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

[Stefano Stabellini <sstabellini@kernel.org>]

On Wed, 14 Jul 2021, Julien Grall wrote:
> 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.

Ah right, I remember what you are talking about. I'll update the patch.