2014-12-11 22:22:30

by Kees Cook

[permalink] [raw]
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 <[email protected]>
---
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


2014-12-12 04:26:47

by Rusty Russell

[permalink] [raw]
Subject: Re: [PATCH] param: do not set store func without write perm

Kees Cook <[email protected]> writes:
> 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.

Hmm, fair enough. The use of the acronym DAC here is a bit weird;
I would have just said:

/* If no perms, it's not writable even if root chmods it! */
if ((kp->perm & (S_IWUSR | S_IWGRP | S_IWOTH)) != 0)
new->attrs[num].mattr.store = param_attr_store;

Applied (with fuzz fixed, since I reworked that code in my tree).

Thanks,
Rusty.

>
> 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 <[email protected]>
> ---
> 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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