Re: [PATCH] LSM: general protection fault in legacy_parse_param

[Christian Brauner <brauner@kernel.org>]

On Tue, Jan 25, 2022 at 05:18:02PM -0500, Paul Moore wrote:
> On Tue, Oct 12, 2021 at 10:27 AM Casey Schaufler <casey@schaufler-ca.com> wrote:
> > On 10/12/2021 3:32 AM, Christian Brauner wrote:
> > > On Mon, Oct 11, 2021 at 03:40:22PM -0700, Casey Schaufler wrote:
> > >> The usual LSM hook "bail on fail" scheme doesn't work for cases where
> > >> a security module may return an error code indicating that it does not
> > >> recognize an input.  In this particular case Smack sees a mount option
> > >> that it recognizes, and returns 0. A call to a BPF hook follows, which
> > >> returns -ENOPARAM, which confuses the caller because Smack has processed
> > >> its data.
> > >>
> > >> Reported-by: syzbot+d1e3b1d92d25abf97943@syzkaller.appspotmail.com
> > >> Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
> > >> ---
> > > Thanks!
> > > Note, I think that we still have the SELinux issue we discussed in the
> > > other thread:
> > >
> > >       rc = selinux_add_opt(opt, param->string, &fc->security);
> > >       if (!rc) {
> > >               param->string = NULL;
> > >               rc = 1;
> > >       }
> > >
> > > SELinux returns 1 not the expected 0. Not sure if that got fixed or is
> > > queued-up for -next. In any case, this here seems correct independent of
> > > that:
> >
> > The aforementioned SELinux change depends on this patch. As the SELinux
> > code is today it blocks the problem seen with Smack, but introduces a
> > different issue. It prevents the BPF hook from being called.
> >
> > So the question becomes whether the SELinux change should be included
> > here, or done separately. Without the security_fs_context_parse_param()
> > change the selinux_fs_context_parse_param() change results in messy
> > failures for SELinux mounts.
> 
> FWIW, this patch looks good to me, so:
> 
> Acked-by: Paul Moore <paul@paul-moore.com>
> 
> ... and with respect to the SELinux hook implementation returning 1 on
> success, I don't have a good answer and looking through my inbox I see
> David Howells hasn't responded either.  I see nothing in the original
> commit explaining why, so I'm going to say let's just change it to
> zero and be done with it; the good news is that if we do it now we've


It was originally supposed to return 1 but then this got changed but - a
classic - the documentation wasn't.

> got almost a full cycle in linux-next to see what falls apart.  As far

Sweet!