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

[Christian Borntraeger]

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?
>
> 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.

Halil, I think we can do this as an addon patch as it makes
sense to have this callback pointer protected independent of
this patch. Agree?