Re: [PATCH 2/3] KVM: Always flush async #PF workqueue when vCPU is being destroyed

[David Matlack <dmatlack@google.com>]

On 2023-10-18 01:46 PM, Sean Christopherson wrote:
> Always flush the per-vCPU async #PF workqueue when a vCPU is clearing its
> completion queue, i.e. when a VM and all its vCPUs is being destroyed.

nit:

  ... or when the guest toggles CR0.PG or async_pf support.

> KVM must ensure that none of its workqueue callbacks is running when the
> last reference to the KVM _module_ is put.  Gifting a reference to the
> associated VM prevents the workqueue callback from dereferencing freed
> vCPU/VM memory, but does not prevent the KVM module from being unloaded
> before the callback completes.
> 
> Drop the misguided VM refcount gifting, as calling kvm_put_kvm() from
> async_pf_execute() if kvm_put_kvm() flushes the async #PF workqueue will
> result in deadlock.  async_pf_execute() can't return until kvm_put_kvm()
> finishes, and kvm_put_kvm() can't return until async_pf_execute() finishes:
>
[...]
> 
> Note, commit 5f6de5cbebee ("KVM: Prevent module exit until all VMs are
> freed") *tried* to fix the module refcounting issue by having VMs grab a
> reference to the module, but that only made the bug slightly harder to hit
> as it gave async_pf_execute() a bit more time to complete before the KVM
> module could be unloaded.

Blegh! Thanks for the fix.

> 
> Fixes: af585b921e5d ("KVM: Halt vcpu if page it tries to access is swapped out")
> Cc: stable@vger.kernel.org
> Cc: David Matlack <dmatlack@google.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: David Matlack <dmatlack@google.com>