[QUESTION] Full user space process isolation?

[Dr. Greg]

On Tue, Jul 04, 2023 at 05:18:43PM +0200, Petr Tesarik wrote:

Good morning, I hope the week is going well for everyone.

> On 7/3/2023 5:28 PM, Roberto Sassu wrote:
> > On Mon, 2023-07-03 at 17:06 +0200, Jann Horn wrote:
> >> On Thu, Jun 22, 2023 at 4:45???PM Roberto Sassu
> >> <roberto.sassu at huaweicloud.com> wrote:
> >>> 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.
> >>
> >> FWIW, the kernel has some infrastructure for this already, see
> >> CONFIG_USERMODE_DRIVER and kernel/usermode_driver.c, with a usage
> >> example in net/bpfilter/.
> > 
> > Thanks, I actually took that code to make a generic UMD management
> > library, that can be used by all use cases:
> > 
> > https://lore.kernel.org/linux-kernel/20230317145240.363908-1-roberto.sassu@huaweicloud.com/
> > 
> >>> 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 believe that actually making it that hard would probably mean that
> >> you'd have to ensure that the process doesn't use swap (in other
> >> words, it would have to run with all memory locked), because root can
> >> choose where swapped pages are stored. Other than that, if you mark it
> >> as a kthread so that no ptrace access is allowed, you can probably get
> >> pretty close. But if you do anything like that, please leave some way
> >> (like a kernel build config option or such) to enable debugging for
> >> these processes.
> > 
> > I didn't think about the swapping part... thanks!
> > 
> > Ok to enable debugging with a config option.
> > 
> >> But I'm not convinced that it makes sense to try to draw a security
> >> boundary between fully-privileged root (with the ability to mount
> >> things and configure swap and so on) and the kernel - my understanding
> >> is that some kernel subsystems don't treat root-to-kernel privilege
> >> escalation issues as security bugs that have to be fixed.
> > 
> > Yes, that is unfortunately true, and in that case the trustworthy UMD
> > would not make things worse. On the other hand, on systems where that
> > separation is defined, the advantage would be to run more exploitable
> > code in user space, leaving the kernel safe.
> > 
> > I'm thinking about all the cases where the code had to be included in
> > the kernel to run at the same privilege level, but would not use any of
> > the kernel facilities (e.g. parsers).
> 
> Thanks for reminding me of kexec-tools. The complete image for booting a
> new kernel was originally prepared in user space. With kernel lockdown,
> all this code had to move into the kernel, adding a new syscall and lots
> of complexity to build purgatory code, etc. Yet, this new implementation
> in the kernel does not offer all features of kexec-tools, so both code
> bases continue to exist and are happily diverging...
> 
> > If the boundary is extended to user space, some of these components
> > could be moved away from the kernel, and the functionality would be the
> > same without decreasing the security.

> All right, AFAICS your idea is limited to relatively simple cases
> for now. I mean, allowing kexec-tools to run in user space is not
> easily possible when UID 0 is not trusted, because kexec needs to
> open various files and make various other syscalls, which would
> require a complex LSM policy. It looks technically possible to write
> one, but then the big question is if it would be simpler to review
> and maintain than adding more kexec-tools features to the kernel.

You either need to develop and maintain a complex system-wide LSM
policy or you need a security model that is specifically tuned and
then scoped to the needs of the workload running on behalf of the
kernel as a UID=0 userspace process.

As I noted in my e-mail to Roberto, our TSEM LSM brings forward the
ability to do both, as a useful side effect of the need to limit model
complexity when the objective is to have a single functional
description of the security state of a system.

> Anyway, I can sense a general desire to run less code in the most
> privileged system environment. Robert's proposal is one of few that
> go in this direction. What are the alternatives?

As I noted above, TSEM brings the ability to provide highly specific
and narrowly scoped security policy to a process heirarchy
ie. workload.

However, regardless of the technology applied, in order to pursue
Roberto's UMD model of having a uid=0 process run tasks on behalf of
the kernel, there would seem to be a need to define what the security
objectives are.

>From the outside looking in, there would seem to be a need to address
two primary issues:

1: Trust/constrain what the UMD process can do.

2: Constrain what the system at large can do to the UMD process.

As we have seen before, requirement 1 implies a definition of what it
means for a process to be 'trusted'.

In the absence of formal verification, which appears to be a
non-starter in practice, this would seem to imply defining a standard
for the allowed security behavior of the UMD workload.

>From our perspective, with TSEM, we define 'trusted' for a workload to
mean that it has not requested a security behavior inconsistent with
what the workload has been unit tested to.  If a process does this, its
ability to execute additional security behaviors is curtailed.

With respect to requirement two.

Here is the ASCII art diagram of Roberto's proposed system:

     r/w  ^                             kernel space
----------|-----------------------------------------
          v (pipe)                        user space
 +-----------------+       +-----------------------+
 | trustworthy UMD |---X---| rest of the processes |
 +-----------------+       +-----------------------+

Casey noted that he believed the Linux LSM had sufficient coverage to
provide the necessary security controls for this model.  He
specifically mentioned that it had support for network traffic
controls and labeling.

I haven't seen a reply from Roberto to my e-mail questioning what the
following means:

---X---

But I get the sense that it means that any other process in userspace
couldn't have any impact, or I assume visibility, into what the UID=0
process is doing on behalf of the kernel.  I don't think it means that
there is supposed to be some type of highly controlled traffic between
the UMD and other processes.

We will see what comments Roberto has on this.

This arguably may be the most difficult requirement to meet if our
interpretation of this requirement is correct, particularly so if this
involves a confidentiality requirement, perhaps a bit less so if there
is only a requirement of integrity of execution.

As I mentioned in a previous e-mail, depending on the requirements,
issue 2 starts to look a lot like protected enclave technologies such
as SGX.  As history has shown, providing a protected execution
environment, against the rest of the system, is a somewhat formidable
undertaking, with probably a requirement for hardware support if SGX
and/or TDX are any examples.

So, I believe that TSEM brings useful technology to the table, but
regardless of technology, it would seem there is a need to
specifically define the security requirements for the UMD model.

> Petr T

Have a good day.

As always,
Dr. Greg

The Quixote Project - Flailing at the Travails of Cybersecurity