linux-kernel - Re: [PATCH v3 22/28] KVM: x86/mmu: Zap defunct roots via asynchronous worker

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Date:   Wed, 2 Mar 2022 17:35:35 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        David Hildenbrand <david@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, David Matlack <dmatlack@...gle.com>,
        Ben Gardon <bgardon@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>
Subject: Re: [PATCH v3 22/28] KVM: x86/mmu: Zap defunct roots via
 asynchronous worker

On Wed, Mar 02, 2022, Paolo Bonzini wrote:
> However, I think we now need a module_get/module_put when creating/destroying
> a VM; the workers can outlive kvm_vm_release and therefore any reference
> automatically taken by VFS's fops_get/fops_put.

Haven't read the rest of the patch, but this caught my eye.  We _already_ need
to handle this scenario.  As you noted, any worker, i.e. anything that takes a
reference via kvm_get_kvm() without any additional guarantee that the module can't
be unloaded is suspect. x86 is mostly fine, though kvm_setup_async_pf() is likely
affected, and other architectures seem to have bugs.

Google has an internal patch that addresses this.  I believe David is going to post
the fix... David?