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SubjectRe: [PATCH v4 2/7] x86: kvm: svm: propagate errors from skip_emulated_instruction()
On Tue, Aug 13, 2019 at 03:53:30PM +0200, Vitaly Kuznetsov wrote:
> @@ -3899,20 +3898,25 @@ static int task_switch_interception(struct vcpu_svm *svm)
> if (reason != TASK_SWITCH_GATE ||
> int_type == SVM_EXITINTINFO_TYPE_SOFT ||
> (int_type == SVM_EXITINTINFO_TYPE_EXEPT &&
> - (int_vec == OF_VECTOR || int_vec == BP_VECTOR)))
> - skip_emulated_instruction(&svm->vcpu);
> + (int_vec == OF_VECTOR || int_vec == BP_VECTOR))) {
> + if (skip_emulated_instruction(&svm->vcpu) != EMULATE_DONE)

This isn't broken in the current code, but it's flawed in the sense that
it assumes skip_emulated_instruction() never returns EMULATE_USER_EXIT.

Note, both SVM and VMX make the opposite assumption when handling
kvm_task_switch() and kvm_inject_realmode_interrupt().

More below...

> + goto fail;
> + }
>
> if (int_type != SVM_EXITINTINFO_TYPE_SOFT)
> int_vec = -1;
>
> if (kvm_task_switch(&svm->vcpu, tss_selector, int_vec, reason,
> - has_error_code, error_code) == EMULATE_FAIL) {
> - svm->vcpu.run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
> - svm->vcpu.run->internal.suberror = KVM_INTERNAL_ERROR_EMULATION;
> - svm->vcpu.run->internal.ndata = 0;
> - return 0;
> - }
> + has_error_code, error_code) == EMULATE_FAIL)
> + goto fail;
> +
> return 1;
> +
> +fail:
> + svm->vcpu.run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
> + svm->vcpu.run->internal.suberror = KVM_INTERNAL_ERROR_EMULATION;
> + svm->vcpu.run->internal.ndata = 0;
> + return 0;
> }
>
> static int cpuid_interception(struct vcpu_svm *svm)

...

> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index c6d951cbd76c..e8f797fe9d9e 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6383,9 +6383,11 @@ static void kvm_vcpu_do_singlestep(struct kvm_vcpu *vcpu, int *r)
> int kvm_skip_emulated_instruction(struct kvm_vcpu *vcpu)
> {
> unsigned long rflags = kvm_x86_ops->get_rflags(vcpu);
> - int r = EMULATE_DONE;
> + int r;
>
> - kvm_x86_ops->skip_emulated_instruction(vcpu);
> + r = kvm_x86_ops->skip_emulated_instruction(vcpu);
> + if (unlikely(r != EMULATE_DONE))
> + return 0;

x86_emulate_instruction() doesn't set vcpu->run->exit_reason when emulation
fails with EMULTYPE_SKIP, i.e. this will exit to userspace with garbage in
the exit_reason.

handle_ept_misconfig() also has the same (pre-existing) flaw.

Given the handle_ept_misconfig() bug and that kvm_emulate_instruction()
sets vcpu->run->exit_reason when it returns EMULATE_FAIL in the normal
case, I think it makes sense to fix the issue in x86_emulate_instruction().
That would also eliminate the need to worry about EMULATE_USER_EXIT in
task_switch_interception(), e.g. the SVM code can just return 0 when it
gets a non-EMULATE_DONE return type.

E.g.:

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 07ab14d73094..73b86f81ed9c 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6201,7 +6201,8 @@ static int handle_emulation_failure(struct kvm_vcpu *vcpu, int emulation_type)
if (emulation_type & EMULTYPE_NO_UD_ON_FAIL)
return EMULATE_FAIL;

- if (!is_guest_mode(vcpu) && kvm_x86_ops->get_cpl(vcpu) == 0) {
+ if ((!is_guest_mode(vcpu) && kvm_x86_ops->get_cpl(vcpu) == 0) ||
+ (emulation_type & EMULTYPE_SKIP)) {
vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
vcpu->run->internal.suberror = KVM_INTERNAL_ERROR_EMULATION;
vcpu->run->internal.ndata = 0;
@@ -6525,8 +6526,6 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
return EMULATE_DONE;
if (ctxt->have_exception && inject_emulated_exception(vcpu))
return EMULATE_DONE;
- if (emulation_type & EMULTYPE_SKIP)
- return EMULATE_FAIL;
return handle_emulation_failure(vcpu, emulation_type);
}
}

As for the kvm_task_switch() handling and other cases, I think it's
possible to rework all of the functions and callers that return/handle
EMULATE_DONE to instead return 0/1, i.e. contain EMULATE_* to x86.c.
I'll put together a series, I think you've suffered more than enough scope
creep as it is :-)

>
> /*
> * rflags is the old, "raw" value of the flags. The new value has
> --
> 2.20.1
>

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