netdev - Re: [Patch net v3] cls_flower: call nla_ok() before nla

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Date:   Thu, 14 Jan 2021 14:50:04 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Cong Wang <cong.wang@...edance.com>,
        syzbot <syzbot+2624e3778b18fc497c92@...kaller.appspotmail.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Xin Long <lucien.xin@...il.com>, Jiri Pirko <jiri@...nulli.us>
Subject: Re: [Patch net v3] cls_flower: call nla_ok() before nla_next()

On Thu, Jan 14, 2021 at 2:30 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Thu, 14 Jan 2021 13:57:13 -0800 Cong Wang wrote:
> > On Thu, Jan 14, 2021 at 1:36 PM Jakub Kicinski <kuba@...nel.org> wrote:
> > >
> > > On Thu, 14 Jan 2021 13:07:49 -0800 Cong Wang wrote:
> > > > -                     if (msk_depth)
> > > > -                             nla_opt_msk = nla_next(nla_opt_msk, &msk_depth);
> > > >                       break;
> > > >               default:
> > > >                       NL_SET_ERR_MSG(extack, "Unknown tunnel option type");
> > > >                       return -EINVAL;
> > > >               }
> > > > +
> > > > +             if (!nla_opt_msk)
> > > > +                     continue;
> > >
> > > Why the switch from !msk_depth to !nla_opt_msk?
> >
> > It is the same, when nla_opt_msk is NULL, msk_depth is 0.
> > Checking nla_opt_msk is NULL is more readable to express that
> > mask is not provided.
> >
> > >
> > > Seems like previously providing masks for only subset of options
> > > would have worked.
> >
> > I don't think so, every type has this check:
> >
> >                         if (key->enc_opts.len != mask->enc_opts.len) {
> >                                 NL_SET_ERR_MSG(extack, "Key and mask
> > miss aligned");
> >                                 return -EINVAL;
> >                         }
> >
> > which guarantees the numbers are aligned.
> >
> > Thanks.
>
> static int fl_set_vxlan_opt(const struct nlattr *nla, struct fl_flow_key *key,
>                             int depth, int option_len,
>                             struct netlink_ext_ack *extack)
> {
>         struct nlattr *tb[TCA_FLOWER_KEY_ENC_OPT_VXLAN_MAX + 1];
>         struct vxlan_metadata *md;
>         int err;
>
>         md = (struct vxlan_metadata *)&key->enc_opts.data[key->enc_opts.len];
>         memset(md, 0xff, sizeof(*md));
>
>         if (!depth)
>                 return sizeof(*md);
>                 ^^^^^^^^^^^^^^^^^^^
>
> The mask is filled with all 1s if attribute is not provided.

Hmm, then what is the length comparison check for?

fl_set_vxlan_opt() either turns negative or sizeof(*md), and negitve
is already checked when it returns, so when we hit the length comparison
it is always equal. So it must be redundant.

(Note, I am only talking about the vxlan case you pick here, because the
geneve case is different, as it returns different sizes.)

Thanks.