Re: [for-6.0 v5 08/13] securable guest memory: Introduce sgm "ready" flag

[Cornelia Huck <cohuck@redhat.com>]

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On Thu, 17 Dec 2020 16:38:20 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Mon, Dec 14, 2020 at 06:00:36PM +0100, Cornelia Huck wrote:
> > On Fri,  4 Dec 2020 16:44:10 +1100
> > David Gibson <david@gibson.dropbear.id.au> wrote:
> >   
> > > The platform specific details of mechanisms for implementing securable
> > > guest memory may require setup at various points during initialization.
> > > Thus, it's not really feasible to have a single sgm initialization hook,
> > > but instead each mechanism needs its own initialization calls in arch or
> > > machine specific code.
> > > 
> > > However, to make it harder to have a bug where a mechanism isn't properly
> > > initialized under some circumstances, we want to have a common place,
> > > relatively late in boot, where we verify that sgm has been initialized if
> > > it was requested.
> > > 
> > > This patch introduces a ready flag to the SecurableGuestMemory base type
> > > to accomplish this, which we verify just before the machine specific
> > > initialization function.
> > > 
> > > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > > ---
> > >  hw/core/machine.c                     | 8 ++++++++
> > >  include/exec/securable-guest-memory.h | 2 ++
> > >  target/i386/sev.c                     | 2 ++
> > >  3 files changed, 12 insertions(+)
> > > 
> > > diff --git a/hw/core/machine.c b/hw/core/machine.c
> > > index 816ea3ae3e..a67a27d03c 100644
> > > --- a/hw/core/machine.c
> > > +++ b/hw/core/machine.c
> > > @@ -1155,6 +1155,14 @@ void machine_run_board_init(MachineState *machine)
> > >      }
> > >  
> > >      if (machine->sgm) {
> > > +        /*
> > > +         * Where securable guest memory is initialized depends on the
> > > +         * specific mechanism in use.  But, we need to make sure it's
> > > +         * ready by now.  If it isn't, that's a bug in the
> > > +         * implementation of that sgm mechanism.
> > > +         */
> > > +        assert(machine->sgm->ready);  
> > 
> > Under which circumstances might we arrive here with 'ready' not set?
> > 
> > - programming error, setup is happening too late -> assert() seems
> >   appropriate  
> 
> Yes, this is designed to catch programming errors.  In particular I'm
> concerned about:
>   * Re-arranging the init code, and either entirely forgetting the sgm
>     setup, or accidentally moving it too late
>   * The sgm setup is buried in the machine setup code, conditional on
>     various things, and changes mean we no longer either call it or
>     (correctly) fail
>   * User has specified an sgm scheme designed for a machine type other
>     than the one they selected.  The arch/machine init code hasn't
>     correctly accounted for that possibility and ignores it, instead
>     of correctly throwing an error
>  
> > - we tried to set it up, but some error happened -> should we rely on
> >   the setup code to error out first? (i.e. we won't end up here, unless
> >   there's a programming error, in which case the assert() looks
> >   fine)  
> 
> Yes, that's my intention.
> 
> >   Is there a possible use case for "we could not set it up, but we
> >   support an unsecured guest (as long as it is clear what happens)"?  
> 
> I don't think so.  My feeling is that if you specify that you want the
> feature, qemu needs to either give it to you, or fail, not silently
> degrade the features presented to the guest.

Yes, that should align with what QEMU is doing elsewhere.

> 
> >   Likely only for guests that transition themselves, but one could
> >   argue that QEMU should simply be invoked a second time without the
> >   sgm stuff being specified in the error case.  
> 
> Right - I think whatever error we give here is likely to be easier to
> diagnose than the guest itself throwing an error when it fails to
> transition to secure mode (plus we should catch it always, rather than
> only if we run a guest which tries to go secure).

Yes, that makes sense.

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