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

[Thomas Garnier]

On Thu, Apr 21, 2016 at 6:30 AM, 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)
>

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
>
>