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

On Wed, Jan 26, 2022 at 2:24 AM Christian Brauner [off-list ref] wrote:

On Tue, Jan 25, 2022 at 05:18:02PM -0500, Paul Moore wrote:

quoted

On Tue, Oct 12, 2021 at 10:27 AM Casey Schaufler [off-list ref] wrote:

quoted

On 10/12/2021 3:32 AM, Christian Brauner wrote:

quoted

On Mon, Oct 11, 2021 at 03:40:22PM -0700, Casey Schaufler wrote:

quoted

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: <redacted>
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.

I'm shocked! :)

Thanks Christian.

-- 
paul-moore.com