Re: [PATCH v1 4/9] KVM: x86: Add new hypercall to set EPT permissions

[Sean Christopherson]

On Fri, May 05, 2023, Micka�l Sala�n wrote:
> 
> On 05/05/2023 18:44, Sean Christopherson wrote:
> > On Fri, May 05, 2023, Micka�l Sala�n wrote:
> > > Add a new KVM_HC_LOCK_MEM_PAGE_RANGES hypercall that enables a guest to
> > > set EPT permissions on a set of page ranges.
> > 
> > IMO, manipulation of protections, both for memory (this patch) and CPU state
> > (control registers in the next patch) should come from userspace.  I have no
> > objection to KVM providing plumbing if necessary, but I think userspace needs to
> > to have full control over the actual state.
> 
> By user space, do you mean the host user space or the guest user space?

Host userspace, a.k.a. the VMM.  Definitely not guest userspace.

> About the guest user space, I see several issues to delegate this kind of
> control:
> - These are restrictions only relevant to the kernel.
> - The threat model is to protect against user space as early as possible.
> - It would be more complex for no obvious gain.
> 
> This patch series is an extension of the kernel self-protections mechanisms,
> and they are not configured by user space.
> 
> 
> > 
> > One of the things that caused Intel's control register pinning series to stall
> > out was how to handle edge cases like kexec() and reboot.  Deferring to userspace
> > means the kernel doesn't need to define policy, e.g. when to unprotect memory,
> > and avoids questions like "should userspace be able to overwrite pinned control
> > registers".
> 
> The idea is to authenticate every changes. For kexec, the VMM (or something
> else) would have to authenticate the new kernel. Do you have something else
> in mind that could legitimately require such memory or CR changes?

I think we're on the same page, the VMM (host userspace) would need to ack any
changes.

FWIW, SMM is another wart as entry to SMM clobbers CRs.  Now that CONFIG_KVM_SMM
is a thing, the easiest solution would be to disallow coexistence with SMM, though
that might not be an option for many use cases (IIUC, QEMU-based deployments use
SMM to implement secure boot).

> > And like the confidential VM use case, keeping userspace in the loop is a big
> > beneifit, e.g. the guest can't circumvent protections by coercing userspace into
> > writing to protected memory .
> 
> I don't understand this part. Are you talking about the host user space? How
> the guest could circumvent protections?

Host userspace.  Guest configures a device buffer in write-protected memory, gets
a host (synthetic) device to write into the memory.