linux-kernel - Re: [PATCH 2/3] x86/mm/pti: Don't clear permissions in pti_clone

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Date:   Tue, 7 Aug 2018 12:38:24 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>, Jiri Kosina <jkosina@...e.cz>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Brian Gerst <brgerst@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Eduardo Valentin <eduval@...zon.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        Hugh Dickins <hughd@...gle.com>,
        Kees Cook <keescook@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <llong@...hat.com>, Pavel Machek <pavel@....cz>,
        "David H . Gutteridge" <dhgutteridge@...patico.ca>,
        Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH 2/3] x86/mm/pti: Don't clear permissions in pti_clone_pmd()

On Tue, Aug 7, 2018 at 11:34 AM, Dave Hansen <dave.hansen@...el.com> wrote:
> On 08/07/2018 03:24 AM, Joerg Roedel wrote:
>> The function sets the global-bit on cloned PMD entries,
>> which only makes sense when the permissions are identical
>> between the user and the kernel page-table.
>>
>> Further, only write-permissions are cleared for entry-text
>> and kernel-text sections, which are not writeable anyway.
>
> I think this patch is correct, but I'd be curious if Andy remembers why
> we chose to clear _PAGE_RW on these things.  It might have been that we
> were trying to say that the *entry* code shouldn't write to this stuff,
> regardless of whether the normal kernel can.
>
> But, either way, I agree with the logic here that Global pages must
> share permissions between both mappings, so feel free to add my Ack.  I
> just want to make sure Andy doesn't remember some detail I'm forgetting.

I suspect it's because we used to (and maybe still do) initialize the
user tables before mark_read_only().