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

From: Tony Krowiak
Date: Tue May 18 2021 - 14:14:10 EST

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.
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