linux-kernel - Re: [PATCH v2 2/2] sysctl: handle overflow for file-max

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Date:   Tue, 16 Oct 2018 16:49:37 -0400
From:   Waiman Long <longman@...hat.com>
To:     Christian Brauner <christian@...uner.io>, keescook@...omium.org,
        linux-kernel@...r.kernel.org
Cc:     ebiederm@...ssion.com, mcgrof@...nel.org,
        akpm@...ux-foundation.org, joe.lawrence@...hat.com,
        linux@...inikbrodowski.net, viro@...iv.linux.org.uk,
        adobriyan@...il.com, linux-api@...r.kernel.org
Subject: Re: [PATCH v2 2/2] sysctl: handle overflow for file-max

On 10/16/2018 03:53 PM, 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 sets the max and min value for file-max and returns -EINVAL
> when a long int is exceeded. Any higher value cannot currently be used as
> the percpu counters are long ints and not unsigned integers. This behavior
> also aligns with other tuneables that return -EINVAL when their range is
> exceeded. See e.g. [1], [2] and others.
>
> [1]: fb910c42cceb ("sysctl: check for UINT_MAX before unsigned int min/max")
> [2]: 196851bed522 ("s390/topology: correct topology mode proc handler")
>
> Cc: Kees Cook <keescook@...omium.org>
> Signed-off-by: Christian Brauner <christian@...uner.io>
> ---
> v2->v1:
> - consistenly fail on overflow
> v0->v1:
> - if max value is < than ULONG_MAX use max as upper bound
> - (Dominik) remove double "the" from commit message
> ---
>  kernel/sysctl.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 7d98e02e5d72..0874001e5435 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 long_max = LONG_MAX;
>  static int one_hundred = 100;
>  static int one_thousand = 1000;
>  #ifdef CONFIG_PRINTK
> @@ -1696,6 +1697,8 @@ static struct ctl_table fs_table[] = {
>  		.maxlen		= sizeof(files_stat.max_files),
>  		.mode		= 0644,
>  		.proc_handler	= proc_doulongvec_minmax,
> +		.extra1		= &zero,
> +		.extra2		= &long_max,
>  	},
>  	{
>  		.procname	= "nr_open",
> @@ -2797,6 +2800,10 @@ static int __do_proc_doulongvec_minmax(void *data, struct ctl_table *table, int
>  				break;
>  			if (neg)
>  				continue;
> +			if ((max && val > *max) || (min && val < *min)) {
> +				err = -EINVAL;
> +				break;
> +			}
>  			val = convmul * val / convdiv;

Should the conversion be done before the min/max check?

>  			if ((min && val < *min) || (max && val > *max))
>  				continue;

You may be able to drop the above statement.

Cheers,
Longman