Re: [PATCH v1 2/2] sysctl: handle overflow for file-max

[Waiman Long <longman@redhat.com>]

On 10/15/2018 06:55 AM, Christian Brauner wrote:
> Currently, when writing
>
> echo 18446744073709551616 > /proc/sys/fs/file-max
>
> /proc/sys/fs/file-max will overflow and be set to 0. That quickly
> crashes the system.
> This commit explicitly caps the value for file-max to ULONG_MAX.
>
> Note, this isn't technically necessary since proc_get_long() will already
> return ULONG_MAX. However, two reason why we still should do this:
> 1. it makes it explicit what the upper bound of file-max is instead of
>    making readers of the code infer it from proc_get_long() themselves
> 2. other tunebles than file-max may want to set a lower max value than
>    ULONG_MAX and we need to enable __do_proc_doulongvec_minmax() to handle
>    such cases too
>
> Cc: Kees Cook <keescook@chromium.org>
> Signed-off-by: Christian Brauner <christian@brauner.io>
> ---
> v0->v1:
> - if max value is < than ULONG_MAX use max as upper bound
> - (Dominik) remove double "the" from commit message
> ---
>  kernel/sysctl.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 97551eb42946..226d4eaf4b0e 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -127,6 +127,7 @@ static int __maybe_unused one = 1;
>  static int __maybe_unused two = 2;
>  static int __maybe_unused four = 4;
>  static unsigned long one_ul = 1;
> +static unsigned long ulong_max = ULONG_MAX;
>  static int one_hundred = 100;
>  static int one_thousand = 1000;
>  #ifdef CONFIG_PRINTK
> @@ -1696,6 +1697,7 @@ static struct ctl_table fs_table[] = {
>  		.maxlen		= sizeof(files_stat.max_files),
>  		.mode		= 0644,
>  		.proc_handler	= proc_doulongvec_minmax,
> +		.extra2		= &ulong_max,

What is the point of having a maximum value of ULONG_MAX anyway? No
value you can put into a ulong type can be bigger than that.

>  	},
>  	{
>  		.procname	= "nr_open",
> @@ -2795,6 +2797,8 @@ static int __do_proc_doulongvec_minmax(void *data, struct ctl_table *table, int
>  				break;
>  			if (neg)
>  				continue;
> +			if (max && val > *max)
> +				val = *max;
>  			val = convmul * val / convdiv;
>  			if ((min && val < *min) || (max && val > *max))
>  				continue;

This does introduce a change in behavior. Previously the out-of-bound
value is ignored, now it is capped at its maximum. This is a
user-visible change.

Cheers,
Longman