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 <keesc...@chromium.org>
---
kernel/params.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/kernel/params.c b/kernel/params.c
index db97b791390f..fd50ce9f1bbf 100644
--- a/kernel/params.c
+++ b/kernel/params.c
@@ -647,7 +647,9 @@ static __modinit int add_sysfs_param(struct module_kobject
*mk,
sysfs_attr_init(&new->attrs[num].mattr.attr);
new->attrs[num].param = kp;
new->attrs[num].mattr.show = param_attr_show;
- new->attrs[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)
+ new->attrs[num].mattr.store = param_attr_store;
new->attrs[num].mattr.attr.name = (char *)name;
new->attrs[num].mattr.attr.mode = kp->perm;
new->num = num+1;
--
1.9.1
--
Kees Cook
Chrome OS Security
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