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

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Date:   Thu, 5 Jan 2017 08:40:29 -0800
From:   Thomas Garnier <thgarnie@...gle.com>
To:     Arjan van de Ven <arjan@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        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>,
        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>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [RFC] x86/mm/KASLR: Remap GDTs at fixed location

On Thu, Jan 5, 2017 at 7:08 AM, Arjan van de Ven <arjan@...ux.intel.com> wrote:
> On 1/5/2017 12:11 AM, Ingo Molnar wrote:
>>
>>
>> * 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.
>>>
>>> The document was changed to mention GDT remapping for KASLR. This patch
>>> also include dump page tables support.
>>>
>>> This patch was tested on multiple hardware configurations and for
>>> hibernation support.
>>
>>
>>>  void kernel_randomize_memory(void);
>>> +void kernel_randomize_smp(void);
>>> +void* kaslr_get_gdt_remap(int cpu);
>>
>>
>> Yeah, no fundamental objections from me to the principle, but I get some
>> bad vibes
>> from the naming here: seeing that kernel_randomize_smp() actually makes
>> things
>> less random.
>>
>
> kernel_unrandomize_smp() ...
>

That seems like a better name.

> one request.. can we make sure this unrandomization is optional?
>

Well, it happens only when KASLR memory randomization is enabled. Do
you think it should have a separate config option?

-- 
Thomas