Re: [RFC PATCH 08/35] KVM: SVM: Prevent debugging under SEV-ES

From: Tom Lendacky
Date: Thu Sep 17 2020 - 12:30:34 EST

On 9/16/20 5:50 PM, Sean Christopherson wrote:
On Wed, Sep 16, 2020 at 03:27:13PM -0500, Tom Lendacky wrote:
On 9/16/20 11:49 AM, Sean Christopherson wrote:
On Wed, Sep 16, 2020 at 11:38:38AM -0500, Tom Lendacky wrote:

On 9/16/20 11:02 AM, Sean Christopherson wrote:
On Wed, Sep 16, 2020 at 10:11:10AM -0500, Tom Lendacky wrote:
On 9/15/20 3:13 PM, Tom Lendacky wrote:
On 9/15/20 11:30 AM, Sean Christopherson wrote:
I don't quite follow the "doesn't mean debugging can't be done in the future".
Does that imply that debugging could be supported for SEV-ES guests, even if
they have an encrypted VMSA?

Almost anything can be done with software. It would require a lot of
hypervisor and guest code and changes to the GHCB spec, etc. So given
that, probably just the check for arch.guest_state_protected is enough for
now. I'll just need to be sure none of the debugging paths can be taken
before the VMSA is encrypted.

So I don't think there's any guarantee that the KVM_SET_GUEST_DEBUG ioctl
couldn't be called before the VMSA is encrypted, meaning I can't check the
arch.guest_state_protected bit for that call. So if we really want to get
rid of the allow_debug() op, I'd need some other way to indicate that this
is an SEV-ES / protected state guest.

Would anything break if KVM "speculatively" set guest_state_protected before
LAUNCH_UPDATE_VMSA? E.g. does KVM need to emulate before LAUNCH_UPDATE_VMSA?

Yes, the way the code is set up, the guest state (VMSA) is initialized in
the same way it is today (mostly) and that state is encrypted by the
LAUNCH_UPDATE_VMSA call. I check the guest_state_protected bit to decide
on whether to direct the updates to the real VMSA (before it's encrypted)
or the GHCB (that's the get_vmsa() function from patch #5).

Ah, gotcha. Would it work to set guest_state_protected[*] from time zero,
and move vmsa_encrypted to struct vcpu_svm? I.e. keep vmsa_encrypted, but
use it only for guiding get_vmsa() and related behavior.

It is mainly __set_sregs() that needs to know when to allow the register
writes and when not to. During guest initialization, __set_sregs is how
some of the VMSA is initialized by Qemu.

Hmm. I assume that also means KVM_SET_REGS and KVM_GET_XCRS are also legal
before the VMSA is encrypted? If so, then the current behavior of setting
vmsa_encrypted "late" make sense. KVM_SET_FPU/XSAVE can be handled by not
allocating guest_fpu, i.e. they can be disallowed from time zero without
adding an SEV-ES specific check.

Which brings us back to KVM_SET_GUEST_DEBUG. What would happen if that were
allowed prior to VMSA encryption? If LAUNCH_UPDATE_VMSA acts as a sort of
reset, one thought would be to allow KVM_SET_GUEST_DEBUG and then sanitize
KVM's state during LAUNCH_UPDATE_VMSA. Or perhaps even better, disallow
LAUNCH_UPDATE_VMSA if vcpu->guest_debug!=0. That would allow using debug
capabilities up until LAUNCH_UPDATE_VMSA without adding much burden to KVM.

I think the vcpu->guest_debug check before the LAUNCH_UPDATE_VMSA would be good. I'll remove the allow_debug() op and replace it with the guest_state_protected check in its place.