KVM: x86: Emulate only IN/OUT instructions when accessing VMware backdoor

Access to VMware backdoor ports is done by one of the IN/OUT/INS/OUTS
instructions. These ports must be allowed access even if TSS I/O
permission bitmap don't allow it.

To handle this, VMX/SVM will be changed in future commits
to intercept #GP which was raised by such access and
handle it by calling x86 emulator to emulate instruction.
If it was one of these instructions, the x86 emulator already handles
it correctly (Since commit "KVM: x86: Always allow access to VMware
backdoor I/O ports") by not checking these ports against TSS I/O
permission bitmap.

One may wonder why checking for specific instructions is necessary
as we can just forward all #GPs to the x86 emulator.
There are multiple reasons for doing so:

1. We don't want the x86 emulator to be reached easily
by guest by just executing an instruction that raises #GP as that
exposes the x86 emulator as a bigger attack surface.

2. The x86 emulator is incomplete and therefore certain instructions
that can cause #GP cannot be emulated. Such an example is "INT x"
(opcode 0xcd) which reaches emulate_int() which can only emulate
the instruction if vCPU is in real-mode.

Signed-off-by: Liran Alon <liran.alon@oracle.com>
Reviewed-by: Nikita Leshenko <nikita.leshchenko@oracle.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -1194,6 +1194,7 @@ enum emulation_result {
 #define EMULTYPE_RETRY		    (1 << 3)
 #define EMULTYPE_NO_REEXECUTE	    (1 << 4)
 #define EMULTYPE_NO_UD_ON_FAIL	    (1 << 5)
+#define EMULTYPE_VMWARE		    (1 << 6)
 int x86_emulate_instruction(struct kvm_vcpu *vcpu, unsigned long cr2,
 			    int emulation_type, void *insn, int insn_len);
 

arch/x86/kvm/x86.c

@@ -5930,6 +5930,30 @@ static bool kvm_vcpu_check_breakpoint(struct kvm_vcpu *vcpu, int *r)
 	return false;
 }
 
+static bool is_vmware_backdoor_opcode(struct x86_emulate_ctxt *ctxt)
+{
+	if (ctxt->opcode_len != 1)
+		return false;
+
+	switch (ctxt->b) {
+	case 0xe4:	/* IN */
+	case 0xe5:
+	case 0xec:
+	case 0xed:
+	case 0xe6:	/* OUT */
+	case 0xe7:
+	case 0xee:
+	case 0xef:
+	case 0x6c:	/* INS */
+	case 0x6d:
+	case 0x6e:	/* OUTS */
+	case 0x6f:
+		return true;
+	}
+
+	return false;
+}
+
 int x86_emulate_instruction(struct kvm_vcpu *vcpu,
 			    unsigned long cr2,
 			    int emulation_type,
@@ -5986,6 +6010,10 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
 		}
 	}
 
+	if ((emulation_type & EMULTYPE_VMWARE) &&
+	    !is_vmware_backdoor_opcode(ctxt))
+		return EMULATE_FAIL;
+
 	if (emulation_type & EMULTYPE_SKIP) {
 		kvm_rip_write(vcpu, ctxt->_eip);
 		if (ctxt->eflags & X86_EFLAGS_RF)