kdb: Allow access to sensitive commands to be restricted by default

Currently kiosk mode must be explicitly requested by the bootloader or
userspace. It is convenient to be able to change the default value in a
similar manner to CONFIG_MAGIC_SYSRQ_DEFAULT_MASK.
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Cc: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>

kernel/debug/kdb/kdb_main.c

@@ -47,7 +47,7 @@
 #undef	MODULE_PARAM_PREFIX
 #define	MODULE_PARAM_PREFIX "kdb."
 
-static int kdb_cmd_enabled;
+static int kdb_cmd_enabled = CONFIG_KDB_DEFAULT_ENABLE;
 module_param_named(cmd_enable, kdb_cmd_enabled, int, 0600);
 
 #define GREP_LEN 256

lib/Kconfig.kgdb

@@ -73,6 +73,31 @@ config KGDB_KDB
 	help
 	  KDB frontend for kernel
 
+config KDB_DEFAULT_ENABLE
+	hex "KDB: Select kdb command functions to be enabled by default"
+	depends on KGDB_KDB
+	default 0x1
+	help
+	  Specifiers which kdb commands are enabled by default. This may
+	  be set to 1 or 0 to enable all commands or disable almost all
+	  commands.
+
+	  Alternatively the following bitmask applies:
+
+	    0x0002 - allow arbitrary reads from memory and symbol lookup
+	    0x0004 - allow arbitrary writes to memory
+	    0x0008 - allow current register state to be inspected
+	    0x0010 - allow current register state to be modified
+	    0x0020 - allow passive inspection (backtrace, process list, lsmod)
+	    0x0040 - allow flow control management (breakpoint, single step)
+	    0x0080 - enable signalling of processes
+	    0x0100 - allow machine to be rebooted
+
+	  The config option merely sets the default at boot time. Both
+	  issuing 'echo X > /sys/module/kdb/parameters/cmd_enable' or
+          setting with kdb.cmd_enable=X kernel command line option will
+	  override the default settings.
+
 config KDB_KEYBOARD
 	bool "KGDB_KDB: keyboard as input device"
 	depends on VT && KGDB_KDB