Re: [PATCH 0/2] Support userspace hypercalls for TDX

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jul 03, 2024 at 11:35:59PM +0000, Tim Merrifield wrote:
> VMCALL and VMMCALL instructions are used by x86 guests to request services
> from the host VMM. Both VMCALL and VMMCALL are not restricted to CPL 0.
> This allows userspace software like open-vm-tools to communicate directly
> with the VMM.
> 
> In the context of confidential VMs, direct communication with the host may
> violate the security model. Existing binaries that make use of hypercalls
> and are not hardened against malicious hypervisors can become a possible
> attack surface. For this reason, user-level VMCALLs are not currently
> forwarded to the host on TDX VMs. This breaks any user-level software that
> use these instructions.
> 
> But if user-level software is aware of the risks and has been hardened to
> address any known violations of the security model, then it seems
> reasonable to allow hypercalls from this process to proceed.

And how are we to ascertain the software using these hooks is deemed
secure? What security risks are there for the kernel if a malicious
userspace process asks for these rights?

The kernel must assume malice on the part of userspace.