Re: [Patch net] cls_flower: call nla_ok() before nla_next()

[Jakub Kicinski <kuba@kernel.org>]

On Wed, 13 Jan 2021 23:20:24 -0800 Cong Wang wrote:
> On Tue, Jan 12, 2021 at 5:38 PM Jakub Kicinski <kuba@kernel.org> wrote:
> >
> > On Mon, 11 Jan 2021 18:55:48 -0800 Cong Wang wrote:  
> > > From: Cong Wang <cong.wang@bytedance.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@syzkaller.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@netronome.com>
> > > Cc: Jamal Hadi Salim <jhs@mojatatu.com>
> > > Cc: Xin Long <lucien.xin@gmail.com>
> > > Cc: Jiri Pirko <jiri@resnulli.us>
> > > Signed-off-by: Cong Wang <cong.wang@bytedance.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.

Not parse, just validate. Policy can be NULL, then nla_validate
basically only checks nla_ok(). But my previous suggestion to call 
nla_validate_nested_deprecated() would not work, we'd need 
__nla_validate(NL_VALIDATE_TRAILING), so yeah, maybe that's more
complex for a read to understand than just calling nla_ok()...