Re: [RFC PATCH v1 0/9] Hypervisor-Enforced Kernel Integrity

From: Mickaël Salaün
Date: Thu May 25 2023 - 09:25:46 EST

On 24/05/2023 23:04, Trilok Soni wrote:
On 5/5/2023 8:20 AM, Mickaël Salaün wrote:

This patch series is a proof-of-concept that implements new KVM features
(extended page tracking, MBEC support, CR pinning) and defines a new API to
protect guest VMs. No VMM (e.g., Qemu) modification is required.

The main idea being that kernel self-protection mechanisms should be delegated
to a more privileged part of the system, hence the hypervisor. It is still the
role of the guest kernel to request such restrictions according to its

Only for the guest kernel images here? Why not for the host OS kernel?

As explained in the Future work section, protecting the host would be useful, but that doesn't really fit with the KVM model. The Protected KVM project is a first step to help in this direction [11].

In a nutshell, KVM is close to a type-2 hypervisor, and the host kernel is also part of the hypervisor.

Embedded devices w/ Android you have mentioned below supports the host
OS as well it seems, right?

What do you mean?

Do we suggest that all the functionalities should be implemented in the
Hypervisor (NS-EL2 for ARM) or even at Secure EL like Secure-EL1 (ARM).

KVM runs in EL2. TrustZone is mainly used to enforce DRM, which means that we may not control the related code.

This patch series is dedicated to hypervisor-enforced kernel integrity, then KVM.

I am hoping that whatever we suggest the interface here from the Guest
to the Hypervisor becomes the ABI right?

Yes, hypercalls are part of the KVM ABI.

# Current limitations

The main limitation of this patch series is the statically enforced
permissions. This is not an issue for kernels without module but this needs to
be addressed. Mechanisms that dynamically impact kernel executable memory are
not handled for now (e.g., kernel modules, tracepoints, eBPF JIT), and such
code will need to be authenticated. Because the hypervisor is highly
privileged and critical to the security of all the VMs, we don't want to
implement a code authentication mechanism in the hypervisor itself but delegate
this verification to something much less privileged. We are thinking of two
ways to solve this: implement this verification in the VMM or spawn a dedicated
special VM (similar to Windows's VBS). There are pros on cons to each approach:
complexity, verification code ownership (guest's or VMM's), access to guest
memory (i.e., confidential computing).

Do you foresee the performance regressions due to lot of tracking here?

The performance impact of execution prevention should be negligible because once configured the hypervisor do nothing except catch illegitimate access attempts.

Production kernels do have lot of tracepoints and we use it as feature
in the GKI kernel for the vendor hooks implementation and in those cases
every vendor driver is a module.

As explained in this section, dynamic kernel modifications such as tracepoints or modules are not currently supported by this patch series. Handling tracepoints is possible but requires more work to define and check legitimate changes. This proposal is still useful for static kernels though.

Separate VM further fragments this
design and delegates more of it to proprietary solutions?

What do you mean? KVM is not a proprietary solution.

For dynamic checks, this would require code not run by KVM itself, but either the VMM or a dedicated VM. In this case, the dynamic authentication code could come from the guest VM or from the VMM itself. In the former case, it is more challenging from a security point of view but doesn't rely on external (proprietary) solution. In the latter case, open-source VMMs should implement the specification to provide the required service (e.g. check kernel module signature).

The goal of the common API layer provided by this RFC is to share code as much as possible between different hypervisor backends.

Do you have any performance numbers w/ current RFC?

No, but the only hypervisor performance impact is at boot time and should be negligible. I'll try to get some numbers for the hardware-enforcement impact, but it should be negligible too.