Re: [PATCH 3/5] KVM: Reject ioctls only if the VM is bugged, not simply marked dead

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

On Tue, Jul 29, 2025 at 12:33:38PM -0700, Sean Christopherson wrote:
>Relax the protection against interacting with a buggy KVM to only reject
>ioctls if the VM is bugged, i.e. allow userspace to invoke ioctls if KVM
>deliberately terminated the VM.  Drop kvm.vm_dead as there are no longer
>any readers, and KVM shouldn't rely on vm_dead for functional correctness.
>The only functional guarantees provided by kvm_vm_dead() come by way of
>KVM_REQ_VM_DEAD, which ensures that vCPU won't re-enter the guest.

If ioctls are allowed for dead VMs, would it be possible for userspace to
create a new vCPU and attempt to enter a dead VM? is this something KVM
should prevent?

>
>Practically speaking, this only affects x86, which uses kvm_vm_dead() to
>prevent running a VM whose resources have been partially freed or has run
>one or more of its vCPUs into an architecturally defined state.  In these
						  ^^^ undefined?

>cases, there is no (known) danger to KVM, the goal is purely to prevent
>entering the guest.
>
>As evidenced by commit ecf371f8b02d ("KVM: SVM: Reject SEV{-ES} intra host
>migration if vCPU creation is in-flight"), the restriction on invoking
>ioctls only blocks _new_ ioctls.  I.e. KVM mustn't rely on blocking ioctls
>for functional safety (whereas KVM_REQ_VM_DEAD is guaranteed to prevent
>vCPUs from entering the guest).