* [PATCH] KVM: allow struct kvm to outlive the file descriptors
@ 2022-03-02 17:43 Paolo Bonzini
2022-03-02 17:47 ` Sean Christopherson
0 siblings, 1 reply; 3+ messages in thread
From: Paolo Bonzini @ 2022-03-02 17:43 UTC (permalink / raw)
To: linux-kernel, kvm; +Cc: dmatlack, seanjc
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@redhat.com>
---
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);
+
return kvm;
out_err:
@@ -1220,6 +1223,7 @@ static void kvm_destroy_vm(struct kvm *kvm)
preempt_notifier_dec();
hardware_disable_all();
mmdrop(mm);
+ module_put(THIS_MODULE);
}
void kvm_get_kvm(struct kvm *kvm)
--
2.31.1
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] KVM: allow struct kvm to outlive the file descriptors
2022-03-02 17:43 [PATCH] KVM: allow struct kvm to outlive the file descriptors Paolo Bonzini
@ 2022-03-02 17:47 ` Sean Christopherson
2022-03-02 17:48 ` Paolo Bonzini
0 siblings, 1 reply; 3+ messages in thread
From: Sean Christopherson @ 2022-03-02 17:47 UTC (permalink / raw)
To: Paolo Bonzini; +Cc: linux-kernel, kvm, dmatlack
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@redhat.com>
> ---
> 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.
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] KVM: allow struct kvm to outlive the file descriptors
2022-03-02 17:47 ` Sean Christopherson
@ 2022-03-02 17:48 ` Paolo Bonzini
0 siblings, 0 replies; 3+ messages in thread
From: Paolo Bonzini @ 2022-03-02 17:48 UTC (permalink / raw)
To: Sean Christopherson; +Cc: linux-kernel, kvm, dmatlack
On 3/2/22 18:47, Sean Christopherson wrote:
>> + /* 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.
>
Got it, I'll wait for David to post the right thing. :)
Paolo
^ permalink raw reply [flat|nested] 3+ messages in thread
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