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

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Date:   Wed, 19 May 2021 13:22:56 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Halil Pasic <pasic@...ux.ibm.com>
Cc:     Tony Krowiak <akrowiak@...ux.ibm.com>, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, cohuck@...hat.com,
        pasic@...ux.vnet.ibm.com, jjherne@...ux.ibm.com, jgg@...dia.com,
        alex.williamson@...hat.com, kwankhede@...dia.com,
        stable@...r.kernel.org, Tony Krowiak <akrowiak@...y.rr.com>
Subject: Re: [PATCH v2] s390/vfio-ap: fix memory leak in mdev remove callback



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@...ibm.com> wrote:
>>
>>> On 18.05.21 17:33, Halil Pasic wrote:
>>>> On Tue, 18 May 2021 15:59:36 +0200
>>>> Christian Borntraeger <borntraeger@...ibm.com> 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;