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Subject: Re: [PATCH v6 2/2] s390/vfio-ap: r/w lock for PQAP interception
 handler function pointer
From:   Tony Krowiak <akrowiak@linux.ibm.com>
To:     linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org
Cc:     borntraeger@de.ibm.com, cohuck@redhat.com,
        pasic@linux.vnet.ibm.com, jjherne@linux.ibm.com, jgg@nvidia.com,
        alex.williamson@redhat.com, kwankhede@nvidia.com,
        frankja@linux.ibm.com, david@redhat.com, imbrenda@linux.ibm.com,
        hca@linux.ibm.com
References: <20210621155714.1198545-1-akrowiak@linux.ibm.com>
 <20210621155714.1198545-3-akrowiak@linux.ibm.com>
Message-ID: <51311ce0-51e5-3d26-a621-afa820cee085@linux.ibm.com>
Date:   Thu, 1 Jul 2021 11:13:06 -0400
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On 6/21/21 11:57 AM, Tony Krowiak wrote:
> The function pointer to the interception handler for the PQAP instruction
> can get changed during the interception process. Let's add a
> semaphore to struct kvm_s390_crypto to control read/write access to the
> function pointer contained therein.
>
> The semaphore must be locked for write access by the vfio_ap device driver
> when notified that the KVM pointer has been set or cleared. It must be
> locked for read access by the interception framework when the PQAP
> instruction is intercepted.
>
> Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
> Cc: stable@vger.kernel.org
> ---
>   arch/s390/include/asm/kvm_host.h      |  8 +++-----
>   arch/s390/kvm/kvm-s390.c              |  1 +
>   arch/s390/kvm/priv.c                  | 10 ++++++----
>   drivers/s390/crypto/vfio_ap_ops.c     | 23 +++++++++++++++++------
>   drivers/s390/crypto/vfio_ap_private.h |  2 +-
>   5 files changed, 28 insertions(+), 16 deletions(-)
>
> diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
> index 8925f3969478..36441669ffba 100644
> --- a/arch/s390/include/asm/kvm_host.h
> +++ b/arch/s390/include/asm/kvm_host.h
> @@ -803,14 +803,12 @@ struct kvm_s390_cpu_model {
>   	unsigned short ibc;
>   };
>   
> -struct kvm_s390_module_hook {
> -	int (*hook)(struct kvm_vcpu *vcpu);
> -	struct module *owner;
> -};
> +typedef int (*crypto_hook)(struct kvm_vcpu *vcpu);
>   
>   struct kvm_s390_crypto {
>   	struct kvm_s390_crypto_cb *crycb;
> -	struct kvm_s390_module_hook *pqap_hook;
> +	struct rw_semaphore pqap_hook_rwsem;
> +	crypto_hook *pqap_hook;
>   	__u32 crycbd;
>   	__u8 aes_kw;
>   	__u8 dea_kw;
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index 1296fc10f80c..418d910df569 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -2606,6 +2606,7 @@ static void kvm_s390_crypto_init(struct kvm *kvm)
>   {
>   	kvm->arch.crypto.crycb = &kvm->arch.sie_page2->crycb;
>   	kvm_s390_set_crycb_format(kvm);
> +	init_rwsem(&kvm->arch.crypto.pqap_hook_rwsem);
>   
>   	if (!test_kvm_facility(kvm, 76))
>   		return;
> diff --git a/arch/s390/kvm/priv.c b/arch/s390/kvm/priv.c
> index 9928f785c677..6bed9406c1f3 100644
> --- a/arch/s390/kvm/priv.c
> +++ b/arch/s390/kvm/priv.c
> @@ -610,6 +610,7 @@ static int handle_io_inst(struct kvm_vcpu *vcpu)
>   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) {
> -		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;
> +		ret = pqap_hook(vcpu);
>   		if (!ret && vcpu->run->s.regs.gprs[1] & 0x00ff0000)
>   			kvm_s390_set_psw_cc(vcpu, 3);
> +		up_read(&vcpu->kvm->arch.crypto.pqap_hook_rwsem);
>   		return ret;
>   	}
> +	up_read(&vcpu->kvm->arch.crypto.pqap_hook_rwsem);
>   	/*
>   	 * A vfio_driver must register a hook.
>   	 * No hook means no driver to enable the SIE CRYCB and no queues.
> diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
> index 122c85c22469..742277bc8d1c 100644
> --- a/drivers/s390/crypto/vfio_ap_ops.c
> +++ b/drivers/s390/crypto/vfio_ap_ops.c
> @@ -352,8 +352,7 @@ static int vfio_ap_mdev_create(struct mdev_device *mdev)
>   	vfio_ap_matrix_init(&matrix_dev->info, &matrix_mdev->matrix);
>   	init_waitqueue_head(&matrix_mdev->wait_for_kvm);
>   	mdev_set_drvdata(mdev, matrix_mdev);
> -	matrix_mdev->pqap_hook.hook = handle_pqap;
> -	matrix_mdev->pqap_hook.owner = THIS_MODULE;
> +	matrix_mdev->pqap_hook = handle_pqap;
>   	mutex_lock(&matrix_dev->lock);
>   	list_add(&matrix_mdev->node, &matrix_dev->mdev_list);
>   	mutex_unlock(&matrix_dev->lock);
> @@ -1115,15 +1114,20 @@ static int vfio_ap_mdev_set_kvm(struct ap_matrix_mdev *matrix_mdev,
>   		}
>   
>   		kvm_get_kvm(kvm);
> +		matrix_mdev->kvm = kvm;
>   		matrix_mdev->kvm_busy = true;
>   		mutex_unlock(&matrix_dev->lock);
> +
> +		down_write(&matrix_mdev->kvm->arch.crypto.pqap_hook_rwsem);
> +		kvm->arch.crypto.pqap_hook = &matrix_mdev->pqap_hook;

Setting the hook using a field in matrix_mdev while not
holding the matrix_dev->lock opens us up to a possible race
condition.

> +		up_write(&matrix_mdev->kvm->arch.crypto.pqap_hook_rwsem);
> +
>   		kvm_arch_crypto_set_masks(kvm,
>   					  matrix_mdev->matrix.apm,
>   					  matrix_mdev->matrix.aqm,
>   					  matrix_mdev->matrix.adm);
> +
>   		mutex_lock(&matrix_dev->lock);
> -		kvm->arch.crypto.pqap_hook = &matrix_mdev->pqap_hook;
> -		matrix_mdev->kvm = kvm;
>   		matrix_mdev->kvm_busy = false;
>   		wake_up_all(&matrix_mdev->wait_for_kvm);
>   	}
> @@ -1189,10 +1193,17 @@ static void vfio_ap_mdev_unset_kvm(struct ap_matrix_mdev *matrix_mdev)
>   	if (matrix_mdev->kvm) {
>   		matrix_mdev->kvm_busy = true;
>   		mutex_unlock(&matrix_dev->lock);
> -		kvm_arch_crypto_clear_masks(matrix_mdev->kvm);
> +
> +		if (matrix_mdev->kvm->arch.crypto.crycbd) {
> +			down_write(&matrix_mdev->kvm->arch.crypto.pqap_hook_rwsem);
> +			matrix_mdev->kvm->arch.crypto.pqap_hook = NULL;
> +			up_write(&matrix_mdev->kvm->arch.crypto.pqap_hook_rwsem);
> +
> +			kvm_arch_crypto_clear_masks(matrix_mdev->kvm);
> +		}
> +
>   		mutex_lock(&matrix_dev->lock);
>   		vfio_ap_mdev_reset_queues(matrix_mdev->mdev);
> -		matrix_mdev->kvm->arch.crypto.pqap_hook = NULL;
>   		kvm_put_kvm(matrix_mdev->kvm);
>   		matrix_mdev->kvm = NULL;
>   		matrix_mdev->kvm_busy = false;
> diff --git a/drivers/s390/crypto/vfio_ap_private.h b/drivers/s390/crypto/vfio_ap_private.h
> index f82a6396acae..e12218e5a629 100644
> --- a/drivers/s390/crypto/vfio_ap_private.h
> +++ b/drivers/s390/crypto/vfio_ap_private.h
> @@ -86,7 +86,7 @@ struct ap_matrix_mdev {
>   	bool kvm_busy;
>   	wait_queue_head_t wait_for_kvm;
>   	struct kvm *kvm;
> -	struct kvm_s390_module_hook pqap_hook;
> +	crypto_hook pqap_hook;
>   	struct mdev_device *mdev;
>   };
>