Re: [RFC PATCH v3 3/5] KVM: x86: Add notifications for Heki policy configuration and violation

From: Sean Christopherson
Date: Fri May 03 2024 - 10:03:33 EST


On Fri, May 03, 2024, Mickaël Salaün wrote:
> Add an interface for user space to be notified about guests' Heki policy
> and related violations.
>
> Extend the KVM_ENABLE_CAP IOCTL with KVM_CAP_HEKI_CONFIGURE and
> KVM_CAP_HEKI_DENIAL. Each one takes a bitmask as first argument that can
> contains KVM_HEKI_EXIT_REASON_CR0 and KVM_HEKI_EXIT_REASON_CR4. The
> returned value is the bitmask of known Heki exit reasons, for now:
> KVM_HEKI_EXIT_REASON_CR0 and KVM_HEKI_EXIT_REASON_CR4.
>
> If KVM_CAP_HEKI_CONFIGURE is set, a VM exit will be triggered for each
> KVM_HC_LOCK_CR_UPDATE hypercalls according to the requested control
> register. This enables to enlighten the VMM with the guest
> auto-restrictions.
>
> If KVM_CAP_HEKI_DENIAL is set, a VM exit will be triggered for each
> pinned CR violation. This enables the VMM to react to a policy
> violation.
>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Madhavan T. Venkataraman <madvenka@xxxxxxxxxxxxxxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Sean Christopherson <seanjc@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> Cc: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxx>
> Link: https://lore.kernel.org/r/20240503131910.307630-4-mic@xxxxxxxxxxx
> ---
>
> Changes since v1:
> * New patch. Making user space aware of Heki properties was requested by
> Sean Christopherson.

No, I suggested having userspace _control_ the pinning[*], not merely be notified
of pinning.

: 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.
:
: 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".
:
: 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 stand by that suggestion, because I don't see a sane way to handle things like
kexec() and reboot without having a _much_ more sophisticated policy than would
ever be acceptable in KVM.

I think that can be done without KVM having any awareness of CR pinning whatsoever.
E.g. userspace just needs to ability to intercept CR writes and inject #GPs Off
the cuff, I suspect the uAPI could look very similar to MSR filtering. E.g I bet
userspace could enforce MSR pinning without any new KVM uAPI at all.

[*] https://lore.kernel.org/all/ZFUyhPuhtMbYdJ76@xxxxxxxxxx