Re: [PATCH RESEND 1/2] KVM: Prevent module exit until all VMs are freed

[Sean Christopherson <seanjc@google.com>]

On Thu, Mar 03, 2022, David Matlack wrote:
> Tie the lifetime the KVM module to the lifetime of each VM via
> kvm.users_count. This way anything that grabs a reference to the VM via
> kvm_get_kvm() cannot accidentally outlive the KVM module.
> 
> Prior to this commit, the lifetime of the KVM module was tied to the
> lifetime of /dev/kvm file descriptors, VM file descriptors, and vCPU
> file descriptors by their respective file_operations "owner" field.
> This approach is insufficient because references grabbed via
> kvm_get_kvm() do not prevent closing any of the aforementioned file
> descriptors.
> 
> This fixes a long standing theoretical bug in KVM that at least affects
> async page faults. kvm_setup_async_pf() grabs a reference via
> kvm_get_kvm(), and drops it in an asynchronous work callback. Nothing
> prevents the VM file descriptor from being closed and the KVM module
> from being unloaded before this callback runs.
> 
> Fixes: af585b921e5d ("KVM: Halt vcpu if page it tries to access is swapped out")

And (or)

  Fixes: 3d3aab1b973b ("KVM: set owner of cpu and vm file operations")

because the above is x86-centric, at a glance PPC and maybe s390 have issues
beyond async #PF.

> Cc: stable@vger.kernel.org
> Suggested-by: Ben Gardon <bgardon@google.com>
> [ Based on a patch from Ben implemented for Google's kernel. ]
> Signed-off-by: David Matlack <dmatlack@google.com>
> ---
>  virt/kvm/kvm_main.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 35ae6d32dae5..b59f0a29dbd5 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -117,6 +117,8 @@ EXPORT_SYMBOL_GPL(kvm_debugfs_dir);
>  
>  static const struct file_operations stat_fops_per_vm;
>  
> +static struct file_operations kvm_chardev_ops;
> +
>  static long kvm_vcpu_ioctl(struct file *file, unsigned int ioctl,
>  			   unsigned long arg);
>  #ifdef CONFIG_KVM_COMPAT
> @@ -1131,6 +1133,11 @@ static struct kvm *kvm_create_vm(unsigned long type)
>  	preempt_notifier_inc();
>  	kvm_init_pm_notifier(kvm);
>  
> +	if (!try_module_get(kvm_chardev_ops.owner)) {

The "try" aspect is unnecessary.  Stealing from Paolo's version, 

	/* KVM is pinned via open("/dev/kvm"), the fd passed to this ioctl(). */
	__module_get(kvm_chardev_ops.owner);

> +		r = -ENODEV;
> +		goto out_err;
> +	}
> +
>  	return kvm;
>  
>  out_err:
> @@ -1220,6 +1227,7 @@ static void kvm_destroy_vm(struct kvm *kvm)
>  	preempt_notifier_dec();
>  	hardware_disable_all();
>  	mmdrop(mm);
> +	module_put(kvm_chardev_ops.owner);
>  }
>  
>  void kvm_get_kvm(struct kvm *kvm)
> 
> base-commit: b13a3befc815eae574d87e6249f973dfbb6ad6cd
> prerequisite-patch-id: 38f66d60319bf0bc9bf49f91f0f9119e5441629b
> prerequisite-patch-id: 51aa921d68ea649d436ea68e1b8f4aabc3805156
> -- 
> 2.35.1.616.g0bdcbb4464-goog
>