Re: [PATCH v3 3/6] KVM: SVM: implement force_intercept_exceptions_mask

Maxim Levitsky
2022-02-08T14:35:34Z

On Thu, 2021-09-02 at 17:34 +0000, Sean Christopherson wrote:
> On Wed, Aug 11, 2021, Maxim Levitsky wrote:
> > On Wed, 2021-08-11 at 15:29 +0300, Maxim Levitsky wrote:
> > > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > > index e45259177009..19f54b07161a 100644
> > > --- a/arch/x86/kvm/svm/svm.c
> > > +++ b/arch/x86/kvm/svm/svm.c
> > > @@ -233,6 +233,8 @@ static const u32 msrpm_ranges[] = {0, 0xc0000000, 0xc0010000};
> > > #define MSRS_RANGE_SIZE 2048
> > > #define MSRS_IN_RANGE (MSRS_RANGE_SIZE * 8 / 2)
> > >
> > > +static int svm_handle_invalid_exit(struct kvm_vcpu *vcpu, u64 exit_code);
> > > +
> > > u32 svm_msrpm_offset(u32 msr)
> > > {
> > > u32 offset;
> > > @@ -1153,6 +1155,22 @@ static void svm_recalc_instruction_intercepts(struct kvm_vcpu *vcpu,
> > > }
> > > }
> > >
> > > +static void svm_init_force_exceptions_intercepts(struct vcpu_svm *svm)
> > > +{
> > > + int exc;
> > > +
> > > + svm->force_intercept_exceptions_mask = force_intercept_exceptions_mask;
>
> Ah, the param is being snapshotted on vCPU creation, hence the writable module
> param. That works, though it'd be better to snapshot it on a per-VM basic, not
> per-vCPU, and do so in common x86 code so that the param doesn't need to be
> exported.

I have nothing against that.

>
> > > + for (exc = 0 ; exc < 32 ; exc++) {
>
> for_each_set_bit()
I used a helper function instead, IMHO a bit cleaner.

>
> > > + if (!(svm->force_intercept_exceptions_mask & (1 << exc)))
> > > + continue;
> > > +
> > > + /* Those are defined to have undefined behavior in the SVM spec */
> > > + if (exc != 2 && exc != 9)
>
> Maybe add a pr_warn_once() to let the user know they done messed up?
>
> And given that there are already intercepts with undefined behavior, it's probably
> best to disallow intercepting anything we aren't 100% postive will be handled
> correctly, e.g. intercepting vector 31 is nonsensical at this time.

Or I think I'll just drop this check altogether - this is a debug feature anyway.

>
> > > + continue;
> > > + set_exception_intercept(svm, exc);
>
> ...
>
> > > +static int gen_exc_interception(struct kvm_vcpu *vcpu)
> > > +{
> > > + /*
> > > + * Generic exception intercept handler which forwards a guest exception
> > > + * as-is to the guest.
> > > + * For exceptions that don't have a special intercept handler.
> > > + *
> > > + * Used only for 'force_intercept_exceptions_mask' KVM debug feature.
> > > + */
> > > + struct vcpu_svm *svm = to_svm(vcpu);
> > > + int exc = svm->vmcb->control.exit_code - SVM_EXIT_EXCP_BASE;
> > > +
> > > + /* SVM doesn't provide us with an error code for the #DF */
> > > + u32 err_code = exc == DF_VECTOR ? 0 : svm->vmcb->control.exit_info_1;
>
> Might be better to handle this in the x86_exception_has_error_code() path to
> avoid confusing readers with respect to exceptions that don't have an error code,
> e.g.
>
> else if (x86_exception_has_error_code(exc)) {
> /* SVM doesn't provide the error code on #DF :-( */
> if (exc == DF_VECTOR)
> kvm_queue_exception_e(vcpu, exc, 0);
> else
> kvm_queue_exception_e(vcpu, exc, svm->vmcb->control.exit_info_1);
> } else {
> ...
> }
>
> Alternatively, can we zero svm->vmcb->control.exit_info_1 on #DF to make it more
> obvious that SVM leaves stale data in exit_info_1 (assuming that's true)? E.g.
>
> ...
>
> if (exc == TS_VECTOR) {
> ...
> } else if (x86_exception_has_error_code(exc)) {
> /* SVM doesn't provide the error code on #DF :-( */
> if (exc == DF_VECTOR)
> svm->vmcb->control.exit_info_1 = 0;
>
> kvm_queue_exception_e(vcpu, exc, svm->vmcb->control.exit_info_1);
> } else {
> ...
> }

Makes sense.

