Re: [Xen-devel] Radical proposal: ship not-fully-tidied shim as 4.10.1

[Anthony Liguori <anthony@xxxxxxxxxxxxx>]

On Mon, Jan 8, 2018 at 1:01 PM, Rich Persaud <persaur@xxxxxxxxx> wrote:
>> On Jan 8, 2018, at 12:45, Ian Jackson <ian.jackson@xxxxxxxxxxxxx> wrote:
>>
>> AIUI we have a series for pv-in-pvh shim which is nearing completion
>> in the sense that it will have been well-tested (especially the
>> hypervisor parts) and has good functionality.  (Wei is handling the
>> assembly of this series.)
>>
>> The series, however, needs proper review and tidying up.
>> Specifically, it needs the kind of tidying up that fixes code
>> structure and style issues that will hinder future Xen development.
>> I.e. the kind of technical debt which does not directly cause bugs now
>> but will cause trouble (including bugs) in the future.
>>
>> IMO that kind of tidying up is definitely essential for
>> xen.git#master.  However, it is much less of an issue for Xen 4.10.
>> Xen 4.10, as a stable branch, will get much more limited further
>> development.  Failure to tidy things up there will make backporting
>> other changes more awkward but the overall impact is both lower and
>> time-bound.
>>
>> Currently the Xen Project has no published resolution for PV guests
>> that can't be booted as, or converted to, PVH or HVM.  (And HVM guests
>> bring their own problems.)  We need to provide our users with more
>> good options as quickly as possible.
>>
>> I would like to suggest that a good way of doing this would be to ship
>> the shim series as 4.10.1 within the next very few days.  It needs
>> some minor bugfixing (build breakage etc.) but is basically ready for
>> use.
>>
>> Speaking as a sysadmin (even, a very conservative sysadmin many of
>> whose systems are running Debian oldstable), I have already taken a
>> decision to rapidly advance to new software, in one context, because
>> of these vulnerabilities - and take and fix whatever impact that has.
>> I think many of our users would like to make the same choice.
>>
>> Releaseing 4.10.1 this week with pv-in-pvh support would give many of
>> our users with PV guests an immediately deployable update, even though
>> of course the version bump to get to 4.10 may be disruptive.
>>
>> Doing this would be a departure from our uusual non-security-bug
>> process of committing changes to xen.git#staging, and then backporting
>> only after the patches have been sitting in xen.git#master for some
>> time.  It's also a departure from our usual security-bug process of
>> developing and testing and committing patches for all supported
>> versions in parallel.
>>
>> But this is not a usual situation.  This time, we don't have the time
>> to wait.
>>
>> Opinions ?
>>
>> Ian.
>
> On a similarly pragmatic note: would a variation of Anthony's vixen patch 
> series be suitable for pre-PVH Xen 4.6 - 4.9?  These versions are currently 
> documented as security-supported (Oct 2018 - July 2020).
>
> There are production environments where upgrading to Xen 4.10 in a timeframe 
> of days or weeks is not practical.
>
> Will PCI passthrough for PV guests be supported in any of the solutions that 
> are being considered?  If not, it would be helpful to document this in the 
> Spectre/Meltdown XSA and/or FAQ, including timeline or complexity estimates 
> for the return of security support for Xen PV driver domains.  SUPPORT.md 
> will also need an update.

It's not particularly hard to plumb through I think but if you are
using PCI passthrough for PV, then you really shouldn't worry about
Spectre/Meltdown.  That PV guest can already read all of physical
memory (since no IOMMU is used) and they can also write to all
physical memory which is far worse than what you can do with
Spectre/Meltdown.

So if you're using PCI passthrough for PV, just keep using normal PV.

Regards,

Anthony Liguori

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