[PATCH v2] KVM: x86: respect singlestep when emulating instruction

[PATCH v2] KVM: x86: respect singlestep when emulating instruction

[Felipe Franciosi]

When userspace configures KVM_GUESTDBG_SINGLESTEP, KVM will manage the
presence of X86_EFLAGS_TF via kvm_set/get_rflags on vcpus. The actual
rflag bit is therefore hidden from callers.

That includes init_emulate_ctxt() which uses the value returned from
kvm_get_flags() to set ctxt->tf. As a result, x86_emulate_instruction()
will skip a single step, leaving singlestep_rip stale and not returning
to userspace.

This resolves the issue by observing the vcpu guest_debug configuration
alongside ctxt->tf in x86_emulate_instruction(), performing the single
step if set.

Signed-off-by: Felipe Franciosi <felipe@nutanix.com>
---
 arch/x86/kvm/x86.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c17e6eb9ad43..64cb183636da 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6919,7 +6919,7 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
 		if (!ctxt->have_exception ||
 		    exception_type(ctxt->exception.vector) == EXCPT_TRAP) {
 			kvm_rip_write(vcpu, ctxt->eip);
-			if (r && ctxt->tf)
+			if (r && (ctxt->tf || (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)))
 				r = kvm_vcpu_do_singlestep(vcpu);
 			if (kvm_x86_ops.update_emulated_instruction)
 				kvm_x86_ops.update_emulated_instruction(vcpu);
-- 
2.20.1

Re: [PATCH v2] KVM: x86: respect singlestep when emulating instruction

[Paolo Bonzini]

On 19/05/20 10:11, Felipe Franciosi wrote:
> When userspace configures KVM_GUESTDBG_SINGLESTEP, KVM will manage the
> presence of X86_EFLAGS_TF via kvm_set/get_rflags on vcpus. The actual
> rflag bit is therefore hidden from callers.
> 
> That includes init_emulate_ctxt() which uses the value returned from
> kvm_get_flags() to set ctxt->tf. As a result, x86_emulate_instruction()
> will skip a single step, leaving singlestep_rip stale and not returning
> to userspace.
> 
> This resolves the issue by observing the vcpu guest_debug configuration
> alongside ctxt->tf in x86_emulate_instruction(), performing the single
> step if set.
> 
> Signed-off-by: Felipe Franciosi <felipe@nutanix.com>
> ---
>  arch/x86/kvm/x86.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index c17e6eb9ad43..64cb183636da 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6919,7 +6919,7 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>  		if (!ctxt->have_exception ||
>  		    exception_type(ctxt->exception.vector) == EXCPT_TRAP) {
>  			kvm_rip_write(vcpu, ctxt->eip);
> -			if (r && ctxt->tf)
> +			if (r && (ctxt->tf || (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)))
>  				r = kvm_vcpu_do_singlestep(vcpu);
>  			if (kvm_x86_ops.update_emulated_instruction)
>  				kvm_x86_ops.update_emulated_instruction(vcpu);
> 

Queued, thanks.

Paolo