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

Pablo Neira Ayuso
2018-10-03T01:28:47Z

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!