KVM: nVMX: Decoding memory operands of VMX instructions

This patch includes a utility function for decoding pointer operands of VMX
instructions issued by L1 (a guest hypervisor)
Signed-off-by: Nadav Har'El <nyh@il.ibm.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/kvm/vmx.c

@@ -4342,6 +4342,59 @@ static int handle_vmoff(struct kvm_vcpu *vcpu)
 	return 1;
 }
 
+/*
+ * Decode the memory-address operand of a vmx instruction, as recorded on an
+ * exit caused by such an instruction (run by a guest hypervisor).
+ * On success, returns 0. When the operand is invalid, returns 1 and throws
+ * #UD or #GP.
+ */
+static int get_vmx_mem_address(struct kvm_vcpu *vcpu,
+				 unsigned long exit_qualification,
+				 u32 vmx_instruction_info, gva_t *ret)
+{
+	/*
+	 * According to Vol. 3B, "Information for VM Exits Due to Instruction
+	 * Execution", on an exit, vmx_instruction_info holds most of the
+	 * addressing components of the operand. Only the displacement part
+	 * is put in exit_qualification (see 3B, "Basic VM-Exit Information").
+	 * For how an actual address is calculated from all these components,
+	 * refer to Vol. 1, "Operand Addressing".
+	 */
+	int  scaling = vmx_instruction_info & 3;
+	int  addr_size = (vmx_instruction_info >> 7) & 7;
+	bool is_reg = vmx_instruction_info & (1u << 10);
+	int  seg_reg = (vmx_instruction_info >> 15) & 7;
+	int  index_reg = (vmx_instruction_info >> 18) & 0xf;
+	bool index_is_valid = !(vmx_instruction_info & (1u << 22));
+	int  base_reg       = (vmx_instruction_info >> 23) & 0xf;
+	bool base_is_valid  = !(vmx_instruction_info & (1u << 27));
+
+	if (is_reg) {
+		kvm_queue_exception(vcpu, UD_VECTOR);
+		return 1;
+	}
+
+	/* Addr = segment_base + offset */
+	/* offset = base + [index * scale] + displacement */
+	*ret = vmx_get_segment_base(vcpu, seg_reg);
+	if (base_is_valid)
+		*ret += kvm_register_read(vcpu, base_reg);
+	if (index_is_valid)
+		*ret += kvm_register_read(vcpu, index_reg)<<scaling;
+	*ret += exit_qualification; /* holds the displacement */
+
+	if (addr_size == 1) /* 32 bit */
+		*ret &= 0xffffffff;
+
+	/*
+	 * TODO: throw #GP (and return 1) in various cases that the VM*
+	 * instructions require it - e.g., offset beyond segment limit,
+	 * unusable or unreadable/unwritable segment, non-canonical 64-bit
+	 * address, and so on. Currently these are not checked.
+	 */
+	return 0;
+}
+
 /*
  * The exit handlers return 1 if the exit was handled fully and guest execution
  * may resume.  Otherwise they set the kvm_run parameter to indicate what needs

arch/x86/kvm/x86.c

@@ -3848,7 +3848,7 @@ static int kvm_fetch_guest_virt(struct x86_emulate_ctxt *ctxt,
 					  exception);
 }
 
-static int kvm_read_guest_virt(struct x86_emulate_ctxt *ctxt,
+int kvm_read_guest_virt(struct x86_emulate_ctxt *ctxt,
 			       gva_t addr, void *val, unsigned int bytes,
 			       struct x86_exception *exception)
 {
@@ -3858,6 +3858,7 @@ static int kvm_read_guest_virt(struct x86_emulate_ctxt *ctxt,
 	return kvm_read_guest_virt_helper(addr, val, bytes, vcpu, access,
 					  exception);
 }
+EXPORT_SYMBOL_GPL(kvm_read_guest_virt);
 
 static int kvm_read_guest_virt_system(struct x86_emulate_ctxt *ctxt,
 				      gva_t addr, void *val, unsigned int bytes,

arch/x86/kvm/x86.h

@@ -81,4 +81,8 @@ int kvm_inject_realmode_interrupt(struct kvm_vcpu *vcpu, int irq, int inc_eip);
 
 void kvm_write_tsc(struct kvm_vcpu *vcpu, u64 data);
 
+int kvm_read_guest_virt(struct x86_emulate_ctxt *ctxt,
+	gva_t addr, void *val, unsigned int bytes,
+	struct x86_exception *exception);
+
 #endif