>
>
> > > +
> > > + if (!(svm->force_intercept_exceptions_mask & (1 << exc)))
>
> BIT(exc)
I added a helper function in common x86 code for this.

>
> > > + return svm_handle_invalid_exit(vcpu, svm->vmcb->control.exit_code);
> > > +
> > > + if (exc == TS_VECTOR) {
> > > + /*
> > > + * SVM doesn't provide us with an error code to be able to
> > > + * re-inject the #TS exception, so just disable its
> > > + * intercept, and let the guest re-execute the instruction.
> > > + */
> > > + vmcb_clr_intercept(&svm->vmcb01.ptr->control,
> > > + INTERCEPT_EXCEPTION_OFFSET + TS_VECTOR);
>
> Maybe just disallow intercepting #TS altogether? Or does this fall into your
> Win98 use case? :-)

Win98 does indeed generate few #TS exceptions but so far I haven't noticed
any issues related to task switches. Anyway I would like to intercept
as much as possible since this is a debug feature. A single interception
is still better that nothing.

>
> > > + recalc_intercepts(svm);
> > > + } else if (x86_exception_has_error_code(exc))
> > > + kvm_queue_exception_e(vcpu, exc, err_code);
> > > + else
> > > + kvm_queue_exception(vcpu, exc);
> > > + return 1;
> > > +}
> > > +
> > > static bool is_erratum_383(void)
> > > {
> > > int err, i;
> > > @@ -3065,6 +3131,10 @@ static int (*const svm_exit_handlers[])(struct kvm_vcpu *vcpu) = {
> > > [SVM_EXIT_WRITE_DR5] = dr_interception,
> > > [SVM_EXIT_WRITE_DR6] = dr_interception,
> > > [SVM_EXIT_WRITE_DR7] = dr_interception,
> > > +
> > > + [SVM_EXIT_EXCP_BASE ...
> > > + SVM_EXIT_EXCP_BASE + 31] = gen_exc_interception,
>
> This generates a Sparse warning due to the duplicate initializer. IMO that's a
> very good warning as I have zero idea how the compiler actually handles this
> particular scenario, e.g. do later entries take priority, is it technically
> "undefined" behavior, etc...
>
> arch/x86/kvm/svm/svm.c:3065:10: warning: Initializer entry defined twice
> arch/x86/kvm/svm/svm.c:3067:29: also defined here
>
> I don't have a clever solution though :-('

Good catch. I thought that this would make sense but standards never make sense.
I'll do this manually.

>
> > > +
> > > [SVM_EXIT_EXCP_BASE + DB_VECTOR] = db_interception,
> > > [SVM_EXIT_EXCP_BASE + BP_VECTOR] = bp_interception,
> > > [SVM_EXIT_EXCP_BASE + UD_VECTOR] = ud_interception,
> > > diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> > > index 524d943f3efc..187ada7c5b03 100644
> > > --- a/arch/x86/kvm/svm/svm.h
> > > +++ b/arch/x86/kvm/svm/svm.h
> > > @@ -196,6 +196,7 @@ struct vcpu_svm {
> > > bool ghcb_sa_free;
> > >
> > > bool guest_state_loaded;
> > > + u32 force_intercept_exceptions_mask;
> > > };
> > >
> > > struct svm_cpu_data {
> > > @@ -351,8 +352,11 @@ static inline void clr_exception_intercept(struct vcpu_svm *svm, u32 bit)
> > > struct vmcb *vmcb = svm->vmcb01.ptr;
> > >
> > > WARN_ON_ONCE(bit >= 32);
> > > - vmcb_clr_intercept(&vmcb->control, INTERCEPT_EXCEPTION_OFFSET + bit);
> > >
> > > + if ((1 << bit) & svm->force_intercept_exceptions_mask)
>
> BIT(bit)
Fixed with helper function as well.

>
> > > + return;
> > > +
> > > + vmcb_clr_intercept(&vmcb->control, INTERCEPT_EXCEPTION_OFFSET + bit);
> > > recalc_intercepts(svm);
> > > }

Thanks for the review!
Best regards,
Maxim Levitsky