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

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Message-ID: <20181016154040.znumdxkhaudewlku@brauner.io>
Date:   Tue, 16 Oct 2018 17:40:41 +0200
From:   Christian Brauner <christian@...uner.io>
To:     Waiman Long <longman@...hat.com>
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 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