[PATCH 4/4] x86,module: Detect CRn and DRn manipulation

[Peter Zijlstra]

Since we now have infrastructure to analyze module text, disallow
modules that write to CRn and DRn registers.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 arch/x86/kernel/module.c |   21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

--- a/arch/x86/kernel/module.c
+++ b/arch/x86/kernel/module.c
@@ -266,6 +266,22 @@ static bool insn_is_vmx(struct insn *ins
 	return false;
 }
 
+static bool insn_is_mov_CRn(struct insn *insn)
+{
+	if (insn->opcode.bytes[0] == 0x0f && insn->opcode.bytes[1] == 0x22)
+		return true;
+
+	return false;
+}
+
+static bool insn_is_mov_DRn(struct insn *insn)
+{
+	if (insn->opcode.bytes[0] == 0x0f && insn->opcode.bytes[1] == 0x23)
+		return true;
+
+	return false;
+}
+
 static int decode_module(struct module *mod, void *text, void *text_end, bool sld_safe)
 {
 	bool allow_vmx = sld_safe || !split_lock_enabled();
@@ -285,6 +301,11 @@ static int decode_module(struct module *
 			return -ENOEXEC;
 		}
 
+		if (insn_is_mov_CRn(&insn) || insn_is_mov_DRn(&insn)) {
+			pr_err("Module writes to CRn or DRn, please use the proper accessors: %s\n", mod->name);
+			return -ENOEXEC;
+		}
+
 		text += insn.length;
 	}