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

[Xu Yilun]

On Wed, Oct 18, 2023 at 01:46:23PM -0700, 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.
> 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:
> 
>  WARNING: CPU: 8 PID: 251 at virt/kvm/kvm_main.c:1435 kvm_put_kvm+0x2d/0x320 [kvm]
>  Modules linked in: vhost_net vhost vhost_iotlb tap kvm_intel kvm irqbypass
>  CPU: 8 PID: 251 Comm: kworker/8:1 Tainted: G        W          6.6.0-rc1-e7af8d17224a-x86/gmem-vm #119
>  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
>  Workqueue: events async_pf_execute [kvm]
>  RIP: 0010:kvm_put_kvm+0x2d/0x320 [kvm]
>  Call Trace:
>   <TASK>
>   async_pf_execute+0x198/0x260 [kvm]
>   process_one_work+0x145/0x2d0
>   worker_thread+0x27e/0x3a0
>   kthread+0xba/0xe0
>   ret_from_fork+0x2d/0x50
>   ret_from_fork_asm+0x11/0x20
>   </TASK>
>  ---[ end trace 0000000000000000 ]---
>  INFO: task kworker/8:1:251 blocked for more than 120 seconds.
>        Tainted: G        W          6.6.0-rc1-e7af8d17224a-x86/gmem-vm #119
>  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>  task:kworker/8:1     state:D stack:0     pid:251   ppid:2      flags:0x00004000
>  Workqueue: events async_pf_execute [kvm]
>  Call Trace:
>   <TASK>
>   __schedule+0x33f/0xa40
>   schedule+0x53/0xc0
>   schedule_timeout+0x12a/0x140
>   __wait_for_common+0x8d/0x1d0
>   __flush_work.isra.0+0x19f/0x2c0
>   kvm_clear_async_pf_completion_queue+0x129/0x190 [kvm]
>   kvm_arch_destroy_vm+0x78/0x1b0 [kvm]
>   kvm_put_kvm+0x1c1/0x320 [kvm]
>   async_pf_execute+0x198/0x260 [kvm]
>   process_one_work+0x145/0x2d0
>   worker_thread+0x27e/0x3a0
>   kthread+0xba/0xe0
>   ret_from_fork+0x2d/0x50
>   ret_from_fork_asm+0x11/0x20
>   </TASK>
> 
> If kvm_clear_async_pf_completion_queue() actually flushes the workqueue,
> then there's no need to gift async_pf_execute() a reference because all
> invocations of async_pf_execute() will be forced to complete before the
> vCPU and its VM are destroyed/freed.  And that in turn fixes the module
> unloading bug as __fput() won't do module_put() on the last vCPU reference
> until the vCPU has been freed, e.g. if closing the vCPU file also puts the
> last reference to the KVM module.
> 
> 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.
> 
> Fixes: af585b921e5d ("KVM: Halt vcpu if page it tries to access is swapped out")
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: David Matlack <dmatlack@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  virt/kvm/async_pf.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/virt/kvm/async_pf.c b/virt/kvm/async_pf.c
> index e033c79d528e..7aeb9d1f43b1 100644
> --- a/virt/kvm/async_pf.c
> +++ b/virt/kvm/async_pf.c
> @@ -87,7 +87,6 @@ static void async_pf_execute(struct work_struct *work)
>  	__kvm_vcpu_wake_up(vcpu);
>  
>  	mmput(mm);
> -	kvm_put_kvm(vcpu->kvm);
>  }
>  
>  void kvm_clear_async_pf_completion_queue(struct kvm_vcpu *vcpu)
> @@ -114,7 +113,6 @@ void kvm_clear_async_pf_completion_queue(struct kvm_vcpu *vcpu)
>  #else
>  		if (cancel_work_sync(&work->work)) {
>  			mmput(work->mm);
> -			kvm_put_kvm(vcpu->kvm); /* == work->vcpu->kvm */
>  			kmem_cache_free(async_pf_cache, work);
>  		}
>  #endif
> @@ -126,7 +124,19 @@ void kvm_clear_async_pf_completion_queue(struct kvm_vcpu *vcpu)
>  			list_first_entry(&vcpu->async_pf.done,
>  					 typeof(*work), link);
>  		list_del(&work->link);
> +
> +		spin_unlock(&vcpu->async_pf.lock);
> +
> +		/*
> +		 * The async #PF is "done", but KVM must wait for the work item
> +		 * itself, i.e. async_pf_execute(), to run to completion.  If
> +		 * KVM is a module, KVM must ensure *no* code owned by the KVM
> +		 * (the module) can be run after the last call to module_put(),
> +		 * i.e. after the last reference to the last vCPU's file is put.
> +		 */
> +		flush_work(&work->work);

I see the flush_work() is inside the check:

  while (!list_empty(&vcpu->async_pf.done))

Is it possible all async_pf are already completed but the work item,
i.e. async_pf_execute, is not completed before this check? That the
work is scheduled out after kvm_arch_async_page_present_queued() and
all APF_READY requests have been handled. In this case the work
synchronization will be skipped...

Thanks,
Yilun

>  		kmem_cache_free(async_pf_cache, work);
> +		spin_lock(&vcpu->async_pf.lock);
>  	}
>  	spin_unlock(&vcpu->async_pf.lock);
>  
> @@ -186,7 +196,6 @@ bool kvm_setup_async_pf(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>  	work->arch = *arch;
>  	work->mm = current->mm;
>  	mmget(work->mm);
> -	kvm_get_kvm(work->vcpu->kvm);
>  
>  	INIT_WORK(&work->work, async_pf_execute);
>  
> -- 
> 2.42.0.655.g421f12c284-goog
>