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

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Date:   Tue, 16 Oct 2018 11:44:40 -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 11:40 AM, Christian Brauner wrote:
> On Tue, Oct 16, 2018 at 11:34:07AM -0400, Waiman Long wrote:
>> On 10/16/2018 11:29 AM, Christian Brauner wrote:
>>> On Tue, Oct 16, 2018 at 11:25:42AM -0400, Waiman Long wrote:
>>>> 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,
>>> Sure, I'll update linux manpages and I can CC stable on the next round.
>>>
>>>> but not the min value. So there is inconsistency. I would say you either
>>>> do both, or none of them.
>>> The min value is 0. I don't think it needs to be set explicitly. I just
>>> kept the max value because it is != ULONG_MAX but LONG_MAX for file-max.
>> I think you are making the change with just one use case in mind. This
>> is a generic function that can be used by many different callers. So any
>> change you make has to be applicable to all use cases. You just can't
>> assume min is always 0 in all the other use cases.
> So, any caller that calls {do_}proc_doulongvec_minmax() must want an
> unsigned long lest they are calling the wrong function.
> The smallest value for an unsigned long is 0. So if any caller wants to
> get into a situation where the caller needs to be capped they need to be
> able to set the value to lower than 0 which they can't since they are
> requesting an unsigned. So I'm not sure it makes sense.
>
> Christian

There may be use cases where the developer may want a min value that is
bigger than 0. As I said, you can't just make an assumption here.
Otherwise, what is the point of the following check:

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

Cheers,
Longman