Re: [PATCH 70/70] x86/sev-es: Add NMI state tracking

Joerg Roedel
2020-03-19T16:07:55Z

Hi Andy,

On Thu, Mar 19, 2020 at 08:35:59AM -0700, Andy Lutomirski wrote:
> On Thu, Mar 19, 2020 at 2:14 AM Joerg Roedel <joro@8bytes.org> wrote:
> >
> > From: Joerg Roedel <jroedel@suse.de>
> >
> > Keep NMI state in SEV-ES code so the kernel can re-enable NMIs for the
> > vCPU when it reaches IRET.
>
> IIRC I suggested just re-enabling NMI in C from do_nmi(). What was
> wrong with that approach?

If I understand the code correctly a nested NMI will just reset the
interrupted NMI handler to start executing again at 'restart_nmi'.
The interrupted NMI handler could be in the #VC handler, and it is not
safe to just jump back to the start of the NMI handler from somewhere
within the #VC handler.

So I decided to not allow NMI nesting for SEV-ES and only re-enable the
NMI window when the first NMI returns. This is not implemented in this
patch, but I will do that once Thomas' entry-code rewrite is upstream.

> This causes us to pop the NMI frame off the stack. Assuming the NMI
> restart logic is invoked (which is maybe impossible?), we get #DB,
> which presumably is actually delivered. And we end up on the #DB
> stack, which might already have been in use, so we have a potential
> increase in nesting. Also, #DB may be called from an unexpected
> context.

An SEV-ES hypervisor is required to intercept #DB, which means that the
#DB exception actually ends up being a #VC exception. So it will not end
up on the #DB stack.

> Now somehow #DB is supposed to invoke #VC, which is supposed to do the
> magic hypercall, and all of this is supposed to be safe? Or is #DB
> unconditionally redirected to #VC? What happens if we had no stack
> (e.g. we interrupted SYSCALL) or we were already in #VC to begin with?

Yeah, as I said above, the #DB is redirected to #VC, as the hypervisor
has to intercept #DB.

The stack-problem is the one that prevents the Single-step-over-iret
approach right now, because the NMI can hit while in kernel mode and on
entry stack, which the generic entry code (besided NMI) does not handle.
Getting a #VC exception there (like after an IRET to that state) breaks
things.

Last, in this version of the patch-set the #VC handler became
nesting-safe. It detects whether the per-cpu GHCB is in use and
safes/restores its contents in this case.

> I think there are two credible ways to approach this:
>
> 1. Just put the NMI unmask in do_nmi(). The kernel *already* knows
> how to handle running do_nmi() with NMIs unmasked. This is much, much
> simpler than your code.

Right, and I thought about that, but the implication is that the
complexity is moved somewhere else, namely into the #VC handler, which
then has to be restartable.

> 2. Have an entirely separate NMI path for the
> SEV-ES-on-misdesigned-CPU case. And have very clear documentation for
> what prevents this code from being executed on future CPUs (Zen3?)
> that have this issue fixed for real?

That sounds like a good alternative, I will investigate this approach.
The NMI handler should be much simpler as it doesn't need to allow NMI
nesting. The question is, does the C code down the NMI path depend on
the NMI handlers stack frame layout (e.g. the in-nmi flag)?

Regards,

Joerg