netdev - Re: [Patch net v2] cls_flower: call nla_ok() before nla

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Date:   Thu, 14 Jan 2021 12:24:19 -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 v2] cls_flower: call nla_ok() before nla_next()

On Thu, Jan 14, 2021 at 12:16 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Thu, 14 Jan 2021 12:03:16 -0800 Cong Wang wrote:
> > On Thu, Jan 14, 2021 at 10:38 AM Jakub Kicinski <kuba@...nel.org> wrote:
> > >
> > > On Thu, 14 Jan 2021 08:38:22 -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")
> > >
> > > > @@ -1340,9 +1341,6 @@ static int fl_set_enc_opt(struct nlattr **tb, struct fl_flow_key *key,
> > > >                               NL_SET_ERR_MSG(extack, "Key and mask miss aligned");
> > > >                               return -EINVAL;
> > > >                       }
> > > > -
> > > > -                     if (msk_depth)
> > > > -                             nla_opt_msk = nla_next(nla_opt_msk, &msk_depth);
> > > >                       break;
> > > >               case TCA_FLOWER_KEY_ENC_OPTS_ERSPAN:
> > > >                       if (key->enc_opts.dst_opt_type) {
> > > > @@ -1373,14 +1371,17 @@ static int fl_set_enc_opt(struct nlattr **tb, struct fl_flow_key *key,
> > > >                               NL_SET_ERR_MSG(extack, "Key and mask miss aligned");
> > > >                               return -EINVAL;
> > > >                       }
> > > > -
> > > > -                     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_ok(nla_opt_msk, msk_depth)) {
> > > > +                     NL_SET_ERR_MSG(extack, "Mask attribute is invalid");
> > > > +                     return -EINVAL;
> > > > +             }
> > > > +             nla_opt_msk = nla_next(nla_opt_msk, &msk_depth);
> > >
> > > we lost the if (msk_depth) now, nla_opt_msk may be NULL -
> > > neither nla_ok() nor nla_next() take NULL
> >
> > How is "if (msk_depth)" lost when nla_ok() has a stricter one?
> >
> > 1156 static inline int nla_ok(const struct nlattr *nla, int remaining)
> > 1157 {
> > 1158         return remaining >= (int) sizeof(*nla) &&
> > 1159                nla->nla_len >= sizeof(*nla) &&
> > 1160                nla->nla_len <= remaining;
> > 1161 }
> >
> > Line 1156 assures msk_depth is not only non-zero but also larger
> > than the nla struct size, and clearly nla won't be dereferenced unless
> > this check is passed.
>
> Fair, depth will but 0 so first check already fails, but nla_next()
> would crash since it tries to access the length of the attribute
> unconditionally.

nla_next() is only called when nla_ok() returns true, which is not
the case for msk_depth==0, therefore NULL won't crash here.

The only problem is we become too strict to reject optionally missing
masks, we should not even call nla_ok() here, otherwise it would
break user-space. So,

+               if (!nla_opt_msk)
+                       continue;

Thanks.