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

[Mickaël Salaün]

On 25/05/2023 20:34, Trilok Soni wrote:
> On 5/25/2023 6:25 AM, Mickaël Salaün wrote:
>>
>> On 24/05/2023 23:04, Trilok Soni wrote:
>>> On 5/5/2023 8:20 AM, Mickaël Salaün wrote:
>>>> Hi,
>>>>
>>>> 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?
> 
> I think you have answered this above w/ pKVM and I was referring the
> host protection as well w/ Heki. The link/references below refers to the
> Android OS it seems and not guest VM.
> 
>>
>>
>>>
>>> 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.
> 
> Sure. I just hope that they are extensible enough to support for other
> Hypervisors too. I am not sure if they are on this list like ACRN / Xen
> and see if it fits their need too.

KVM, Hyper-V and Xen mailing lists are CCed. The KVM hypercalls are 
specific to KVM, but this patch series also include a common guest API 
intended to be used with all hypervisors.


> 
> Is there any other Hypervisor you plan to test this feature as well?

We're also working on Hyper-V.

> 
>>
>>>
>>>
>>>>
>>>> # 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.
> 
> Yes, if you are using the static kernel only and not considering the
> other dynamic patching features like explained. They need to be thought
> upon differently to reduce the likely impact.

What do you mean? We plan to support dynamic code, and performance is of 
course part of the requirement.


> 
>>
>>
>>> 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.
> 
> Ah, I was referring the VBS Windows VM mentioned in the above text. Is
> it open-source? The reference of VM (or dedicated VM) didn't mention
> that VM itself will be open-source running Linux kernel.

This patch series is dedicated to KVM. Windows VBS was only mentioned as 
a comparable (but much more advanced) set of features. Everything 
required to use this new KVM features is and will be open-source. There 
is nothing to worry about licensing, the goal is to make it widely and 
freely available to protect users.


> 
>>
>> 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.
> 
> Thanks. Please share the data once you have it ready.

It's on my todo list, but again, that should not be an issue and I even 
doubt the difference to be measurable.