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

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Message-ID: <feea2aff-91ff-89a6-9d7c-5402a1d6a27f@intel.com>
Date:   Tue, 7 Aug 2018 11:34:46 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, 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>, aliguori@...zon.com,
        daniel.gruss@...k.tugraz.at, hughd@...gle.com, keescook@...gle.com,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <llong@...hat.com>, Pavel Machek <pavel@....cz>,
        "David H . Gutteridge" <dhgutteridge@...patico.ca>, jroedel@...e.de
Subject: Re: [PATCH 2/3] x86/mm/pti: Don't clear permissions in
 pti_clone_pmd()

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.