Re: [PATCH 1/2] s390/vfio-ap: r/w lock for PQAP interception handler function pointer

[Halil Pasic <pasic@linux.ibm.com>]

On Thu, 19 Aug 2021 09:36:34 -0400
Tony Krowiak <akrowiak@linux.ibm.com> wrote:

> >>>    static int handle_pqap(struct kvm_vcpu *vcpu)
> >>>    {
> >>>    	struct ap_queue_status status = {};
> >>> +	crypto_hook pqap_hook;
> >>>    	unsigned long reg0;
> >>>    	int ret;
> >>>    	uint8_t fc;
> >>> @@ -657,15 +658,16 @@ static int handle_pqap(struct kvm_vcpu *vcpu)
> >>>    	 * Verify that the hook callback is registered, lock the owner
> >>>    	 * and call the hook.
> >>>    	 */
> >>> +	down_read(&vcpu->kvm->arch.crypto.pqap_hook_rwsem);
> >>>    	if (vcpu->kvm->arch.crypto.pqap_hook) {                     <--- HERE
> >>> -		if (!try_module_get(vcpu->kvm->arch.crypto.pqap_hook->owner))
> >>> -			return -EOPNOTSUPP;
> >>> -		ret = vcpu->kvm->arch.crypto.pqap_hook->hook(vcpu);
> >>> -		module_put(vcpu->kvm->arch.crypto.pqap_hook->owner);
> >>> +		pqap_hook = *vcpu->kvm->arch.crypto.pqap_hook;  
> >> Dont we have to check for NULL here? If not can you add a comment why?  
> > I believe we did the necessary check on the line I just marked with
> > "<--- HERE".
> >
> > I find that "*" operator confusing in this context as it doesn't do
> > any good for us. I believe this situation is described in 6.5.3.2.4 of
> > the c11 standard. For convenience I will cite from the corresponding
> > draft:
> > "The unary * operator denotes indirection. If the operand points to a
> > function, the result is a function designator; if it points to an
> > object, the result is an lvalue designating the object. If the operand
> > has type ‘‘pointer to type’’, the result has type ‘‘type’’. If an
> > invalid value has been assigned to the pointer, the behavior of the
> > unary * operator is undefined."
> >
> > Frankly I also fail to see the benefit of introducing the local variable
> > named "pqap_hook", but back then I decided to not complain about style.  
> 
> The vcpu->kvm->arch.crypto.pqap_hook is a pointer to a function
> pointer. The actual function pointer is stored in matrix_mdev->pqap_hook,
> the reason being that the handle_pqap function in vfio_ap_ops.c
> retrieves the matrix_mdev via a container_of macro. The dereferencing
> of the vcpu->kvm->arch.crypto.pqap_hook into a local variable was
> to get the function pointer. There may have been a more stylish
> way of doing this, but the functionality is there.

You are right, and I was wrong. But then we do have to distinct pointer
deferences, and we check for NULL only once.

I still do believe we do not have a potential null pointer dereference
here, but the reason for that is that vfio-ap (the party that manages
these pointers) guarantees that whenever
vcpu->kvm->arch.crypto.pqap_hook != NULL is true, 
*vcpu->kvm->arch.crypto.pqap_hook != NULL is also true (and also that
the function pointer is a valid one). Which is the case, because we
set matrix_mdev->pqap_hook in vfio_ap_mdev_create() and don't touch
it any more.

In my opinion it is worth a comment.


> 
> >
> > Regards,
> > Halil
> >  
> >>  
> >>> +		ret = pqap_hook(vcpu);

BTW the second dereference takes place here.

If we wanted, we could make sure we don't dereference a null pointer
here but I think that would be an overkill.

Regards,
Halil  
> >> [...]