linux-kernel - Re: [PATCH v7 6/6] mm/memfd: security hook for memfd

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Date:   Tue, 13 Dec 2022 14:22:19 -0500
From:   Paul Moore <paul@...l-moore.com>
To:     Jeff Xu <jeffxu@...gle.com>
Cc:     jeffxu@...omium.org, skhan@...uxfoundation.org,
        keescook@...omium.org, akpm@...ux-foundation.org,
        dmitry.torokhov@...il.com, dverkamp@...omium.org, hughd@...gle.com,
        jorgelo@...omium.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-mm@...ck.org,
        jannh@...gle.com, linux-hardening@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH v7 6/6] mm/memfd: security hook for memfd_create

On Tue, Dec 13, 2022 at 10:00 AM Jeff Xu <jeffxu@...gle.com> wrote:
> On Fri, Dec 9, 2022 at 10:29 AM Paul Moore <paul@...l-moore.com> wrote:
> > On Fri, Dec 9, 2022 at 11:05 AM <jeffxu@...omium.org> wrote:
> > >
> > > From: Jeff Xu <jeffxu@...gle.com>
> > >
> > > The new security_memfd_create allows lsm to check flags of
> > > memfd_create.
> > >
> > > The security by default system (such as chromeos) can use this
> > > to implement system wide lsm to allow only non-executable memfd
> > > being created.
> > >
> > > Signed-off-by: Jeff Xu <jeffxu@...gle.com>
> > > Reported-by: kernel test robot <lkp@...el.com>
> > > ---
> > >  include/linux/lsm_hook_defs.h | 1 +
> > >  include/linux/lsm_hooks.h     | 4 ++++
> > >  include/linux/security.h      | 6 ++++++
> > >  mm/memfd.c                    | 5 +++++
> > >  security/security.c           | 5 +++++
> > >  5 files changed, 21 insertions(+)
> >
> > We typically require at least one in-tree LSM implementation to
> > accompany a new LSM hook.  Beyond simply providing proof that the hook
> > has value, it helps provide a functional example both for reviewers as
> > well as future LSM implementations.  Also, while the BPF LSM is
> > definitely "in-tree", its nature is such that the actual
> > implementation lives out-of-tree; something like SELinux, AppArmor,
> > Smack, etc. are much more desirable from an in-tree example
> > perspective.
>
> Thanks for the comments.
> Would that be OK if I add a new LSM in the kernel  to block executable
> memfd creation ?

If you would be proposing the LSM only to meet the requirement of
providing an in-tree LSM example, no that would definitely *not* be
okay.

Proposing a new LSM involves documenting a meaningful security model,
implementing it, developing tests, going through a (likely multi-step)
review process, and finally accepting the long term maintenance
responsibilities of this new LSM.  If you are proposing a new LSM
because you feel the current LSMs do not provide a security model
which meets your needs, then yes, proposing a new LSM might be a good
idea.  However, if you are proposing a new LSM because you don't want
to learn how to add a new hook to an existing LSM, then I suspect you
are misguided/misinformed with the amount of work involved in
submitting a new LSM.

> Alternatively,  it might be possible to add this into SELinux or
> landlock, it will be a larger change.

It will be a much smaller change than submitting a new LSM, and it
would have infinitely more value to the community than a throw-away
LSM where the only use-case is getting your code merged upstream.

-- 
paul-moore.com