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        moritz.lipp@...k.tugraz.at,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        michael.schwarz@...k.tugraz.at,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH 02/23] x86, kaiser: do not set _PAGE_USER for init_mm page tables

On Wed, Nov 1, 2017 at 2:28 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Wed, 1 Nov 2017, Andy Lutomirski wrote:
>
>> On Wed, Nov 1, 2017 at 2:11 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
>> > On Tue, 31 Oct 2017, Dave Hansen wrote:
>> >
>> >>
>> >> init_mm is for kernel-exclusive use.  If someone is allocating page
>> >> tables in it, do not set _PAGE_USER on them.  This ensures that
>> >> we do *not* set NX on these page tables in the KAISER code.
>> >
>> > This changelog is confusing at best.
>> >
>> > Why is this a kaiser issue? Nothing should ever create _PAGE_USER entries
>> > in init_mm, right?
>>
>> The vsyscall page is _PAGE_USER and lives in init_mm via the fixmap.
>
> Groan, forgot about that abomination, but still there is no point in having
> it marked PAGE_USER in the init_mm at all, kaiser or not.
>

How can it be PAGE_USER in user mms but not init_mm?  It's the same page table.

> Thanks,
>
>         tglx