linux-kernel - Re: [RFC v1 3/4] x86, boot: Implement ASLR for kernel memory sections (x86

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Message-ID: <CAJcbSZFiz+QjRUuj6_aVusQFOdkm85_8VF_wqe6GqTJbMZ82Xg@mail.gmail.com>
Date:	Thu, 21 Apr 2016 08:11:32 -0700
From:	Thomas Garnier <thgarnie@...gle.com>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:	"H . Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...e.de>,
	Andy Lutomirski <luto@...nel.org>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Kees Cook <keescook@...omium.org>,
	Stephen Smalley <sds@...ho.nsa.gov>,
	Seth Jennings <sjennings@...iantweb.net>,
	Kefeng Wang <wangkefeng.wang@...wei.com>,
	Jonathan Corbet <corbet@....net>,
	Matt Fleming <matt@...eblueprint.co.uk>,
	Toshi Kani <toshi.kani@....com>,
	Alexander Kuleshov <kuleshovmail@...il.com>,
	Alexander Popov <alpopov@...ecurity.com>,
	Joerg Roedel <jroedel@...e.de>, Dave Young <dyoung@...hat.com>,
	Baoquan He <bhe@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Mark Salter <msalter@...hat.com>, x86@...nel.org,
	LKML <linux-kernel@...r.kernel.org>, linux-doc@...r.kernel.org,
	Greg Thelen <gthelen@...gle.com>,
	kernel-hardening@...ts.openwall.com
Subject: Re: [RFC v1 3/4] x86, boot: Implement ASLR for kernel memory sections (x86_64)

On Thu, Apr 21, 2016 at 6:30 AM, Boris Ostrovsky
<boris.ostrovsky@...cle.com> wrote:
>
>
> On 04/15/2016 06:03 PM, Thomas Garnier wrote:
>>
>> +void __init kernel_randomize_memory(void)
>> +{
>> +       size_t i;
>> +       unsigned long addr = memory_rand_start;
>> +       unsigned long padding, rand, mem_tb;
>> +       struct rnd_state rnd_st;
>> +       unsigned long remain_padding = memory_rand_end -
>> memory_rand_start;
>> +
>> +       if (!kaslr_enabled())
>> +               return;
>> +
>> +       /* Take the additional space when Xen is not active. */
>> +       if (!xen_domain())
>> +               page_offset_base -= __XEN_SPACE;
>
>
> This should be !xen_pv_domain(). Xen HVM guests are no different from bare
> metal as far as address ranges are concerned. (Technically it's probably
> !xen_pv_domain() && !xen_pvh_domain() but we can ignore PVH for now since it
> is being replaced by an HVM-type guest)
>

In my test KASLR was disabled on Xen so I should just remove this
check. I kept it in case it might change in the future.

> Having said that, I am not sure I understand why page_offset_base is
> shifted. I thought 0xffff800000000000 - 0xffff87ffffffffff is not supposed
> to be used by anyone, whether we are running under a hypervisor or not.
>

It is shifted to get the most space possible, it increases the entropy
available. Do you know why we should not use 0xffff800000000000 -
0xffff87ffffffffff?

> -boris
>
>