Messages in this thread | | | Date | Tue, 16 Oct 2018 17:40:41 +0200 | From | Christian Brauner <> | Subject | Re: [PATCH v1 2/2] sysctl: handle overflow for file-max |
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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@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.
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
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