On Fri, Mar 17, 2023 at 7:53 AM Roberto Sassu roberto.sassu@huaweicloud.com 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.