linux-kernel - Re: [PATCH] x86/mm/pti: Move user W+X check into pti

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Message-ID: <20180809111603.aqon7xqmanvoycbu@suse.de>
Date:   Thu, 9 Aug 2018 13:16:03 +0200
From:   Joerg Roedel <jroedel@...e.de>
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@...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>
Subject: Re: [PATCH] x86/mm/pti: Move user W+X check into pti_finalize()

Hi Dave,

On Wed, Aug 08, 2018 at 08:54:37AM -0700, Dave Hansen wrote:
> One bit of information missing from the changelog: Could you clarify how
> there are any entries in the user page tables for the code to complain?
> Before pti_init(), I would have expected the user page tables to be empty.

The W+X check runs at the end of mark_readonly() in x86, which is after
pti_init() already put kernel mappings into the user page-table. Problem
is that the cloned entries are still W+X mapped, which is fixed in
pti_finalize() running _after_ mark_readonly().

Regards,

	Joerg