linux-kernel - Re: [PATCH 27/37] x86/mm/pti: Keep permissions when cloning kernel text in pti_clone_kernel

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Message-ID: <CAGXu5jLN_rzmfgM-Xne836ip+qMc8T1QX=mhozo3NFLNssgUfw@mail.gmail.com>
Date:   Mon, 23 Apr 2018 10:06:20 -0700
From:   Kees Cook <keescook@...gle.com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     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>,
        Dave Hansen <dave.hansen@...el.com>,
        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>,
        Anthony Liguori <aliguori@...zon.com>,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        Hugh Dickins <hughd@...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 27/37] x86/mm/pti: Keep permissions when cloning kernel
 text in pti_clone_kernel_text()

On Mon, Apr 23, 2018 at 8:47 AM, Joerg Roedel <joro@...tes.org> wrote:
> From: Joerg Roedel <jroedel@...e.de>
>
> Mapping the kernel text area to user-space makes only sense
> if it has the same permissions as in the kernel page-table.
> If permissions are different this will cause a TLB reload
> when using the kernel page-table, which is as good as not
> mapping it at all.
>
> On 64-bit kernels this patch makes no difference, as the
> whole range cloned by pti_clone_kernel_text() is mapped RO
> anyway. On 32 bit there are writeable mappings in the range,
> so just keep the permissions as they are.

Why are there R/W text mappings in this range? I find that to be
unexpected. Shouldn't CONFIG_DEBUG_WX already complain if that were
true?

-Kees

-- 
Kees Cook
Pixel Security