KVM: x86: Exit to userspace if fastpath triggers one on instruction skip

Exit to userspace if a fastpath handler triggers such an exit, which can
happen when skipping the instruction, e.g. due to userspace
single-stepping the guest via KVM_GUESTDBG_SINGLESTEP or because of an
emulation failure.

Fixes: 404d5d7b ("KVM: X86: Introduce more exit_fastpath_completion enum values")
Link: https://lore.kernel.org/r/20240802195120.325560-4-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/include/asm/kvm_host.h

@@ -211,6 +211,7 @@ enum exit_fastpath_completion {
 	EXIT_FASTPATH_NONE,
 	EXIT_FASTPATH_REENTER_GUEST,
 	EXIT_FASTPATH_EXIT_HANDLED,
+	EXIT_FASTPATH_EXIT_USERSPACE,
 };
 typedef enum exit_fastpath_completion fastpath_t;
 

arch/x86/kvm/x86.c

@@ -2206,8 +2206,10 @@ fastpath_t handle_fastpath_set_msr_irqoff(struct kvm_vcpu *vcpu)
 	}
 
 	if (handled) {
-		kvm_skip_emulated_instruction(vcpu);
-		ret = EXIT_FASTPATH_REENTER_GUEST;
+		if (!kvm_skip_emulated_instruction(vcpu))
+			ret = EXIT_FASTPATH_EXIT_USERSPACE;
+		else
+			ret = EXIT_FASTPATH_REENTER_GUEST;
 		trace_kvm_msr_write(msr, data);
 	} else {
 		ret = EXIT_FASTPATH_NONE;
@@ -11196,6 +11198,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 	if (vcpu->arch.apic_attention)
 		kvm_lapic_sync_from_vapic(vcpu);
 
+	if (unlikely(exit_fastpath == EXIT_FASTPATH_EXIT_USERSPACE))
+		return 0;
+
 	r = kvm_x86_call(handle_exit)(vcpu, exit_fastpath);
 	return r;