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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: Thu, 31 Jan 2019 16:35:35 +0000
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- Cc: Juergen Gross <jgross@xxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
- Delivery-date: Thu, 31 Jan 2019 16:38:40 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 31/01/2019 14:25, Jan Beulich wrote:
>>>> On 31.01.19 at 14:59, <andrew.cooper3@xxxxxxxxxx> wrote:
>> At the time XSA-273 was published, shadowing dom0 had proved to be unstable,
>> which is why dom0 was unprotected by default. The instability was identified
>> to be problems with shadowing PV superpages, and fixed.
>>
>> In hindsight, this patch should have been posted at the same time.
>>
>> There is now no legitimate reason to handle dom0 differently to domu when it
>> comes to pv-l1tf protections.
> I'm not entirely convinced by this statement: Crashing Dom0
> (and hence the entire host) because of a failure to enable
> shadow mode on it is not a good thing imo. What's wrong
> with sticking to the current default, just for reasons other
> than the original one? Anything malicious running in Dom0
> has easier (or at least different) ways of getting at the same
> information.
This statement is only true of the dom0 kernel (and root userspace,
given the questionable behaviour of /proc/xen/privcmd).
It does not hold for regular dom0 userspace - in particular, L1TF was
discovered because of an accidental mprotect(PROT_NONE), meaning that
this is a viable attack vector for a depriv qemu.
As to crashing, that is only if you compile SHADOW out, and I remain to
be convinced that compiling shadow out of Xen is a viable option at the
moment.
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.
~Andrew
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