Re: [RFC PATCH 08/21] KVM: x86: Add kvm_x86_ops hook to short circuit emulation

[Andy Lutomirski]

On Mon, Aug 19, 2019 at 3:01 PM Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:
>
> On Thu, Aug 15, 2019 at 05:47:12PM -0700, Andy Lutomirski wrote:
> >
> >
> > >> On Jul 29, 2019, at 7:49 PM, Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote:
> > >>
> > >> On Sat, Jul 27, 2019 at 10:38:03AM -0700, Andy Lutomirski wrote:
> > >> On Fri, Jul 26, 2019 at 10:52 PM Sean Christopherson
> > >> <sean.j.christopherson@xxxxxxxxx> wrote:
> > >>>
> > >>> Similar to the existing AMD #NPF case where emulation of the current
> > >>> instruction is not possible due to lack of information, virtualization
> > >>> of Intel SGX will introduce a scenario where emulation is not possible
> > >>> due to the VMExit occurring in an SGX enclave.  And again similar to
> > >>> the AMD case, emulation can be initiated by kvm_mmu_page_fault(), i.e.
> > >>> outside of the control of the vendor-specific code.
> > >>>
> > >>> While the cause and architecturally visible behavior of the two cases
> > >>> is different,  e.g. Intel SGX will inject a #UD whereas AMD #NPF is a
> > >>> clean resume or complete shutdown, the impact on the common emulation
> > >>> code is identical: KVM must stop emulation immediately and resume the
> > >>> guest.
> > >>>
> > >>> Replace the exisiting need_emulation_on_page_fault() with a more generic
> > >>> is_emulatable() kvm_x86_ops callback, which is called unconditionally
> > >>> by x86_emulate_instruction().
> > >>
> > >> Having recently noticed that emulate_ud() is broken when the guest's
> > >> TF is set, I suppose I should ask: does your new code function
> > >> sensibly when TF is set?
> > >
> > > Barring a VMX fault injection interaction I'm not thinking of, yes.  The
> > > SGX reaction to the #UD VM-Exit is to inject a #UD and resume the guest,
> > > pending breakpoints shouldn't be affected in any way (unless some other
> > > part of KVM mucks with them, e.g. when guest single-stepping is enabled).
> >
> > What I mean is: does the code actually do what you think it does if TF is
> > set?  Right now, as I understand it, the KVM emulation code has a bug in
> > which some emulated faults also inject #DB despite the fact that the
> > instruction faulted, and the #DB seems to take precedence over the original
> > fault.  This confuses the guest.
>
> Yes.  The proposed change is to inject the #UD instead of calling into the
> emulator, and by inspection I've verified that all code that injects a #DB
> is either contained within the emulator or is mutually exclusive with an
> intercepted #UD.  It's a qualified yes because I don't have an actual
> testcase to verify my literacy.  I'll look into adding a test, either to
> the selftest/x86/sgx or to kvm-unit-tests.

I wrote one, and it fails:

# ./tools/testing/selftests/x86/syscall_arg_fault_32
[RUN]    SYSENTER with invalid state
[OK]    Seems okay
[RUN]    SYSCALL with invalid state
[SKIP]    Illegal instruction
[RUN]    SYSENTER with TF and invalid state
[OK]    Seems okay
[RUN]    SYSCALL with TF and invalid state
[WARN]    Got stuck single-stepping -- you probably have a KVM bug

emulate_ud() is buggy.