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

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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;



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