Re: Linux guest kernel threat model for Confidential Computing

[Carlos Bilbao]

On 1/25/23 6:28 AM, Reshetova, Elena wrote:
> Hi Greg, 
> 
> You mentioned couple of times (last time in this recent thread:
> https://lore.kernel.org/all/Y80WtujnO7kfduAZ@xxxxxxxxx/) that we ought to start
> discussing the updated threat model for kernel, so this email is a start in this direction. 
> 
> (Note: I tried to include relevant people from different companies, as well as linux-coco
> mailing list, but I hope everyone can help by including additional people as needed).
> 
> As we have shared before in various lkml threads/conference presentations
> ([1], [2], [3] and many others), for the Confidential Computing guest kernel, we have a 
> change in the threat model where guest kernel doesn’t anymore trust the hypervisor. 
> This is a big change in the threat model and requires both careful assessment of the 
> new (hypervisor <-> guest kernel) attack surface, as well as careful design of mitigations
> and security validation techniques. This is the activity that we have started back at Intel
> and the current status can be found in
> 
> 1) Threat model and potential mitigations: 
> https://intel.github.io/ccc-linux-guest-hardening-docs/security-spec.html> 
> 2) One of the described in the above doc mitigations is "hardening of the enabled
> code". What we mean by this, as well as techniques that are being used are
> described in this document: > https://intel.github.io/ccc-linux-guest-hardening-docs/tdx-guest-hardening.html

Regarding driver hardening, does anyone have a better filtering idea?

The current solution assumes the kernel command line is trusted and cannot
avoid the __init() functions that waste memory. I don't know if the
__exit() routines of the filtered devices are called, but it doesn't sound
much better to allocate memory and free it right after.

> 
> 3) All the tools are open-source and everyone can start using them right away even
> without any special HW (readme has description of what is needed).
> Tools and documentation is here:
> https://github.com/intel/ccc-linux-guest-hardening
> 
> 4) all not yet upstreamed linux patches (that we are slowly submitting) can be found 
> here: https://github.com/intel/tdx/commits/guest-next
> 
> So, my main question before we start to argue about the threat model, mitigations, etc,
> is what is the good way to get this reviewed to make sure everyone is aligned?
> There are a lot of angles and details, so what is the most efficient method? 
> Should I split the threat model from https://intel.github.io/ccc-linux-guest-hardening-docs/security-spec.html
> into logical pieces and start submitting it to mailing list for discussion one by one? 
> Any other methods? 
> 
> The original plan we had in mind is to start discussing the relevant pieces when submitting the code,
> i.e. when submitting the device filter patches, we will include problem statement, threat model link, 
> data, alternatives considered, etc. 
> 
> Best Regards,
> Elena.
> 
> [1] https://lore.kernel.org/all/20210804174322.2898409-1-sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx/
> [2] https://lpc.events/event/16/contributions/1328/
> [3] https://events.linuxfoundation.org/archive/2022/linux-security-summit-north-america/program/schedule/

Thanks,
Carlos