On Wed, Mar 02, 2022, Paolo Bonzini wrote:
> Right now, the kvm module is kept alive by VFS via fops_get/fops_put, but there
> may be cases in which a kvm_get_kvm's matching kvm_put_kvm happens after
> the file descriptor is closed. One case that will be introduced soon is
> when work is delegated to the system work queue; the worker might be
> a bit late and the file descriptor can be closed in the meantime. Ensure
> that the module has not gone away by tying a module reference explicitly
> to the lifetime of the struct kvm.
>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
> virt/kvm/kvm_main.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 64eb99444688..e3f37fc2ebf1 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1131,6 +1131,9 @@ static struct kvm *kvm_create_vm(unsigned long type)
> preempt_notifier_inc();
> kvm_init_pm_notifier(kvm);
>
> + /* This is safe, since we have a reference from open(). */
> + __module_get(THIS_MODULE);
This isn't sufficient. For x86, it only grabs a reference to kvm.ko, not the
vendor module. Instead, we can do:
if (!try_module_get(kvm_chardev_ops.owner))
return ERR_PTR(-EINVAL);
And then on top, revert commit revert ("KVM: set owner of cpu and vm file operations").
vCPUs file descriptors hold reference to the VM, which means they indirectly hold a
reference to the module. So once the "real" bug of struct kvm not holding a reference
to the module is fixed, grabbing a reference when a VM/vCPU inode is opened becomes
unnecessary.
