linux-kernel - Re: [PATCH v2] proc/sysctl: add shared variables for range check

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Date:   Tue, 16 Apr 2019 20:17:41 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Matteo Croce <mcroce@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-fsdevel@...r.kernel.org,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v2] proc/sysctl: add shared variables for range check

On Wed, 17 Apr 2019 02:59:43 +0200 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)\b' |wc -l
>     245
> 
> This patch adds three const variables for the most commonly used values,
> and use them instead of creating a local one for every object file.
> 

Nice.  A few thoughts:

> --- a/arch/s390/appldata/appldata_base.c
> +++ b/arch/s390/appldata/appldata_base.c
> @@ -220,15 +220,13 @@ appldata_timer_handler(struct ctl_table *ctl, int write,
>  			   void __user *buffer, size_t *lenp, loff_t *ppos)
>  {
>  	int timer_active = appldata_timer_active;
> -	int zero = 0;
> -	int one = 1;
>  	int rc;
>  	struct ctl_table ctl_entry = {
>  		.procname	= ctl->procname,
>  		.data		= &timer_active,
>  		.maxlen		= sizeof(int),
> -		.extra1		= &zero,
> -		.extra2		= &one,
> +		.extra1		= (void *)&sysctl_zero,
> +		.extra2		= (void *)&sysctl_one,

The casts are ugly, and by casting away constness they introduce the
risk that some errant could could change the value of 0, 1 and INT_MAX!
Maybe - perhaps trying to do that would cause a segv but still,
they're ugly.

A proper fix would require changing extra1 and extra2 to const void *. 
Perhaps that would be unfeasibly extensive?

> ...
>
> --- 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;

Don't these require EXPORT_SYMBOL()?