Re: [PATCH v2] s390/vfio-ap: fix memory leak in mdev remove callback

[Jason Gunthorpe]

On Wed, May 19, 2021 at 01:22:56PM +0200, Christian Borntraeger wrote:
> 
> 
> On 19.05.21 10:17, Christian Borntraeger wrote:
> > 
> > 
> > On 19.05.21 01:27, Halil Pasic wrote:
> > > On Tue, 18 May 2021 19:01:42 +0200
> > > Christian Borntraeger <borntraeger@xxxxxxxxxx> wrote:
> > > 
> > > > On 18.05.21 17:33, Halil Pasic wrote:
> > > > > On Tue, 18 May 2021 15:59:36 +0200
> > > > > Christian Borntraeger <borntraeger@xxxxxxxxxx> wrote:
> > > [..]
> > > > > > > > 
> > > > > > > > Would it help, if the code in priv.c would read the hook once
> > > > > > > > and then only work on the copy? We could protect that with rcu
> > > > > > > > and do a synchronize rcu in vfio_ap_mdev_unset_kvm after
> > > > > > > > unsetting the pointer?
> > > > > 
> > > > > Unfortunately just "the hook" is ambiguous in this context. We
> > > > > have kvm->arch.crypto.pqap_hook that is supposed to point to
> > > > > a struct kvm_s390_module_hook member of struct ap_matrix_mdev
> > > > > which is also called pqap_hook. And struct kvm_s390_module_hook
> > > > > has function pointer member named "hook".
> > > > 
> > > > I was referring to the full struct.
> > > > > > > 
> > > > > > > I'll look into this.
> > > > > > 
> > > > > > I think it could work. in priv.c use rcu_readlock, save the
> > > > > > pointer, do the check and call, call rcu_read_unlock.
> > > > > > In vfio_ap use rcu_assign_pointer to set the pointer and
> > > > > > after setting it to zero call sychronize_rcu.
> > > > > 
> > > > > In my opinion, we should make the accesses to the
> > > > > kvm->arch.crypto.pqap_hook pointer properly synchronized. I'm
> > > > > not sure if that is what you are proposing. How do we usually
> > > > > do synchronisation on the stuff that lives in kvm->arch?
> > > > 
> > > > RCU is a method of synchronization. We  make sure that structure
> > > > pqap_hook is still valid as long as we are inside the rcu read
> > > > lock. So the idea is: clear pointer, wait until all old readers
> > > > have finished and the proceed with getting rid of the structure.
> > > 
> > > Yes I know that RCU is a method of synchronization, but I'm not
> > > very familiar with it. I'm a little confused by "read the hook
> > > once and then work on a copy". I guess, I would have to read up
> > > on the RCU again to get clarity. I intend to brush up my RCU knowledge
> > > once the patch comes along. I would be glad to have your help when
> > > reviewing an RCU based solution for this.
> > 
> > Just had a quick look. Its not trivial, as the hook function itself
> > takes a mutex and an rcu section must not sleep. Will have a deeper
> > look.
> 
> 
> As a quick hack something like this could work. The whole locking is pretty
> complicated and this makes it even more complex so we might want to do
> a cleanup/locking rework later on.
> 
> 
> index 9928f785c677..fde6e02aab54 100644
> +++ b/arch/s390/kvm/priv.c
> @@ -609,6 +609,7 @@ static int handle_io_inst(struct kvm_vcpu *vcpu)
>   */
>  static int handle_pqap(struct kvm_vcpu *vcpu)
>  {
> +       struct kvm_s390_module_hook *pqap_hook;
>         struct ap_queue_status status = {};
>         unsigned long reg0;
>         int ret;
> @@ -657,14 +658,21 @@ static int handle_pqap(struct kvm_vcpu *vcpu)
>          * Verify that the hook callback is registered, lock the owner
>          * and call the hook.
>          */
> -       if (vcpu->kvm->arch.crypto.pqap_hook) {
> -               if (!try_module_get(vcpu->kvm->arch.crypto.pqap_hook->owner))
> +       rcu_read_lock();
> +       pqap_hook = rcu_dereference(vcpu->kvm->arch.crypto.pqap_hook);
> +       if (pqap_hook) {
> +               if (!try_module_get(pqap_hook->owner)) {

module locking doesn't prevent driver unbinding

> +                       rcu_read_unlock();
>                         return -EOPNOTSUPP;
> -               ret = vcpu->kvm->arch.crypto.pqap_hook->hook(vcpu);
> -               module_put(vcpu->kvm->arch.crypto.pqap_hook->owner);
> +               }
> +               rcu_read_unlock();
> +               ret = pqap_hook->hook(vcpu);

So taking the pointer out of the rcu still isn't protected.

Unless this is super performance critical just use a rw sem

Jason