Re: [PATCH] SUPPORT.md: add Dom0less as Supported

[Julien Grall <julien@xen.org>]

Hi Stefano,

On 14/07/2021 20:28, Stefano Stabellini wrote:
> On Wed, 14 Jul 2021, Julien Grall wrote:
>> Hi Stefano,
>>
>> On 14/07/2021 01:39, Stefano Stabellini wrote:
>>> Add Dom0less to SUPPORT.md to clarify its support status. The feature is
>>> mature enough and small enough to make it security supported.
>>>
>>> Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
>>>
>>> diff --git a/SUPPORT.md b/SUPPORT.md
>>> index 317392d8f3..c777f3da72 100644
>>> --- a/SUPPORT.md
>>> +++ b/SUPPORT.md
>>> @@ -832,6 +832,12 @@ OVMF firmware implements the UEFI boot protocol.
>>>          Status, qemu-xen: Supported
>>>    +## Dom0less
>>> +
>>> +Guest creation from the hypervisor at boot without Dom0 intervention.
>>> +
>>> +    Status, ARM: Supported
>>> +
>>
>> After XSA-372, we will not scrubbed memory assigned to dom0less DomU when
>> bootscrub=on.
> 
> Do you mean *before* XSA-372, right?

No, I really meant *after* XSA-372.

> I thought the XSA-372 patches take
> care of the problem?

It didn't. We have an open question for the bootscrub=on one. From the 
commit message of patch #1:

         2) The memory allocated for a domU will not be scrubbed anymore 
when an
         admin select bootscrub=on. This is not something we advertised, 
but if
         this is a concern we can introduce either force scrub for all 
domUs or
         a per-domain flag in the DT. The behavior for bootscrub=off and
         bootscrub=idle (default) has not changed.

>  >
>> Do we want to exclude this combination or mention that XSAs will
>> not be issued if the domU can read secret from unscrubbed memory?
> 
> I could say that if bootscrub=off then we won't issue XSAs for domUs reading
> secrets from unscrubbed memory. But it is kind of obvious anyway? I am
> happy to add it if you think it is good to clarify.

Right, it is pretty clear that bootscrub=off will not scrub the memory 
and the "problem" would not be specific to dom0less.

The one I asked to clarify is bootscrub=on because one may think the 
memory is scrubbed for dom0less domU for all the cases but bootscrub=off.

IIRC when we discussed about it on security@xen.org, we suggested that 
it would be fine to rely on the bootloader to scrub it. But I think this 
needs to be written down rather waiting for it to be re-discovered.

The other solution is to fix it properly.

Cheers,

-- 
Julien Grall