linux-kernel - Re: [PATCH v3] selinux: lsm: fix a missing-check bug in selinux_add_mnt

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Message-ID: <20190611030417.GA4013@ubuntu>
Date:   Mon, 10 Jun 2019 20:04:18 -0700
From:   Gen Zhang <blackgod016574@...il.com>
To:     Paul Moore <paul@...l-moore.com>
Cc:     Ondrej Mosnacek <omosnace@...hat.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Eric Paris <eparis@...isplace.org>, selinux@...r.kernel.org,
        Linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] selinux: lsm: fix a missing-check bug in
 selinux_add_mnt_opt( )

On Mon, Jun 10, 2019 at 03:31:50PM -0400, Paul Moore wrote:
> On Fri, Jun 7, 2019 at 8:11 AM Gen Zhang <blackgod016574@...il.com> wrote:
> >
> > On Fri, Jun 07, 2019 at 10:39:05AM +0200, Ondrej Mosnacek wrote:
> > > On Thu, Jun 6, 2019 at 11:23 AM Gen Zhang <blackgod016574@...il.com> wrote:
> > > > In selinux_add_mnt_opt(), 'val' is allocated by kmemdup_nul(). It returns
> > > > NULL when fails. So 'val' should be checked. And 'mnt_opts' should be
> > > > freed when error.
> > > >
> > > > Signed-off-by: Gen Zhang <blackgod016574@...il.com>
> > > > Fixes: 757cbe597fe8 ("LSM: new method: ->sb_add_mnt_opt()")
> > > > ---
> > > > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> > > > index 3ec702c..4e4c1c6 100644
> > > > --- a/security/selinux/hooks.c
> > > > +++ b/security/selinux/hooks.c
> > > > @@ -1052,15 +1052,23 @@ static int selinux_add_mnt_opt(const char *option, const char *val, int len,
> > > >         if (token == Opt_error)
> > > >                 return -EINVAL;
> > > >
> > > > -       if (token != Opt_seclabel)
> > > > -               val = kmemdup_nul(val, len, GFP_KERNEL);
> > > > +       if (token != Opt_seclabel) {
> > > > +                       val = kmemdup_nul(val, len, GFP_KERNEL);
> > > > +                       if (!val) {
> > > > +                               rc = -ENOMEM;
> > > > +                               goto free_opt;
> > > > +                       }
> > > > +       }
> > > >         rc = selinux_add_opt(token, val, mnt_opts);
> > > >         if (unlikely(rc)) {
> > > >                 kfree(val);
> > > > -               if (*mnt_opts) {
> > > > -                       selinux_free_mnt_opts(*mnt_opts);
> > > > -                       *mnt_opts = NULL;
> > > > -               }
> > > > +               goto free_opt;
> > > > +       }
> > > > +       return rc;
> > >
> > > At this point rc is guaranteed to be 0, so you can just 'return 0' for
> > > clarity. Also, I visually prefer an empty line between a return
> > > statement and a goto label, but I'm not sure what is the
> > > general/maintainer's preference.
> >
> > Am I supposed to revise and send a patch v4 for this, or let the
> > maintainer do this? :-)
> 
> First a few things from my perspective: I don't really care too much
> about the difference between returning "0" and "rc" here, one could
> argue that "0" is cleaner and that "rc" is "safer".  To me it isn't a
> big deal and generally isn't something I would even comment on unless
> there was something else in the patch that needed addressing.  I care
> a more about the style choice of having an empty line between the
> return and the start of the goto targets (vertical whitespace before
> the jump targets is good, please include it), but once again, I'm not
> sure I would comment on that.  The patch subject line is a bit
> confusing in that we already discussed when to use "selinux" and when
> to use "lsm", but I imagine there might be some confusion about using
> both so let me try and clear that up now: don't do it unless you have
> a *really* good reason to do so :)  In this case it is all SELinux
> code so there is no reason why you should be including the "lsm"
> prefix.
Thanks for your comments. I was uncertain of the meaning of "lsm". So I
used"selinux: lsm:". I am aware of that now.

Thanks
Gen
> 
> You've been pretty responsive, so if you don't mind submitting a v4
> with the changes mentioned above, that would be far more preferable to
> me making the changes.  I have some other comments about maintainer
> fixes to patches, but I'll save that for the other thread :)
> 
> -- 
> paul moore
> www.paul-moore.com