[PATCH v7 6/6] mm/memfd: security hook for memfd_create

[Abhinav Saxena]

Paul Moore <paul at paul-moore.com> writes:

> On Tue, Dec 13, 2022 at 10:00 AM Jeff Xu <jeffxu at google.com> wrote:
>> On Fri, Dec 9, 2022 at 10:29 AM Paul Moore <paul at paul-moore.com> wrote:
>> > On Fri, Dec 9, 2022 at 11:05 AM <jeffxu at chromium.org> wrote:
>> > >
>> > > From: Jeff Xu <jeffxu at google.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 at google.com>
>> > > Reported-by: kernel test robot <lkp at intel.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/>