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

[Christian Borntraeger]

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
--- a/arch/s390/kvm/priv.c
+++ 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)) {
+                       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);
+               module_put(pqap_hook->owner);
                if (!ret && vcpu->run->s.regs.gprs[1] & 0x00ff0000)
                        kvm_s390_set_psw_cc(vcpu, 3);
                return ret;
+       } else {
+               rcu_read_unlock();
        }
        /*
         * A vfio_driver must register a hook.
diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
index f90c9103dac2..a7124abd6aed 100644
--- a/drivers/s390/crypto/vfio_ap_ops.c
+++ b/drivers/s390/crypto/vfio_ap_ops.c
@@ -1194,6 +1194,7 @@ static void vfio_ap_mdev_unset_kvm(struct ap_matrix_mdev *matrix_mdev)
                mutex_lock(&matrix_dev->lock);
                vfio_ap_mdev_reset_queues(matrix_mdev->mdev);
                matrix_mdev->kvm->arch.crypto.pqap_hook = NULL;
+               synchronize_rcu();
                kvm_put_kvm(matrix_mdev->kvm);
                matrix_mdev->kvm = NULL;
                matrix_mdev->kvm_busy = false;