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

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Date:	Thu, 21 Apr 2016 08:46:27 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Thomas Garnier <thgarnie@...gle.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>
CC:	x86@...nel.org, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org, 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 April 21, 2016 6:30:24 AM PDT, 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)
>
>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.
>
>-boris

That range is reserved for the hypervisor use.
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