Code: static int handle_emulation_failure(struct kvm_vcpu *vcpu)
{
	int r = EMULATE_DONE;
 
 	++vcpu->stat.insn_emulation_fail;
 	trace_kvm_emulate_insn_failed(vcpu);
	if (!is_guest_mode(vcpu)) {
 		vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
 		vcpu->run->internal.suberror = KVM_INTERNAL_ERROR_EMULATION;
 		vcpu->run->internal.ndata = 0;
		r = EMULATE_FAIL;
	}
	kvm_queue_exception(vcpu, UD_VECTOR);

	return r;
}

Vulnerability Type: DoS 
CWE ID: CWE-362
Summary: Race condition in arch/x86/kvm/x86.c in the Linux kernel before 3.17.4 allows guest OS users to cause a denial of service (guest OS crash) via a crafted application that performs an MMIO transaction or a PIO transaction to trigger a guest userspace emulation error report, a similar issue to CVE-2010-5313.
Commit Message: KVM: x86: Don't report guest userspace emulation error to userspace

Commit fc3a9157d314 ("KVM: X86: Don't report L2 emulation failures to
user-space") disabled the reporting of L2 (nested guest) emulation failures to
userspace due to race-condition between a vmexit and the instruction emulator.
The same rational applies also to userspace applications that are permitted by
the guest OS to access MMIO area or perform PIO.

This patch extends the current behavior - of injecting a #UD instead of
reporting it to userspace - also for guest userspace code.

Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>