Commit 384dea1c authored May 19, 2020 by Felipe Franciosi Committed by Paolo Bonzini Jun 11, 2020

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>

parent 7e464770

arch/x86/kvm/x86.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -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);

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