netdev - Re: [PATCH 0/5] usermode_driver: Add management library and API

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Date:   Wed, 22 Mar 2023 15:27:33 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Roberto Sassu <roberto.sassu@...weicloud.com>
Cc:     Jonathan Corbet <corbet@....net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        David Ahern <dsahern@...nel.org>,
        Shuah Khan <shuah@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Roberto Sassu <roberto.sassu@...wei.com>
Subject: Re: [PATCH 0/5] usermode_driver: Add management library and API

On Wed, Mar 22, 2023 at 5:08 AM Roberto Sassu
<roberto.sassu@...weicloud.com> wrote:
>
> On Tue, 2023-03-21 at 19:23 -0700, Alexei Starovoitov wrote:
> > On Fri, Mar 17, 2023 at 7:53 AM Roberto Sassu
> > <roberto.sassu@...weicloud.com> wrote:
> > > From: Roberto Sassu <roberto.sassu@...wei.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
>
> Thanks. I just ensured that it worked, by doing:
>
> getsockopt(fd, SOL_IP, IPT_SO_GET_INFO, &info, &optlen);
>
> and accepting IPT_SO_GET_INFO in main.c.
>
> > > 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.
>
> What would make it more generic? I made the API message format-
> independent. It has the capability of starting the user space process
> as required, when there is a communication.
>
> > It was a quick hack to get bpfilter off the ground, but certainly
> > not a generic one.
>
> True, it is not generic in the sense that it can accomodate any
> possible use case. The main goal is to move something that was running
> in the kernel to user space, with the same isolation guarantees as if
> the code was executed in the kernel.

They are not the same guarantees.
UMD is exactly equivalent to root process running in user space.
Meaning it can be killed, ptraced, priority inverted, etc

> > > 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.
>
> UMD is better in the sense that it establishes a bidirectional pipe
> between the kernel and the user space process. With that, there is no
> need to further restrict the access to a sysfs file, for example.

If a simple pipe is good enough then you can have a kernel module
that creates it and interacts with the user space process.
Out-of-tree bpftiler can do that, so can you.
PGP is not suitable for kernel git repo either as kernel code or as UMD.