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

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Message-ID: <20181016153332.wi2l4cvngkk4kkyb@brauner.io>
Date:   Tue, 16 Oct 2018 17:33:33 +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 05:29:55PM +0200, 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.

Sorry, s/stable/linux-api@...r.kernel.org/

> 
> > 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.