linux-hardening - [QUESTION] Full user space process isolation?

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Date:   Thu, 22 Jun 2023 16:42:37 +0200
From:   Roberto Sassu <roberto.sassu@...weicloud.com>
To:     Oleg Nesterov <oleg@...hat.com>, Paul Moore <paul@...l-moore.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Eric Paris <eparis@...isplace.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Kees Cook <keescook@...omium.org>,
        Casey Schaufler <casey@...aufler-ca.com>,
        David Howells <dhowells@...hat.com>,
        LuisChamberlain <mcgrof@...nel.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        Petr Tesarik <petrtesarik@...weicloud.com>,
        Christoph Hellwig <hch@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>, Tejun Heo <tj@...nel.org>
Cc:     linux-mm@...ck.org, linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, keyrings@...r.kernel.org,
        linux-integrity@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: [QUESTION] Full user space process isolation?

Hi everyone

I briefly discussed this topic at LSS NA 2023, but I wanted to have an
opinion from a broader audience.

In short:

I wanted to execute some kernel workloads in a fully isolated user
space process, started from a binary statically linked with klibc,
connected to the kernel only through a pipe.

I also wanted that, for the root user, tampering with that process is
as hard as if the same code runs in kernel space.

I would use the fully isolated process to parse and convert unsupported
data formats to a supported one, after the kernel verified the
authenticity of the original format (that already exists and cannot
change).

Preventing tampering of the process ensures that the conversion goes as
expected. Also, the integrity of the binary needs to be verified.

List of wished data formats:

PGP: verify the authenticity of RPM/DEB/... headers
RPM/DEB/... headers: extract reference file checksums for
                    (kernel-based) file integrity check (e.g. with IMA)

Alternative #1:

Write the parsers to run in kernel space. That was rejected due to
security and scalability concerns. If that changed, please let me know.

Alternative #2:

Linux distributions could provide what the kernel supports. However,
from personal experience, the effort seems orders of magnitude higher
than just writing a tiny component to support the original format. And
there is no guarantee that all Linux distributions will do it.

Full process isolation could be achieved in this way:

process -> outside: set seccomp strict profile at process creation
                    so that the process can only read/write/close the
                    pipe and exit, no other system calls are allowed

outside -> process: deny ptrace/kill with the process as target

Anything else?

The only risk I see is that a new feature allowing to interact with
another process is added to the kernel, without the ptrace permission
being asked.

With the restrictions above, can we say that the code inside the
process is as safe (against tampering) to execute as if it runs in
kernel space?

Thanks

Roberto