Re: [Xen-devel] [PATCH RFC v2 00/12] xen/x86: use per-vcpu stacks for 64 bit pv domains

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.01.18 at 15:18, <jgross@xxxxxxxx> wrote:
> On 22/01/18 13:50, Jan Beulich wrote:
>>>>> On 22.01.18 at 13:32, <jgross@xxxxxxxx> wrote:
>>> As a preparation for doing page table isolation in the Xen hypervisor
>>> in order to mitigate "Meltdown" use dedicated stacks, GDT and TSS for
>>> 64 bit PV domains mapped to the per-domain virtual area.
>>>
>>> The per-vcpu stacks are used for early interrupt handling only. After
>>> saving the domain's registers stacks are switched back to the normal
>>> per physical cpu ones in order to be able to address on-stack data
>>> from other cpus e.g. while handling IPIs.
>>>
>>> Adding %cr3 switching between saving of the registers and switching
>>> the stacks will enable the possibility to run guest code without any
>>> per physical cpu mapping, i.e. avoiding the threat of a guest being
>>> able to access other domains data.
>>>
>>> Without any further measures it will still be possible for e.g. a
>>> guest's user program to read stack data of another vcpu of the same
>>> domain, but this can be easily avoided by a little PV-ABI modification
>>> introducing per-cpu user address spaces.
>>>
>>> This series is meant as a replacement for Andrew's patch series:
>>> "x86: Prerequisite work for a Xen KAISER solution".
>> 
>> Considering in particular the two reverts, what I'm missing here
>> is a clear description of the meaningful additional protection this
>> approach provides over the band-aid. For context see also
>> https://lists.xenproject.org/archives/html/xen-devel/2018-01/msg01735.html 
> 
> My approach supports mapping only the following data while the guest is
> running (apart form the guest's own data, of course):
> 
> - the per-vcpu entry stacks of the domain which will contain only the
>   guest's registers saved when an interrupt occurs
> - the per-vcpu GDTs and TSSs of the domain
> - the IDT
> - the interrupt handler code (arch/x86/x86_64/[compat/]entry.S
> 
> All other hypervisor data and code can be completely hidden from the
> guests.

I understand that. What I'm not clear about is: Which parts of
the additionally hidden data are actually necessary (or at least
very desirable) to hide?

Jan


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