linux-kernel - Re: [PATCH v3 2/2] s390/vfio-ap: control access to PQAP(AQIC) interception handler

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Date:   Wed, 19 May 2021 13:16:10 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Tony Krowiak <akrowiak@...ux.ibm.com>
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        borntraeger@...ibm.com, cohuck@...hat.com,
        pasic@...ux.vnet.ibm.com, jjherne@...ux.ibm.com,
        alex.williamson@...hat.com, kwankhede@...dia.com
Subject: Re: [PATCH v3 2/2] s390/vfio-ap: control access to PQAP(AQIC)
 interception handler

On Wed, May 19, 2021 at 11:39:21AM -0400, Tony Krowiak wrote:

> @@ -287,13 +289,17 @@ static int handle_pqap(struct kvm_vcpu *vcpu)
>  	if (!(vcpu->arch.sie_block->eca & ECA_AIV))
>  		return -EOPNOTSUPP;
>  
> -	apqn = vcpu->run->s.regs.gprs[0] & 0xffff;
> -	mutex_lock(&matrix_dev->lock);
> +	rcu_read_lock();
> +	pqap_module_hook = rcu_dereference(vcpu->kvm->arch.crypto.pqap_hook);
> +	if (!pqap_module_hook) {
> +		rcu_read_unlock();
> +		goto set_status;
> +	}
>  
> -	if (!vcpu->kvm->arch.crypto.pqap_hook)
> -		goto out_unlock;
> -	matrix_mdev = container_of(vcpu->kvm->arch.crypto.pqap_hook,
> -				   struct ap_matrix_mdev, pqap_hook);
> +	matrix_mdev = pqap_module_hook->data;
> +	rcu_read_unlock();
> +	mutex_lock(&matrix_dev->lock);

The matrix_mdev pointer was extracted from the pqap_module_hook,
but now there is nothing protecting it since the rcu was dropped and
it gets freed in vfio_ap_mdev_remove.

And, again, module locking doesn't prevent vfio_ap_mdev_remove() from
being called. None of these patches should be combining module locking
with RCU.

Jason