Messages in this thread Patch in this message | | | Subject | Re: [PATCH v2] s390/vfio-ap: fix memory leak in mdev remove callback | From | Christian Borntraeger <> | Date | Wed, 19 May 2021 13:22:56 +0200 |
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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@de.ibm.com> wrote: >> >>> On 18.05.21 17:33, Halil Pasic wrote: >>>> On Tue, 18 May 2021 15:59:36 +0200 >>>> Christian Borntraeger <borntraeger@de.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;
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