From b0a65b0cccd477b2fd8b7adad0ac39433df54829 Mon Sep 17 00:00:00 2001 From: Kees Cook Date: Fri, 12 Dec 2014 13:36:49 +1030 Subject: [PATCH] param: do not set store func without write perm When a module_param is defined without DAC write permissions, it can still be changed at runtime and updated. Drivers using a 0444 permission may be surprised that these values can still be changed. For drivers that want to allow updates, any S_IW* flag will set the "store" function as before. Drivers without S_IW* flags will have the "store" function unset, unforcing a read-only value. Drivers that wish neither "store" nor "get" can continue to use "0" for perms to stay out of sysfs entirely. Old behavior: # cd /sys/module/snd/parameters # ls -l total 0 -r--r--r-- 1 root root 4096 Dec 11 13:55 cards_limit -r--r--r-- 1 root root 4096 Dec 11 13:55 major -r--r--r-- 1 root root 4096 Dec 11 13:55 slots # cat major 116 # echo -1 > major -bash: major: Permission denied # chmod u+w major # echo -1 > major # cat major -1 New behavior: ... # chmod u+w major # echo -1 > major -bash: echo: write error: Input/output error Signed-off-by: Kees Cook Signed-off-by: Rusty Russell --- kernel/params.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/kernel/params.c b/kernel/params.c index 795321aba29f..0af9b2c4e56c 100644 --- a/kernel/params.c +++ b/kernel/params.c @@ -645,7 +645,9 @@ static __modinit int add_sysfs_param(struct module_kobject *mk, sysfs_attr_init(&mk->mp->attrs[mk->mp->num].mattr.attr); mk->mp->attrs[mk->mp->num].param = kp; mk->mp->attrs[mk->mp->num].mattr.show = param_attr_show; - mk->mp->attrs[mk->mp->num].mattr.store = param_attr_store; + /* Do not allow runtime DAC changes to make param writable. */ + if ((kp->perm & (S_IWUSR | S_IWGRP | S_IWOTH)) != 0) + mk->mp->attrs[mk->mp->num].mattr.store = param_attr_store; mk->mp->attrs[mk->mp->num].mattr.attr.name = (char *)name; mk->mp->attrs[mk->mp->num].mattr.attr.mode = kp->perm; mk->mp->num++; -- 2.20.1