On Fri, Mar 17, 2023 at 7:53 AM Roberto Sassu
[off-list ref] wrote:

From: Roberto Sassu <roberto.sassu@huawei.com>

A User Mode Driver (UMD) is a specialization of a User Mode Helper (UMH),
which runs a user space process from a binary blob, and creates a
bidirectional pipe, so that the kernel can make a request to that process,
and the latter provides its response. It is currently used by bpfilter,
although it does not seem to do any useful work.

FYI the new home for bpfilter is here:
https://github.com/facebook/bpfilter

The problem is, if other users would like to implement a UMD similar to
bpfilter, they would have to duplicate the code. Instead, make an UMD
management library and API from the existing bpfilter and sockopt code,
and move it to common kernel code.

Also, define the software architecture and the main components of the
library: the UMD Manager, running in the kernel, acting as the frontend
interface to any user or kernel-originated request; the UMD Loader, also
running in the kernel, responsible to load the UMD Handler; the UMD
Handler, running in user space, responsible to handle requests from the UMD
Manager and to send to it the response.

That doesn't look like a generic interface for UMD.
It was a quick hack to get bpfilter off the ground, but certainly
not a generic one.

I have two use cases, but for sake of brevity I will propose one.

I would like to add support for PGP keys and signatures in the kernel, so
that I can extend secure boot to applications, and allow/deny code
execution based on the signed file digests included in RPM headers.

While I proposed a patch set a while ago (based on a previous work of David
Howells), the main objection was that the PGP packet parser should not run
in the kernel.

That makes a perfect example for using a UMD. If the PGP parser is moved to
user space (UMD Handler), and the kernel (UMD Manager) just instantiates
the key and verifies the signature on already parsed data, this would
address the concern.

I don't think PGP parser belongs to UMD either.
Please do it as a normal user space process and define a proper
protocol for communication between kernel and user space.