arm64: add check_wx_pages debugfs for CHECK_WX

follow the suggestion from
KSPP#35

Signed-off-by: Phong Tran <tranmanphong@gmail.com>

arch/arm64/Kconfig.debug

@@ -48,7 +48,8 @@ config DEBUG_WX
 	  of other unfixed kernel bugs easier.
 
 	  There is no runtime or memory usage effect of this option
-	  once the kernel has booted up - it's a one time check.
+	  once the kernel has booted up - it's a one time check and
+	  can be checked by echo "1" to "check_wx_pages" debugfs in runtime.
 
 	  If in doubt, say "Y".
 

arch/arm64/include/asm/ptdump.h

@@ -24,9 +24,11 @@ struct ptdump_info {
 void ptdump_walk(struct seq_file *s, struct ptdump_info *info);
 #ifdef CONFIG_PTDUMP_DEBUGFS
 void ptdump_debugfs_register(struct ptdump_info *info, const char *name);
+int ptdump_check_wx_init(void);
 #else
 static inline void ptdump_debugfs_register(struct ptdump_info *info,
 					   const char *name) { }
+static inline int ptdump_check_wx_init(void) { return 0; }
 #endif
 void ptdump_check_wx(void);
 #endif /* CONFIG_PTDUMP_CORE */

arch/arm64/mm/dump.c

@@ -378,6 +378,7 @@ static int ptdump_init(void)
 #endif
 	ptdump_initialize();
 	ptdump_debugfs_register(&kernel_ptdump_info, "kernel_page_tables");
+	ptdump_check_wx_init();
 	return 0;
 }
 device_initcall(ptdump_init);

arch/arm64/mm/ptdump_debugfs.c

@@ -20,3 +20,21 @@ void ptdump_debugfs_register(struct ptdump_info *info, const char *name)
 {
 	debugfs_create_file(name, 0400, NULL, info, &ptdump_fops);
 }
+
+static int check_wx_debugfs_set(void *data, u64 val)
+{
+	if (val != 1ULL)
+		return -EINVAL;
+
+	ptdump_check_wx();
+
+	return 0;
+}
+
+DEFINE_SIMPLE_ATTRIBUTE(check_wx_fops, NULL, check_wx_debugfs_set, "%llu\n");
+
+int ptdump_check_wx_init(void)
+{
+	return debugfs_create_file("check_wx_pages", 0200, NULL,
+				   NULL, &check_wx_fops) ? 0 : -ENOMEM;
+}