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

[Christian Borntraeger]

On 18.05.21 20:14, Tony Krowiak wrote:
> On 5/18/21 9:59 AM, Christian Borntraeger 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?
> > >
> > > 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?
>
> I agree that this is a viable option; however, this does not
> guarantee that the matrix_mdev is not freed thus rendering
> the function pointer to the interception handler invalid unless
> that is also included within the rcu_readlock/rcu_read_unlock.

The trick should be the sychronize_rcu. This will put the deleting
code (vfio_ap_mdev_unset_kvm) to sleep until the rcu read section
has finished. So if you first set the pointer to zero, then call
synchronize_rcu the code will only progress until all users of
the old poiner have finished.

> That is not possible given the matrix_mdev is freed within
> the remove callback and the pointer to the structure that
> contains the interception handler function pointer is cleared
> in the vfio_ap_mdev_unset_kvm() function. I am working on
> a patch and should be able to post it before EOD or first thing
> tomorrow.