linux-kernel - Re: [PATCH] LSM: general protection fault in legacy_parse

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Date:   Wed, 26 Jan 2022 17:37:41 -0500
From:   Paul Moore <paul@...l-moore.com>
To:     Christian Brauner <brauner@...nel.org>
Cc:     Casey Schaufler <casey@...aufler-ca.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Christian Brauner <christian@...uner.io>,
        James Morris <jmorris@...ei.org>,
        Linux Security Module list 
        <linux-security-module@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzbot <syzbot+d1e3b1d92d25abf97943@...kaller.appspotmail.com>,
        David Howells <dhowells@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        selinux@...r.kernel.org
Subject: Re: [PATCH] LSM: general protection fault in legacy_parse_param

On Wed, Jan 26, 2022 at 2:24 AM Christian Brauner <brauner@...nel.org> wrote:
>
> 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@...aufler-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@...kaller.appspotmail.com
> > > >> Signed-off-by: Casey Schaufler <casey@...aufler-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@...l-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.

I'm shocked! :)

Thanks Christian.

-- 
paul-moore.com