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From: Boris Ostrovsky <boris.ostrovsky@oracle.com>
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Date: Thu, 21 Apr 2016 09:30:24 -0400
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Subject: [kernel-hardening] Re: [RFC v1 3/4] x86, boot: Implement ASLR for kernel memory sections
 (x86_64)
To: Thomas Garnier <thgarnie@google.com>, "H . Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@suse.de>, Andy Lutomirski <luto@kernel.org>, Dmitry Vyukov <dvyukov@google.com>, Paolo Bonzini <pbonzini@redhat.com>, Dan Williams <dan.j.williams@intel.com>, Kees Cook <keescook@chromium.org>, Stephen Smalley <sds@tycho.nsa.gov>, Seth Jennings <sjennings@variantweb.net>, Kefeng Wang <wangkefeng.wang@huawei.com>, Jonathan Corbet <corbet@lwn.net>, Matt Fleming <matt@codeblueprint.co.uk>, Toshi Kani <toshi.kani@hpe.com>, Alexander Kuleshov <kuleshovmail@gmail.com>, Alexander Popov <alpopov@ptsecurity.com>, Joerg Roedel <jroedel@suse.de>, Dave Young <dyoung@redhat.com>, Baoquan He <bhe@redhat.com>, Dave Hansen <dave.hansen@linux.intel.com>, Mark Salter <msalter@redhat.com>
Cc: x86@kernel.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, gthelen@google.com, kernel-hardening@lists.openwall.com
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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