netdev - Re: [PATCH 1/2] cipso: fix total option length computation

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Message-ID: <CAFqZXNvsumcLSKKRGzvUDmz=6WYfw3a0tG43juBjnUTdbfsDsw@mail.gmail.com>
Date: Wed, 17 Apr 2024 14:49:23 +0200
From: Ondrej Mosnacek <omosnace@...hat.com>
To: Paul Moore <paul@...l-moore.com>
Cc: netdev@...r.kernel.org, linux-security-module@...r.kernel.org
Subject: Re: [PATCH 1/2] cipso: fix total option length computation

On Tue, Apr 16, 2024 at 8:39 PM Paul Moore <paul@...l-moore.com> wrote:
>
> On Apr 16, 2024 Ondrej Mosnacek <omosnace@...hat.com> wrote:
> >
> > As evident from the definition of ip_options_get(), the IP option
> > IPOPT_END is used to pad the IP option data array, not IPOPT_NOP. Yet
> > the loop that walks the IP options to determine the total IP options
> > length in cipso_v4_delopt() doesn't take it into account.
> >
> > Fix it by recognizing the IPOPT_END value as the end of actual options.
> > Also add safety checks in case the options are invalid/corrupted.
> >
> > Fixes: 014ab19a69c3 ("selinux: Set socket NetLabel based on connection endpoint")
> > Signed-off-by: Ondrej Mosnacek <omosnace@...hat.com>
> > ---
> >  net/ipv4/cipso_ipv4.c | 19 ++++++++++++++-----
> >  1 file changed, 14 insertions(+), 5 deletions(-)
> >
> > diff --git a/net/ipv4/cipso_ipv4.c b/net/ipv4/cipso_ipv4.c
> > index 8b17d83e5fde4..75b5e3c35f9bf 100644
> > --- a/net/ipv4/cipso_ipv4.c
> > +++ b/net/ipv4/cipso_ipv4.c
> > @@ -2012,12 +2012,21 @@ static int cipso_v4_delopt(struct ip_options_rcu __rcu **opt_ptr)
> >                * from there we can determine the new total option length */
> >               iter = 0;
> >               optlen_new = 0;
> > -             while (iter < opt->opt.optlen)
> > -                     if (opt->opt.__data[iter] != IPOPT_NOP) {
> > -                             iter += opt->opt.__data[iter + 1];
> > -                             optlen_new = iter;
> > -                     } else
> > +             while (iter < opt->opt.optlen) {
> > +                     if (opt->opt.__data[iter] == IPOPT_END) {
> > +                             break;
> > +                     } else if (opt->opt.__data[iter] == IPOPT_NOP) {
> >                               iter++;
> > +                     } else {
> > +                             if (WARN_ON(opt->opt.__data[iter + 1] < 2))
> > +                                     iter += 2;
> > +                             else
> > +                                     iter += opt->opt.__data[iter + 1];
> > +                             optlen_new = iter;
>
> I worry that WARN_ON(), especially in conjunction with the one below,
> could generate a lot of noise on the console and system logs, let's
> be a bit more selective about what we check and report on.  Presumably
> the options have already gone through a basic sanity check so there
> shouldn't be anything too scary in there.
>
>   if (opt == IPOPT_END) {
>     /* ... */
>   } else if (opt == IPOPT_NOP) {
>     /* ... */
>   } else {
>     iter += opt[iter + 1];
>     optlen_new = iter;
>   }

How about turning it to WARN_ON_ONCE() instead? It's actually the
better choice in this case (alerts to a possible kernel bug, not to an
event that would need to be logged every time), I just used WARN_ON()
instinctively and didn't think of the _ONCE variant.

>
> > +                     }
> > +             }
> > +             if (WARN_ON(optlen_new > opt->opt.optlen))
> > +                     optlen_new = opt->opt.optlen;
>
> This is also probably not really necessary, but it bothers me less.

I would convert this one to WARN_ON_ONCE() as well, or drop both if
you still don't like either of them to be there.

>
> >               hdr_delta = opt->opt.optlen;
> >               opt->opt.optlen = (optlen_new + 3) & ~3;
> >               hdr_delta -= opt->opt.optlen;
> > --
> > 2.44.0
>
> --
> paul-moore.com
>

--
Ondrej Mosnacek
Senior Software Engineer, Linux Security - SELinux kernel
Red Hat, Inc.