configfs: fix kernel infoleak through user-controlled format string

Some modules call config_item_init_type_name() and config_group_init_type_name()
with parameter "name" directly controlled by userspace. These two
functions call config_item_set_name() with this name used as a format
string, which can be used to leak information such as content of the
stack to userspace.

For example, make_netconsole_target() in netconsole module calls
config_item_init_type_name() with the name of a newly-created directory.
This means that the following commands give some unexpected output, with
configfs mounted in /sys/kernel/config/ and on a system with a
configured eth0 ethernet interface:

# modprobe netconsole
# mkdir /sys/kernel/config/netconsole/target_%lx
# echo eth0 > /sys/kernel/config/netconsole/target_%lx/dev_name
# echo 1 > /sys/kernel/config/netconsole/target_%lx/enabled
# echo eth0 > /sys/kernel/config/netconsole/target_%lx/dev_name
# dmesg |tail -n1
[ 142.697668] netconsole: target (target_ffffffffc0ae8080) is
enabled, disable to update parameters

The directory name is correct but %lx has been interpreted in the
internal item name, displayed here in the error message used by
store_dev_name() in drivers/net/netconsole.c.

To fix this, update every caller of config_item_set_name to use "%s"
when operating on untrusted input.

This issue was found using -Wformat-security gcc flag, once a __printf
attribute has been added to config_item_set_name().

Signed-off-by: Nicolas Iooss <[e-mail]>
Acked-by: Greg Kroah-Hartman <[e-mail]>
Acked-by: Felipe Balbi <[e-mail]>
Acked-by: Joel Becker <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/usb/gadget/configfs.c

···
 	if (IS_ERR(fi))
 		return ERR_CAST(fi);
 
-	ret = config_item_set_name(&fi->group.cg_item, name);
+	ret = config_item_set_name(&fi->group.cg_item, "%s", name);
 	if (ret) {
 		usb_put_function_instance(fi);
 		return ERR_PTR(ret);

fs/configfs/item.c

···
 				const char *name,
 				struct config_item_type *type)
 {
-	config_item_set_name(item, name);
+	config_item_set_name(item, "%s", name);
 	item->ci_type = type;
 	config_item_init(item);
 }
···
 void config_group_init_type_name(struct config_group *group, const char *name,
 			 struct config_item_type *type)
 {
-	config_item_set_name(&group->cg_item, name);
+	config_item_set_name(&group->cg_item, "%s", name);
 	group->cg_item.ci_type = type;
 	config_group_init(group);
 }