Patch "KVM: Always flush async #PF workqueue when vCPU is being destroyed" has been added to the 6.6-stable tree

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This is a note to let you know that I've just added the patch titled

    KVM: Always flush async #PF workqueue when vCPU is being destroyed

to the 6.6-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     kvm-always-flush-async-pf-workqueue-when-vcpu-is-bei.patch
and it can be found in the queue-6.6 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 081dc95179b16dec6ecd71a09b68f12727cc5f23
Author: Sean Christopherson <seanjc@xxxxxxxxxx>
Date:   Tue Jan 9 17:15:30 2024 -0800

    KVM: Always flush async #PF workqueue when vCPU is being destroyed
    
    [ Upstream commit 3d75b8aa5c29058a512db29da7cbee8052724157 ]
    
    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
    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@xxxxxxxxxxxxxxx
    Cc: David Matlack <dmatlack@xxxxxxxxxx>
    Reviewed-by: Xu Yilun <yilun.xu@xxxxxxxxx>
    Reviewed-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
    Link: https://lore.kernel.org/r/20240110011533.503302-2-seanjc@xxxxxxxxxx
    Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/virt/kvm/async_pf.c b/virt/kvm/async_pf.c
index e033c79d528e0..28658b9e0d968 100644
--- a/virt/kvm/async_pf.c
+++ b/virt/kvm/async_pf.c
@@ -87,7 +87,27 @@ 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().  Note, flushing the work item
+	 * is always required when the item is taken off the completion queue.
+	 * E.g. even if the vCPU handles the item in the "normal" path, the VM
+	 * could be terminated before async_pf_execute() completes.
+	 *
+	 * 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 +134,6 @@ 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 +145,10 @@ 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);
+		kvm_flush_and_free_async_pf_work(work);
+		spin_lock(&vcpu->async_pf.lock);
 	}
 	spin_unlock(&vcpu->async_pf.lock);
 
@@ -151,7 +173,7 @@ 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 +208,6 @@ 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);
 




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