Re: [PATCH net-next] netfilter: xt_quota: fix the behavior of xt_quota module

Maciej Żenczykowski
2018-10-03T05:07:39Z

Could you clarify what exact behaviour Willem removed?

On Tue, Oct 2, 2018 at 11:43 AM, Pablo Neira Ayuso <pablo@netfilter.org> wrote:
> On Tue, Oct 02, 2018 at 11:28:21AM -0700, Chenbo Feng wrote:
>> On Tue, Oct 2, 2018 at 11:15 AM Pablo Neira Ayuso <pablo@netfilter.org> wrote:
>> >
>> > Hi Chenbo,
>> >
>> > On Tue, Oct 02, 2018 at 10:45:58AM -0700, Chenbo Feng wrote:
>> > > On Tue, Oct 2, 2018 at 3:51 AM Pablo Neira Ayuso <pablo@netfilter.org> wrote:
>> > [...]
>> > > Do you mean the remain field will be zeroed when copying the
>> > > xt_quota_info struct out of the kernel? I believe that is decided by
>> > > the usersize defined in struct xt_match and this patch set it to the
>> > > full struct size. So the whole xt_quota_info struct will be copied
>> > > into userspace including the field stores the remaining quota. The
>> > > userspace will not be aware of it if the ipatbles is not updated but
>> > > it should not modify it as well. I have tested the behavior with
>> > > net-next branch and it seems working. Am I missing something
>> > > recently updated?
>> >
>> > Hm, I see, I overlook that your patch removes this:
>> >
>> > - .usersize = offsetof(struct xt_quota_info, master),
>> >
>> > BTW, is iptables -D command working with your patch?
>> >
>> > Telling this because if .usersize is removed, then IIRC userspace
>> > compares this new remain field with userspace value and deletion will
>> > break.
>> >
>> > Patch that I was referring before is this one from Willem:
>> >
>> > commit f32815d21d4d8287336fb9cef4d2d9e0866214c2
>> > Author: Willem de Bruijn <willemb@google.com>
>> > Date: Mon Jan 2 17:19:40 2017 -0500
>> >
>> > xtables: add xt_match, xt_target and data copy_to_user functions
>> >
>> > xt_entry_target, xt_entry_match and their private data may contain
>> > kernel data.
>> > [...]
>> > Private data is defined in xt_match and xt_target. All matches and
>> > targets that maintain kernel data store this at the tail of their
>> > private structure. Extend xt_match and xt_target with .usersize to
>> > limit how many bytes of data are copied. The remainder is cleared.
>> >
>> > Let me know, thanks !
>>
>> The delete operation is decided by the userspacesize defined in
>> userspace ipatbles. I think it is unrelated to the usersize we talk
>> about here. For old userspace iptables, the userspacesize is
>> offsetof(struct xt_quota_info, master) so it will not compare the rest
>> if the struct. And for new iptables we use offsetof(struct
>> xt_quota_info, remain). Either way the userspace does not consider the
>> remain field when comparing rules so we can do ipatbles rule deletion
>> with or without specifying --remain option or even specify --remain to
>> a wrong number. We decide to make it this way since the --remain field
>> is changing all the time when there is network traffic going on and
>> it's hard to compare the remaining quota for new ipatbles as well.
>
> Thanks for explaining and your patience. Patch looks good then.
>
> Thanks!