[PATCH 0/3] Firmware LSM hook

[Paul Moore]

On Wed, Mar 11, 2026 at 4:20 AM Leon Romanovsky <leon at kernel.org> wrote:
> On Tue, Mar 10, 2026 at 05:40:02PM -0400, Paul Moore wrote:
> > On Tue, Mar 10, 2026 at 3:30 PM Leon Romanovsky <leon at kernel.org> wrote:
> > > On Tue, Mar 10, 2026 at 02:24:40PM -0400, Paul Moore wrote:
> > > > On Tue, Mar 10, 2026 at 5:07 AM Leon Romanovsky <leon at kernel.org> wrote:
> > > > > On Mon, Mar 09, 2026 at 07:10:25PM -0400, Paul Moore wrote:
> > > > > > On Mon, Mar 9, 2026 at 3:37 PM Leon Romanovsky <leon at kernel.org> wrote:
> > > > > > > On Mon, Mar 09, 2026 at 02:32:39PM -0400, Paul Moore wrote:
> > > > > > > > On Mon, Mar 9, 2026 at 7:15 AM Leon Romanovsky <leon at kernel.org> wrote:
> > > >
> > > > ...
> > > >
> > > > > > > > Hi Leon,
> > > > > > > >
> > > > > > > > At the link below, you'll find guidance on submitting new LSM hooks.
> > > > > > > > Please take a look and let me know if you have any questions.
> > > > > > > >
> > > > > > > > https://github.com/LinuxSecurityModule/kernel/blob/main/README.md#new-lsm-hooks
> > > > > > >
> > > > > > > I assume that you are referring to this part:
> > > > > >
> > > > > > I'm referring to all of the guidance, but yes, at the very least that
> > > > > > is something that I think we need to see in a future revision of this
> > > > > > patchset.
> > > > > >
> > > > > > >  * New LSM hooks must demonstrate their usefulness by providing a meaningful
> > > > > > >    implementation for at least one in-kernel LSM. The goal is to demonstrate
> > > > > > >    the purpose and expected semantics of the hooks. Out of tree kernel code,
> > > > > > >    and pass through implementations, such as the BPF LSM, are not eligible
> > > > > > >    for LSM hook reference implementations.
> > > > > > >
> > > > > > > The point is that we are not inspecting a kernel call, but the FW mailbox,
> > > > > > > which has very little meaning to the kernel. From the kernel's perspective,
> > > > > > > all relevant checks have already been performed, but the existing capability
> > > > > > > granularity does not allow us to distinguish between FW_CMD1 and FW_CMD2.
> > > > > >
> > > > > > It might help if you could phrase this differently, as I'm not
> > > > > > entirely clear on your argument.  LSMs are not limited to enforcing
> > > > > > access controls on requests the kernel understands (see the SELinux
> > > > > > userspace object manager concept), and the idea of access controls
> > > > > > with greater granularity than capabilities is one of the main reasons
> > > > > > people look to LSMs for access control (SELinux, AppArmor, Smack,
> > > > > > etc.).
> > > > >
> > > > > I should note that my understanding of LSM is limited, so some parts of my
> > > > > answers may be inaccurate.
> > > > >
> > > > > What I am referring to is a different level of granularity — specifically,
> > > > > the internals of the firmware commands. In the proposed approach, BPF
> > > > > programs would make decisions based on data passed through the mailbox.
> > > > > That mailbox format varies across vendors, and may even differ between
> > > > > firmware versions from the same vendor.
> > > >
> > > > That helps, thank you.
> > > >
> > > > > > > Here we propose a generic interface that can be applied to all FWCTL
> > > > > > > devices without out-of-tree kernel code at all.
> > > > > >
> > > > > > I expected to see a patch implementing some meaningful support for
> > > > > > access controls using these hooks in one of the existing LSMs, I did
> > > > > > not see that in this patchset.
> > > > >
> > > > > In some cases, the mailbox is forwarded from user space unchanged, but
> > > > > in others the kernel modifies it before submitting it to the FW.
> > > >
> > > > Without a standard format, opcode definitions, etc. I suspect
> > > > integrating this into an LSM will present a number of challenges.
> > >
> > > The opcode is relatively easy to extract from the mailbox and pass to the LSM.
> > > All drivers implement some variant of mlx5ctl_validate_rpc()/devx_is_general_cmd()
> > > to validate the opcode. The problem is that this check alone is not sufficient.
> > >
> > > > Instead of performing an LSM access control check before submitting
> > > > the firmware command, it might be easier from an LSM perspective to
> > > > have the firmware call into the kernel/LSM for an access control
> > > > decision before performing a security-relevant action.
> > >
> > > Ultimately, the LSM must make a decision for each executed firmware
> > > command. This will need to be handled one way or another, and will
> > > likely require parsing the mailbox again.
> >
> > As it's unlikely that parsing the mailbox is something that a LSM will
> > want to handle,
>
> I believe this approach offers the cleanest and most natural way to support
> all mailbox‑based devices.
>
> > my suggestion was to leverage the existing mailbox parsing in the firmware
> > and require the firmware to call into the LSM when authorization is needed.
> >
> > > > This removes the challenge of parsing/interpreting the arbitrary firmware commands,
> > > > but it does add some additional complexity of having to generically
> > > > represent the security relevant actions the firmware might request
> > >
> > > The difference here is that the proposed LSM hook is intended to disable
> > > certain functionality provided by the firmware, effectively depending on
> > > the operator’s preferences.
> >
> > My suggestion would also allow a LSM hook to disable certain firmware
> > functionality; however, the firmware itself would need to call the LSM
> > to check if the functionality is authorized.
>
> This suggestion adds an extra call from the FW to the LSM for every command, even
> for systems which don't have LSM at all.

If latency is a concern, I imagine we could create an LSM hook to
report whether any LSMs provided firmware access controls.  The
firmware could then use that hook, potentially caching the result, to
limit its calls into the LSM.

> The FW must pass the already parsed data
> back to the LSM; otherwise, the LSM   has no basis to decide whether to accept or
> reject the request.
>
> For example, consider the MLX5_CMD_OP_QUERY_DCT command handled in
> mlx5ctl_validate_rpc(). DCT in RDMA refers to Dynamically Connected
> Transport, a Mellanox-specific extension that effectively introduces a new
> QP‑type family on top of the standard RC/UC/UD transports. This type does not
> exist for other vendors, each of whom provides its own vendor‑specific
> extensions. All parameters here are tightly coupled to those specific
> commands.
>
> It is unrealistic to expect different firmware implementations to supply
> their data in a common format that would allow the LSM to make a generic
> decision.

That's unfortunate as that would be the easiest path forward.
Regardless, you are welcome to work on whatever implementation you
think makes sense for any of the in-tree LSMs, with that in place we
can take another look at the firmware command hooks.

Good luck.

-- 
paul-moore.com