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

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Date:   Tue, 16 Oct 2018 11:25:42 -0400
From:   Waiman Long <longman@...hat.com>
To:     Christian Brauner <christian@...uner.io>
Cc:     keescook@...omium.org, linux-kernel@...r.kernel.org,
        ebiederm@...ssion.com, mcgrof@...nel.org,
        akpm@...ux-foundation.org, joe.lawrence@...hat.com,
        linux@...inikbrodowski.net, viro@...iv.linux.org.uk
Subject: Re: [PATCH v1 2/2] sysctl: handle overflow for file-max

On 10/16/2018 11:21 AM, Christian Brauner wrote:
> On Tue, Oct 16, 2018 at 11:13:28AM -0400, Waiman Long wrote:
>> 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@...omium.org>
>>> Signed-off-by: Christian Brauner <christian@...uner.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.
> This is changed in the new code to LONG_MAX. See the full thread for
> context. There's also an additional explantion in the commit message.
>
>>>  	},
>>>  	{
>>>  		.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.
> Not completely true though. Try
>
> echo 18446744073709551616 > /proc/sys/fs/file-max
>
> on a system you find acceptable loosing.
> So this is an acceptable user-visible change I'd say. But I'm open to
> other suggestions.

I am not saying this is unacceptable. I just say this is a user-visible
change and so should be documented somehow. BTW, you cap the max value,
but not the min value. So there is inconsistency. I would say you either
do both, or none of them.

Cheers,
Longman