Messages in this thread | | | Subject | Re: [PATCH v1 2/2] sysctl: handle overflow for file-max | From | Waiman Long <> | Date | Tue, 16 Oct 2018 11:34:07 -0400 |
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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
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