Messages in this thread |  | | Subject | Re: [PATCH v1 2/2] sysctl: handle overflow for file-max | From | Waiman Long <> | Date | Tue, 16 Oct 2018 11:44:40 -0400 |
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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@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
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
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