Re: [RFC v1 3/4] x86, boot: Implement ASLR for kernel memory sections (x86_64)

[H. Peter Anvin]

On April 21, 2016 8:52:01 AM PDT, Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:
>On Thu, Apr 21, 2016 at 8:46 AM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>> On April 21, 2016 6:30:24 AM PDT, Boris Ostrovsky
><boris.ostrovsky@xxxxxxxxxx> 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.
>
>I know, I thought I could use it if no hypervisor was used but might
>introduce problems in the future so I will remove it for the next
>iteration.
>
>> --
>> Sent from my Android device with K-9 Mail. Please excuse brevity and
>formatting.

At least in theory the hypervisor can use it even though no PV architecture is advertised to the kernel.  One kind of would hope none would.

I think this range is also used by the kernel pointer checking thing, as it *has* to live right next to the canonical boundary.
-- 
Sent from my Android device with K-9 Mail. Please excuse brevity and formatting.