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

Chenbo Feng
2018-10-03T01:13:16Z

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.