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

Thomas Garnier
2016-04-21T20:18:51Z

Make sense, thanks for the details.

On Thu, Apr 21, 2016 at 1:15 PM, H. Peter Anvin <hpa@zytor.com> wrote:
> On April 21, 2016 8:52:01 AM PDT, Thomas Garnier <thgarnie@google.com> wrote:
>>On Thu, Apr 21, 2016 at 8:46 AM, H. Peter Anvin <hpa@zytor.com> wrote:
>>> On April 21, 2016 6:30:24 AM PDT, Boris Ostrovsky
>><boris.ostrovsky@oracle.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.
>>
>>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.