x86/insn: Add insn_get_modrm_reg_off()

Add a function to the instruction decoder which returns the pt_regs
offset of the register specified in the reg field of the modrm byte.
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Link: https://lkml.kernel.org/r/20200907131613.12703-11-joro@8bytes.org

arch/x86/include/asm/insn-eval.h

@@ -17,6 +17,7 @@
 
 void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs);
 int insn_get_modrm_rm_off(struct insn *insn, struct pt_regs *regs);
+int insn_get_modrm_reg_off(struct insn *insn, struct pt_regs *regs);
 unsigned long insn_get_seg_base(struct pt_regs *regs, int seg_reg_idx);
 int insn_get_code_seg_params(struct pt_regs *regs);
 int insn_fetch_from_user(struct pt_regs *regs,

arch/x86/lib/insn-eval.c

@@ -20,6 +20,7 @@
 
 enum reg_type {
 	REG_TYPE_RM = 0,
+	REG_TYPE_REG,
 	REG_TYPE_INDEX,
 	REG_TYPE_BASE,
 };
@@ -439,6 +440,13 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs,
 			regno += 8;
 		break;
 
+	case REG_TYPE_REG:
+		regno = X86_MODRM_REG(insn->modrm.value);
+
+		if (X86_REX_R(insn->rex_prefix.value))
+			regno += 8;
+		break;
+
 	case REG_TYPE_INDEX:
 		regno = X86_SIB_INDEX(insn->sib.value);
 		if (X86_REX_X(insn->rex_prefix.value))
@@ -807,6 +815,21 @@ int insn_get_modrm_rm_off(struct insn *insn, struct pt_regs *regs)
 	return get_reg_offset(insn, regs, REG_TYPE_RM);
 }
 
+/**
+ * insn_get_modrm_reg_off() - Obtain register in reg part of the ModRM byte
+ * @insn:	Instruction containing the ModRM byte
+ * @regs:	Register values as seen when entering kernel mode
+ *
+ * Returns:
+ *
+ * The register indicated by the reg part of the ModRM byte. The
+ * register is obtained as an offset from the base of pt_regs.
+ */
+int insn_get_modrm_reg_off(struct insn *insn, struct pt_regs *regs)
+{
+	return get_reg_offset(insn, regs, REG_TYPE_REG);
+}
+
 /**
  * get_seg_base_limit() - obtain base address and limit of a segment
  * @insn:	Instruction. Must be valid.