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

[Waiman Long <longman@redhat.com>]

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@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.
>>> 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.

Cheers,
Longman