lkdtm: add "WRITE_KERN" test

Add "WRITE_KERN" crash target to validate that kernel executable memory
is not writable.

Signed-off-by: Kees Cook <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/misc/lkdtm.c

···
 	CT_EXEC_USERSPACE,
 	CT_ACCESS_USERSPACE,
 	CT_WRITE_RO,
+	CT_WRITE_KERN,
 };
 
 static char* cp_name[] = {
···
 	"EXEC_USERSPACE",
 	"ACCESS_USERSPACE",
 	"WRITE_RO",
+	"WRITE_KERN",
 };
 
 static struct jprobe lkdtm;
···
 
 static void do_nothing(void)
 {
+	return;
+}
+
+/* Must immediately follow do_nothing for size calculuations to work out. */
+static void do_overwritten(void)
+{
+	pr_info("do_overwritten wasn't overwritten!\n");
 	return;
 }
 
···
 		pr_info("attempting bad write at %p\n", ptr);
 		*ptr ^= 0xabcd1234;
 
+		break;
+	}
+	case CT_WRITE_KERN: {
+		size_t size;
+		unsigned char *ptr;
+
+		size = (unsigned long)do_overwritten -
+		       (unsigned long)do_nothing;
+		ptr = (unsigned char *)do_overwritten;
+
+		pr_info("attempting bad %zu byte write at %p\n", size, ptr);
+		memcpy(ptr, (unsigned char *)do_nothing, size);
+		flush_icache_range((unsigned long)ptr,
+				   (unsigned long)(ptr + size));
+
+		do_overwritten();
 		break;
 	}
 	case CT_NONE: