linux-kernel - Re: [PATCH 0/3] KVM: Fix KVM-owned file refcounting of KVM module(s)

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Message-ID: <170144720127.838654.18038243366582421190.b4-ty@google.com>
Date:   Fri,  1 Dec 2023 15:30:35 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Al Viro <viro@...iv.linux.org.uk>,
        David Matlack <dmatlack@...gle.com>
Subject: Re: [PATCH 0/3] KVM: Fix KVM-owned file refcounting of KVM module(s)

On Wed, 18 Oct 2023 13:46:21 -0700, Sean Christopherson wrote:
> Clean up a KVM module refcounting mess that Al pointed out in the context
> of the guest_memfd series.  The worst behavior was recently introduced by
> an ill-fated attempt to fix a bug in x86's async #PF code.  Instead of
> fixing the underlying bug of not flushing a workqueue (see patch 2), KVM
> fudged around the bug by gifting every VM a reference to the KVM module.
> 
> That made the reproducer happy (hopefully there was actually a reproducer
> at one point), but it didn't fully fix the use-after-free bug, it just made
> the bug harder to hit.  E.g. as pointed out by Al, if kvm_destroy_vm() is
> preempted after putting the last KVM module reference, KVM can be unloaded
> before kvm_destroy_vm() completes, and scheduling back in the associated
> task will explode (preemption isn't strictly required, it's just the most
> obvious path to failure).
> 
> [...]

Applied 1 and 3 (the .owner fixes) to kvm-x86 fixes.  I'll follow-up with a
separate series to tackle the async #PF mess.

[1/3] KVM: Set file_operations.owner appropriately for all such structures
      https://github.com/kvm-x86/linux/commit/087e15206d6a
[2/3] KVM: Always flush async #PF workqueue when vCPU is being destroyed
      (no commit info)
[3/3] Revert "KVM: Prevent module exit until all VMs are freed"
      https://github.com/kvm-x86/linux/commit/ea61294befd3

--
https://github.com/kvm-x86/linux/tree/next