KVM: x86: respect singlestep when emulating instruction

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@vger.kernel.org
Signed-off-by: Felipe Franciosi <felipe@nutanix.com>
Message-Id: <20200519081048.8204-1-felipe@nutanix.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 87ee76b73d72305fc45e4a39bcc275c4ad036a9b..172843a8c3141e3f9cae679d810a9e79439ff29c 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7010,7 +7010,7 @@ 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);