linux-kernel - Re: [PATCH 1/2] proc/sysctl: add shared variables for range check

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Date:   Wed, 10 Apr 2019 15:18:33 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Matteo Croce <mcroce@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Alexey Dobriyan <adobriyan@...il.com>
Subject: Re: [PATCH 1/2] proc/sysctl: add shared variables for range check

On Mon, Apr 8, 2019 at 3:09 PM Matteo Croce <mcroce@...hat.com> wrote:
>
> In the sysctl code the proc_dointvec_minmax() function is often used to
> validate the user supplied value between an allowed range. This function
> uses the extra1 and extra2 members from struct ctl_table as minimum and
> maximum allowed value.
>
> On sysctl handler declaration, in every source file there are some readonly
> variables containing just an integer which address is assigned to the
> extra1 and extra2 members, so the sysctl range is enforced.
>
> The special values 0, 1 and INT_MAX are very often used as range boundary,
> leading duplication of variables like zero=0, one=1, int_max=INT_MAX in
> different source files:
>
>     $ git grep -E '\.extra[12].*&(zero|one|int_max)' |wc -l
>     261
>
> This patch adds three variables for the most commonly used values, so they
> can be referenced instead of creating a local one for every object file.
>
> Signed-off-by: Matteo Croce <mcroce@...hat.com>

Oh, and for this patch: Yes please; let's get everyone to use these
instead of littering the kernel with (non-const!) copies of the same
thing. :)

Acked-by: Kees Cook <keescook@...omium.org>

-Kees

> ---
>  fs/proc/proc_sysctl.c  | 5 +++++
>  include/linux/sysctl.h | 4 ++++
>  2 files changed, 9 insertions(+)
>
> diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
> index d65390727541..e7a96169fced 100644
> --- a/fs/proc/proc_sysctl.c
> +++ b/fs/proc/proc_sysctl.c
> @@ -21,6 +21,11 @@ static const struct inode_operations proc_sys_inode_operations;
>  static const struct file_operations proc_sys_dir_file_operations;
>  static const struct inode_operations proc_sys_dir_operations;
>
> +/* shared constants to be used in various sysctls */
> +const int sysctl_zero = 0;
> +const int sysctl_one = 1;
> +const int sysctl_int_max = INT_MAX;
> +
>  /* Support for permanently empty directories */
>
>  struct ctl_table sysctl_mount_point[] = {
> diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
> index b769ecfcc3bd..f3b191799747 100644
> --- a/include/linux/sysctl.h
> +++ b/include/linux/sysctl.h
> @@ -37,6 +37,10 @@ struct ctl_table_root;
>  struct ctl_table_header;
>  struct ctl_dir;
>
> +extern const int sysctl_zero;
> +extern const int sysctl_one;
> +extern const int sysctl_int_max;
> +
>  typedef int proc_handler (struct ctl_table *ctl, int write,
>                           void __user *buffer, size_t *lenp, loff_t *ppos);
>
> --
> 2.21.0
>


-- 
Kees Cook