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

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Message-ID: <CAM_iQpUuwpm_uQ76SY+Vz=FN+xq_bhUTScn8NcRNfcn8xehQgQ@mail.gmail.com>
Date:   Wed, 13 Jan 2021 23:20:24 -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] cls_flower: call nla_ok() before nla_next()

On Tue, Jan 12, 2021 at 5:38 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Mon, 11 Jan 2021 18:55:48 -0800 Cong Wang wrote:
> > From: Cong Wang <cong.wang@...edance.com>
> >
> > fl_set_enc_opt() simply checks if there are still bytes left to parse,
> > but this is not sufficent as syzbot seems to be able to generate
> > malformatted netlink messages. nla_ok() is more strict so should be
> > used to validate the next nlattr here.
> >
> > And nla_validate_nested_deprecated() has less strict check too, it is
> > probably too late to switch to the strict version, but we can just
> > call nla_ok() too after it.
> >
> > Reported-and-tested-by: syzbot+2624e3778b18fc497c92@...kaller.appspotmail.com
> > Fixes: 0a6e77784f49 ("net/sched: allow flower to match tunnel options")
> > Fixes: 79b1011cb33d ("net: sched: allow flower to match erspan options")
> > Cc: Pieter Jansen van Vuuren <pieter.jansenvanvuuren@...ronome.com>
> > Cc: Jamal Hadi Salim <jhs@...atatu.com>
> > Cc: Xin Long <lucien.xin@...il.com>
> > Cc: Jiri Pirko <jiri@...nulli.us>
> > Signed-off-by: Cong Wang <cong.wang@...edance.com>
>
> Thanks for keeping up with the syzbot bugs!
>
> > diff --git a/net/sched/cls_flower.c b/net/sched/cls_flower.c
> > index 1319986693fc..e265c443536e 100644
> > --- a/net/sched/cls_flower.c
> > +++ b/net/sched/cls_flower.c
> > @@ -1272,6 +1272,8 @@ static int fl_set_enc_opt(struct nlattr **tb, struct fl_flow_key *key,
> >
> >               nla_opt_msk = nla_data(tb[TCA_FLOWER_KEY_ENC_OPTS_MASK]);
> >               msk_depth = nla_len(tb[TCA_FLOWER_KEY_ENC_OPTS_MASK]);
> > +             if (!nla_ok(nla_opt_msk, msk_depth))
> > +                     return -EINVAL;
>
> Can we just add another call to nla_validate_nested_deprecated()
> here instead of having to worry about each attr individually?

No, we can not parse the nested attr here because different key types
have different attributes.

> See below..
>
> >       }
> >
> >       nla_for_each_attr(nla_opt_key, nla_enc_key,
> > @@ -1308,7 +1310,7 @@ static int fl_set_enc_opt(struct nlattr **tb, struct fl_flow_key *key,
> >                               return -EINVAL;
> >                       }
> >
> > -                     if (msk_depth)
> > +                     if (nla_ok(nla_opt_msk, msk_depth))
> >                               nla_opt_msk = nla_next(nla_opt_msk, &msk_depth);
>
> Should we not error otherwise? if msk_depth && !nla_ok() then the
> message is clearly misformatted. If we don't error out we'll keep
> reusing the same mask over and over, while the intention of this
> code was to have mask per key AFAICT.

Yeah, erroring out sounds better. Will change this in v2.

Thanks!