Re: [PATCH 1/4] KVM: Always flush async #PF workqueue when vCPU is being destroyed

Xu Yilun
2024-01-20T12:44:08Z

On Tue, Jan 09, 2024 at 05:15:30PM -0800, Sean Christopherson wrote:
> Always flush the per-vCPU async #PF workqueue when a vCPU is clearing its
> completion queue, e.g. 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

I'm not sure why __fput() of vCPU fd should be mentioned here. I assume
we just need to say that vCPUs are freed before module_put(KVM the module)
in kvm_destroy_vm(), then the whole logic for module unloading fix is:

1. All workqueue callbacks complete when kvm_clear_async_pf_completion_queue(vcpu)
2. kvm_clear_async_pf_completion_queue(vcpu) must be executed before vCPU free.
3. vCPUs must be freed before module_put(KVM the module).

So all workqueue callbacks complete before module_put(KVM the module).

__fput() of vCPU fd is not the only trigger of kvm_destroy_vm(), that
makes me distracted from reason of the fix.

> last reference to the KVM module.
>
> Note that kvm_check_async_pf_completion() may also take the work item off
> the completion queue and so also needs to flush the work queue, as the
> work will not be seen by kvm_clear_async_pf_completion_queue(). Waiting
> on the workqueue could theoretically delay a vCPU due to waiting for the
> work to complete, but that's a very, very small chance, and likely a very
> small delay. kvm_arch_async_page_present_queued() unconditionally makes a
> new request, i.e. will effectively delay entering the guest, so the
> remaining work is really just:
>
> trace_kvm_async_pf_completed(addr, cr2_or_gpa);
>
> __kvm_vcpu_wake_up(vcpu);
>
> mmput(mm);
>
> and mmput() can't drop the last reference to the page tables if the vCPU is
> still alive, i.e. the vCPU won't get stuck tearing down page tables.
>
> Add a helper to do the flushing, specifically to deal with "wakeup all"
> work items, as they aren't actually work items, i.e. are never placed in a
> workqueue. Trying to flush a bogus workqueue entry rightly makes
> __flush_work() complain (kudos to whoever added that sanity check).
>
> 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@vger.kernel.org
> Cc: David Matlack <dmatlack@google.com>
> Cc: Xu Yilun <yilun.xu@linux.intel.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> virt/kvm/async_pf.c | 37 ++++++++++++++++++++++++++++++++-----
> 1 file changed, 32 insertions(+), 5 deletions(-)
>
> diff --git a/virt/kvm/async_pf.c b/virt/kvm/async_pf.c
> index e033c79d528e..876927a558ad 100644
> --- a/virt/kvm/async_pf.c
> +++ b/virt/kvm/async_pf.c
> @@ -87,7 +87,25 @@ static void async_pf_execute(struct work_struct *work)
> __kvm_vcpu_wake_up(vcpu);
>
> mmput(mm);
> - kvm_put_kvm(vcpu->kvm);
> +}
> +
> +static void kvm_flush_and_free_async_pf_work(struct kvm_async_pf *work)
> +{
> + /*
> + * 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.

Maybe drop the i.e? It is not exactly true, other components like VFIO
may also be the last one to put KVM reference?

> + *
> + * Wake all events skip the queue and go straight done, i.e. don't
> + * need to be flushed (but sanity check that the work wasn't queued).
> + */
> + if (work->wakeup_all)
> + WARN_ON_ONCE(work->work.func);
> + else
> + flush_work(&work->work);
> + kmem_cache_free(async_pf_cache, work);
> }
>
> void kvm_clear_async_pf_completion_queue(struct kvm_vcpu *vcpu)
> @@ -114,7 +132,6 @@ 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 +143,18 @@ 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);
> - kmem_cache_free(async_pf_cache, work);
> +
> + 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.
> + */
> + kvm_flush_and_free_async_pf_work(work);
> + spin_lock(&vcpu->async_pf.lock);
> }
> spin_unlock(&vcpu->async_pf.lock);
>
> @@ -151,7 +179,7 @@ void kvm_check_async_pf_completion(struct kvm_vcpu *vcpu)
>
> list_del(&work->queue);
> vcpu->async_pf.queued--;
> - kmem_cache_free(async_pf_cache, work);
> + kvm_flush_and_free_async_pf_work(work);
> }
> }
>
> @@ -186,7 +214,6 @@ 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);

Overall LGTM, Reviewed-by: Xu Yilun <yilun.xu@intel.com>

>
> --
> 2.43.0.472.g3155946c3a-goog
>