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

[Christian Borntraeger]

On 18.05.21 17:33, Halil Pasic wrote:
> On Tue, 18 May 2021 15:59:36 +0200
> Christian Borntraeger <borntraeger@xxxxxxxxxx> wrote:
>
> > On 18.05.21 15:42, Tony Krowiak wrote:
> > > On 5/18/21 5:30 AM, Christian Borntraeger wrote:
> > > > On 17.05.21 21:10, Halil Pasic wrote:
> > > > > On Mon, 17 May 2021 09:37:42 -0400
> > > > > Tony Krowiak <akrowiak@xxxxxxxxxxxxx> wrote:
> > > > >   
> > > > > > > Because of this, I don't think the rest of your argument is valid.
> > > > > >
> > > > > > Okay, so your concern is that between the point in time the
> > > > > > vcpu->kvm->arch.crypto.pqap_hook pointer is checked in
> > > > > > priv.c and the point in time the handle_pqap() function
> > > > > > in vfio_ap_ops.c is called, the memory allocated for the
> > > > > > matrix_mdev containing the struct kvm_s390_module_hook
> > > > > > may get freed, thus rendering the function pointer invalid.
> > > > > > While not impossible, that seems extremely unlikely to
> > > > > > happen. Can you articulate a scenario where that could
> > > > > > even occur?
> > > > >
> > > > > Malicious userspace. We tend to do the pqap aqic just once
> > > > > in the guest right after the queue is detected. I do agree
> > > > > it ain't very likely to happen during normal operation. But why are
> > > > > you asking?
> > > >
> > > > 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.