[PATCH 5.7 048/163] KVM: x86: respect singlestep when emulating instruction

From: Greg Kroah-Hartman
Date: Tue Jun 16 2020 - 12:12:57 EST


From: Felipe Franciosi <felipe@xxxxxxxxxxx>

commit 384dea1c9183880be183cfaae161d99aafd16df6 upstream.

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.

Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Felipe Franciosi <felipe@xxxxxxxxxxx>
Message-Id: <20200519081048.8204-1-felipe@xxxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
arch/x86/kvm/x86.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6923,7 +6923,7 @@ restart:
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);