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Re: [Xen-devel] [PATCH] x86/pv: Enable pv-l1tf mitigations for dom0 by default
- To: Jan Beulich <JBeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 3 Jul 2019 12:37:31 +0100
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- Cc: Juergen Gross <JGross@xxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
- Delivery-date: Wed, 03 Jul 2019 11:37:38 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 01/02/2019 07:39, Jan Beulich wrote:
>>>> Basically, if you've got an updated dom0 kernel, you'll be fine even
>>>> with this default flipped. If you've forgotten/missed that, then you're
>>>> already wide open (in a lack of defence in depth way) and flipping the
>>>> default here will make things blindly obvious.
>>> Well, for new versions flipping the default may indeed be acceptable
>>> based on this argument. But even then - and even more so for stable
>>> versions - the change in behavior may come as a surprise to people
>>> who have perhaps even deliberately chosen not to upgrade their
>>> kernels.
>> If it were not with the instability, XSA-273 would have gone out with
>> this default.
> I'm not sure this would have been the case - the argument of
> avoiding a host crash would still have been one to consider.
> I've just checked, and I did bring up that aspect back at the
> time already, especially also for the !SHADOW_PAGING case
> (where I also continue to think it would be wrong to crash the
> host by default), irrespective of whether actually building that
> way is a viable option at the moment.
I know what you said at the time, and perhaps it ought to be telling
that you didn't change my mind.
It is frankly exhausting having this argument repeatedly, but my
position is not changing.
To be absolutely clear, I would have gone as far as nacking an attempt
to make it not the default, had it not been for the instability we
ultimately failed to fix within the embargo window.
About ~100% of deployments which are going to take this change will have
a fixed dom0 kernel, so this is a no-op in terms of behaviour.
However, we really do have production downstream users where dom0
doesn't have carte blanch access to guest memory, and therefore the "is
all powerful" argument is false. For those deployments, the current
default is a security risk.
Dom0 is not sufficiently special that the pv-l1tf default warrants being
the odd-feature-out.
~Andrew
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