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

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Message-ID: <CABi2SkUj2WoNpm6repAr0rkmQgFAmfRTYD0k=KcPUXjOd3tvhw@mail.gmail.com>
Date: Sat, 20 Sep 2025 11:58:51 -0700
From: Jeff Xu <jeffxu@...omium.org>
To: Abhinav Saxena <xandfury@...il.com>
Cc: Paul Moore <paul@...l-moore.com>, Jeff Xu <jeffxu@...gle.com>, 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>, 
	Mickaël Salaün <mic@...ikod.net>, 
	Günther Noack <gnoack@...gle.com>, Fan Wu <wufan@...nel.org>, 
	Kees Cook <kees@...nel.org>, "tweek@...gle.com" <tweek@...gle.com>
Subject: Re: [PATCH v7 6/6] mm/memfd: security hook for memfd_create

Hi Abhinav

Thanks for reaching out, it is great that you are interested in
Landlock and IPE use cases for executable memfd.

Adding  the latest discussion/status  that I'm aware of, related to
memfd, for reference

- Thiébaud Weksteen (In CC)  has patch [1] for a new selinux policy
for memfd_create in [1]
- Mickaël Salaün and I  discussed the security hook to block executing
memfd [2].
- Your recent patch in Landlock [3]

[1] https://lore.kernel.org/all/20250918020434.1612137-1-tweek@google.com/
[2] https://lore.kernel.org/all/20240719.sah7oeY9pha4@digikod.net/
[3] https://lore.kernel.org/all/20250719-memfd-exec-v1-0-0ef7feba5821@gmail.com/

Thanks
-Jeff



-Jeff

On Fri, Sep 19, 2025 at 11:10 PM Abhinav Saxena <xandfury@...il.com> wrote:
>
> Paul Moore <paul@...l-moore.com> writes:
>
> > 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.
>
> Hi Paul/everyone!
>
> I am not sure what is the latest here. But it seems both landlock[1] and
> IPE[2] have a use case for memfd_create(2) LSM hook.
>
> I would be happy to work on the use case for such a hook for landlock.
>
> CC’ing maintainers for both LSMs.
>
> -Abhinav
>
> [1] - <https://lore.kernel.org/all/20250719-memfd-exec-v1-0-0ef7feba5821@gmail.com/>
> [2] - <https://lore.kernel.org/linux-security-module/20250129203932.22165-1-wufan@kernel.org/>