KVM: Fix KVM-owned file refcounting of KVM module(s)

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

Posted by Sean Christopherson 2 years, 1 month ago

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).

Then after applying that "fix", we/I made an even bigger goof by relying on
the nonexistent "protection" provided by the VM's reference and removed the
code which guaranteed that the KVM module would be pinned until *after* the
last reference to a KVM-owned file was put.

Undo the mess we created and fix the original async #PF workqueue bug.

Sean Christopherson (3):
  KVM: Set file_operations.owner appropriately for all such structures
  KVM: Always flush async #PF workqueue when vCPU is being destroyed
  Revert "KVM: Prevent module exit until all VMs are freed"

 arch/x86/kvm/debugfs.c |  1 +
 virt/kvm/async_pf.c    | 15 ++++++++++++---
 virt/kvm/kvm_main.c    | 18 ++++++++----------
 3 files changed, 21 insertions(+), 13 deletions(-)


base-commit: 437bba5ad2bba00c2056c896753a32edf80860cc
-- 
2.42.0.655.g421f12c284-goog

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

Posted by Sean Christopherson 2 years ago

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

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