linux-kernel - Re: [RFC] x86/mm/KASLR: Remap GDTs at fixed location

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Date:   Thu, 5 Jan 2017 09:51:15 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Garnier <thgarnie@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Kees Cook <keescook@...omium.org>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>, Dave Hansen <dave@...1.net>,
        Chen Yucong <slaoub@...il.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Michael Ellerman <mpe@...erman.id.au>,
        Juergen Gross <jgross@...e.com>,
        Richard Weinberger <richard@....at>, X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [RFC] x86/mm/KASLR: Remap GDTs at fixed location

On Wed, Jan 4, 2017 at 2:16 PM, Thomas Garnier <thgarnie@...gle.com> wrote:
> Each processor holds a GDT in its per-cpu structure. The sgdt
> instruction gives the base address of the current GDT. This address can
> be used to bypass KASLR memory randomization. With another bug, an
> attacker could target other per-cpu structures or deduce the base of the
> main memory section (PAGE_OFFSET).
>
> In this change, a space is reserved at the end of the memory range
> available for KASLR memory randomization. The space is big enough to hold
> the maximum number of CPUs (as defined by setup_max_cpus). Each GDT is
> mapped at specific offset based on the target CPU. Note that if there is
> not enough space available, the GDTs are not remapped.

Can we remap it read-only?  I.e. use PAGE_KERNEL_RO instead of
PAGE_KERNEL.  After all, the ability to modify the GDT is instant
root.