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

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Message-ID: <aIlzeT+yFG2Tvb3/@intel.com>
Date: Wed, 30 Jul 2025 09:20:57 +0800
From: Chao Gao <chao.gao@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>,
	Paolo Bonzini <pbonzini@...hat.com>, <linux-arm-kernel@...ts.infradead.org>,
	<kvmarm@...ts.linux.dev>, <kvm@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, Adrian Hunter <adrian.hunter@...el.com>,
	Vishal Annapurve <vannapurve@...gle.com>, Xiaoyao Li <xiaoyao.li@...el.com>,
	Rick Edgecombe <rick.p.edgecombe@...el.com>, Nikolay Borisov
	<nik.borisov@...e.com>
Subject: Re: [PATCH 3/5] KVM: Reject ioctls only if the VM is bugged, not
 simply marked dead

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).