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

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 01/22/2018 06:39 PM, Andrew Cooper wrote:
> On 22/01/18 16:51, Jan Beulich wrote:
>>>>> On 22.01.18 at 16:00, <jgross@xxxxxxxx> wrote:
>>> On 22/01/18 15:48, Jan Beulich wrote:
>>>>>>> On 22.01.18 at 15:38, <jgross@xxxxxxxx> wrote:
>>>>> On 22/01/18 15:22, Jan Beulich wrote:
>>>>>>>>> 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?
>>>>> Necessary:
>>>>> - other guests' memory (e.g. physical memory 1:1 mapping)
>>>>> - data from other guests e.g.in stack pages, debug buffers, I/O buffers,
>>>>>   code emulator buffers
>>>>> - other guests' register values e.g. in vcpu structure
>>>> All of this is already being made invisible by the band-aid (with the
>>>> exception of leftovers on the hypervisor stacks across context
>>>> switches, which we've already said could be taken care of by
>>>> memset()ing that area). I'm asking about the _additional_ benefits
>>>> of your approach.
>>> I'm quite sure the performance will be much better as it doesn't require
>>> per physical cpu L4 page tables, but just a shadow L4 table for each
>>> guest L4 table, similar to the Linux kernel KPTI approach.
>> But isn't that model having the same synchronization issues upon
>> guest L4 updates which Andrew was fighting with?
> 
> (Condensing a lot of threads down into one)
> 
> All the methods have L4 synchronisation update issues, until we have a
> PV ABI which guarantees that L4's don't get reused.  Any improvements to
> the shadowing/synchronisation algorithm will benefit all approaches.
> 
> Juergen: you're now adding a LTR into the context switch path which
> tends to be very slow.  I.e. As currently presented, this series
> necessarily has a higher runtime overhead than Jan's XPTI.
> 
> One of my concerns is that this patch series moves further away from the
> secondary goal of my KAISER series, which was to have the IDT and GDT
> mapped at the same linear addresses on every CPU so a) SIDT/SGDT don't
> leak which CPU you're currently scheduled on into PV guests and b) the
> context switch code can drop a load of its slow instructions like LGDT
> and the VMWRITEs to update the VMCS.
> 
> Jan: As to the things not covered by the current XPTI, hiding most of
> the .text section is important to prevent fingerprinting or ROP
> scanning.  This is a defence-in-depth argument, but a guest being easily
> able to identify whether certain XSAs are fixed or not is quite bad. 

I'm afraid we have a fairly different opinion of what is "quite bad".
Suppose we handed users a knob and said, "If you flip this switch,
attackers won't be able to tell if you've fixed XSAs or not without
trying them; but it will slow down your guests 20%."  How many do you
think would flip it, and how many would reckon that an attacker could
probably find out that information anyway?

 -George

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