On Thu, 17 Dec 2020 16:38:20 +1100
David Gibson <david@xxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxx>
> > > ---
> > > 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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