Re: [Xen-devel] [PATCH] x86: Meltdown band-aid against malicious 64-bit PV guests

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 15.01.18 at 17:54, <persaur@xxxxxxxxx> wrote:
> On Jan 12, 2018, at 05:19, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>> 
>> This is a very simplistic change limiting the amount of memory a running
>> 64-bit PV guest has mapped (and hence available for attacking): Only the
>> mappings of stack, IDT, and TSS are being cloned from the direct map
>> into per-CPU page tables. Guest controlled parts of the page tables are
>> being copied into those per-CPU page tables upon entry into the guest.
>> Cross-vCPU synchronization of top level page table entry changes is
>> being effected by forcing other active vCPU-s of the guest into the
>> hypervisor.
>> 
>> The change to context_switch() isn't strictly necessary, but there's no
>> reason to keep switching page tables once a PV guest is being scheduled
>> out.
>> 
>> There is certainly much room for improvement, especially of performance,
>> here - first and foremost suppressing all the negative effects on AMD
>> systems. But in the interest of backportability (including to really old
>> hypervisors, which may not even have alternative patching) any such is
>> being left out here.
> 
> Thanks for releasing this patch to support use cases not covered by the 
> previous mitigations.  Is there a name or acronym we can use to reference 
> this patch in the FAQ, XSA and other support documents?

I'm against any such naming, but XPTI-light would come to mind.

Jan


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