Re: [PATCH 5/5] KVM: TDX: Add sub-ioctl KVM_TDX_TERMINATE_VM

[Chao Gao <chao.gao@intel.com>]

>Oof.  And as Chao pointed out[*], removing the vm_dead check would allow creating
>and running vCPUs in a dead VM, which is most definitely not desirable.  Squashing
>the vCPU creation case is easy enough if we keep vm_dead but still generally allow
>ioctls, and it's probably worth doing that no matter what (to plug the hole where
>pending vCPU creations could succeed):
>
>diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>index d477a7fda0ae..941d2c32b7dc 100644
>--- a/virt/kvm/kvm_main.c
>+++ b/virt/kvm/kvm_main.c
>@@ -4207,6 +4207,11 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
> 
>        mutex_lock(&kvm->lock);
> 
>+       if (kvm->vm_dead) {
>+               r = -EIO;
>+               goto unlock_vcpu_destroy;
>+       }
>+

yes. this addresses my concern.

>        if (kvm_get_vcpu_by_id(kvm, id)) {
>                r = -EEXIST;
>                goto unlock_vcpu_destroy;
>
>And then to ensure vCPUs can't do anything, check KVM_REQ_VM_DEAD after acquiring
>vcpu->mutex.
>
>diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>index 6c07dd423458..883077eee4ce 100644
>--- a/virt/kvm/kvm_main.c
>+++ b/virt/kvm/kvm_main.c
>@@ -4433,6 +4433,12 @@ static long kvm_vcpu_ioctl(struct file *filp,
> 
>        if (mutex_lock_killable(&vcpu->mutex))
>                return -EINTR;
>+
>+       if (kvm_test_request(KVM_REQ_VM_DEAD, vcpu)) {
>+               r = -EIO;
>+               goto out;
>+       }
>+
>        switch (ioctl) {
>        case KVM_RUN: {
>                struct pid *oldpid;
>
>
>That should address all TDVPS paths (I hope), and I _think_ would address all
>MMU-related paths as well?  E.g. prefault requires a vCPU.
>
>Disallowing (most) vCPU ioctls but not all VM ioctls on vm_dead isn't great ABI
>(understatement), but I think we need/want the above changes even if we keep the
>general vm_dead restriction.  And given the extremely ad hoc behavior of taking
>kvm->lock for VM ioctls, trying to enforce vm_dead for "all" VM ioctls seems like
>a fool's errand.
>
>So I'm leaning toward keeping "KVM: Reject ioctls only if the VM is bugged, not
>simply marked dead" (with a different shortlog+changelog), but keeping vm_dead
>(and not introducing kvm_tdx.vm_terminated).

Sounds good to me.

With kvm_tdx.vm_terminated removed, we should consider adding a comment above
the is_hkid_assigned() check in tdx_sept_remove_private_spte() to clarify that
!is_hkid_assigned() indicates the guest has been terminated, allowing private
pages to be reclaimed directly without zapping.