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Subject: Re: [PATCH v3 2/2] s390/vfio-ap: control access to PQAP(AQIC)
 interception handler
To:     Halil Pasic <pasic@linux.ibm.com>
Cc:     linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        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
References: <20210519153921.804887-1-akrowiak@linux.ibm.com>
 <20210519153921.804887-3-akrowiak@linux.ibm.com>
 <20210519192147.2362fe1b.pasic@linux.ibm.com>
From:   Tony Krowiak <akrowiak@linux.ibm.com>
Message-ID: <0b0dd666-174a-281d-d630-148a0a6eaa76@linux.ibm.com>
Date:   Wed, 19 May 2021 19:14:36 -0400
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On 5/19/21 1:21 PM, Halil Pasic wrote:
> On Wed, 19 May 2021 11:39:21 -0400
> Tony Krowiak <akrowiak@linux.ibm.com> wrote:
>
>> There is currently nothing that controls access to the structure that
>> contains the function pointer to the handler that processes interception of
>> the PQAP(AQIC) instruction. If the mdev is removed while the PQAP(AQIC)
>> instruction is being intercepted, there is a possibility that the function
>> pointer to the handler can get wiped out prior to the attempt to call it.
>>
>> This patch utilizes RCU to synchronize access to the kvm_s390_module_hook
>> structure used to process interception of the PQAP(AQIC) instruction.
>>
>> Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
>> ---
>>   arch/s390/include/asm/kvm_host.h  |  1 +
>>   arch/s390/kvm/priv.c              | 47 ++++++++++++++++-----------
>>   drivers/s390/crypto/vfio_ap_ops.c | 54 ++++++++++++++++++++++++-------
>>   3 files changed, 73 insertions(+), 29 deletions(-)
>>
>> diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
>> index 8925f3969478..4987e82d6116 100644
>> --- a/arch/s390/include/asm/kvm_host.h
>> +++ b/arch/s390/include/asm/kvm_host.h
>> @@ -806,6 +806,7 @@ struct kvm_s390_cpu_model {
>>   struct kvm_s390_module_hook {
>>   	int (*hook)(struct kvm_vcpu *vcpu);
>>   	struct module *owner;
>> +	void *data;
> I guess you need this, because you stopped using the member of struct
> ap_mdev_matrix and instead you kzalloc() a new object. Yet I don't
> understand why do you do so?

I did so because the mdev remove callback frees the matrix_mdev
and I thought I could protect against accessing freed storage by
storing the pointer in the pqap_hook structure; however, since we
can't hold the rcu_read_lock while the handle_pqap is executing - due
to wait conditions - this turns out to be a bad idea. I'm not sure at
this point that we can use RCU for this because the freeing of the
matrix_mdev is independent of pqap processing.

>
>>   };
>>   
>>   struct kvm_s390_crypto {
>> diff --git a/arch/s390/kvm/priv.c b/arch/s390/kvm/priv.c
>> index 9928f785c677..2d330dfbdb61 100644
>> --- a/arch/s390/kvm/priv.c
>> +++ b/arch/s390/kvm/priv.c
>> @@ -610,8 +610,11 @@ static int handle_io_inst(struct kvm_vcpu *vcpu)
>>   static int handle_pqap(struct kvm_vcpu *vcpu)
>>   {
>>   	struct ap_queue_status status = {};
>> +	struct kvm_s390_module_hook *pqap_module_hook;
>> +	int (*pqap_hook)(struct kvm_vcpu *vcpu);
>> +	struct module *owner;
>>   	unsigned long reg0;
>> -	int ret;
>> +	int ret = 0;
>>   	uint8_t fc;
>>   
>>   	/* Verify that the AP instruction are available */
>> @@ -657,24 +660,32 @@ static int handle_pqap(struct kvm_vcpu *vcpu)
>>   	 * Verify that the hook callback is registered, lock the owner
>>   	 * and call the hook.
>>   	 */
>> -	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);
>> -		if (!ret && vcpu->run->s.regs.gprs[1] & 0x00ff0000)
>> -			kvm_s390_set_psw_cc(vcpu, 3);
>> -		return ret;
>> +	rcu_read_lock();
>> +	pqap_module_hook = rcu_dereference(vcpu->kvm->arch.crypto.pqap_hook);
>> +	if (pqap_module_hook) {
>> +		pqap_hook = pqap_module_hook->hook;
>> +		owner = pqap_module_hook->owner;
>> +		rcu_read_unlock();
>> +		if (!try_module_get(owner)) {
> Why do this outside the rcu_read lock?

It should be done inside the lock.

>
> What guarantees that the module ain't gone by this time? I don't think
> try_module_get() is guaranteed to give you false if passed in a pointer
> that points to some memory that ain't a struct module any more
> (use-after-free).

That needs to be inside the lock.

>
>> +			ret = -EOPNOTSUPP;
>> +		} else {
>> +			ret = pqap_hook(vcpu);
>> +			module_put(owner);
>> +			if (!ret && vcpu->run->s.regs.gprs[1] & 0x00ff0000)
>> +				kvm_s390_set_psw_cc(vcpu, 3);
>> +		}
>> +	} else {
>> +		rcu_read_unlock();
>> +		/*
>> +		 * A vfio_driver must register a hook.
>> +		 * No hook means no driver to enable the SIE CRYCB and no
>> +		 * queues. We send this response to the guest.
>> +		 */
>> +		status.response_code = 0x01;
>> +		memcpy(&vcpu->run->s.regs.gprs[1], &status, sizeof(status));
>> +		kvm_s390_set_psw_cc(vcpu, 3);
>>   	}
>> -	/*
>> -	 * A vfio_driver must register a hook.
>> -	 * No hook means no driver to enable the SIE CRYCB and no queues.
>> -	 * We send this response to the guest.
>> -	 */
>> -	status.response_code = 0x01;
>> -	memcpy(&vcpu->run->s.regs.gprs[1], &status, sizeof(status));
>> -	kvm_s390_set_psw_cc(vcpu, 3);
>> -	return 0;
>> +	return ret;
>>   }
>>   
>>   static int handle_stfl(struct kvm_vcpu *vcpu)
>> diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
>> index f90c9103dac2..a6aa3f753ac4 100644
>> --- a/drivers/s390/crypto/vfio_ap_ops.c
>> +++ b/drivers/s390/crypto/vfio_ap_ops.c
>> @@ -16,6 +16,7 @@
>>   #include <linux/bitops.h>
>>   #include <linux/kvm_host.h>
>>   #include <linux/module.h>
>> +#include <linux/rcupdate.h>
>>   #include <asm/kvm.h>
>>   #include <asm/zcrypt.h>
>>   
>> @@ -279,6 +280,7 @@ static int handle_pqap(struct kvm_vcpu *vcpu)
>>   	uint64_t status;
>>   	uint16_t apqn;
>>   	struct vfio_ap_queue *q;
>> +	struct kvm_s390_module_hook *pqap_module_hook;
>>   	struct ap_queue_status qstatus = {
>>   			       .response_code = AP_RESPONSE_Q_NOT_AVAIL, };
>>   	struct ap_matrix_mdev *matrix_mdev;
>> @@ -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);
> I agree with Jason's assessment. At this point the matrix_dev pointer
> may point to garbage.
>
> Above, I think we can use the pqap_hook function pointer local to
> handle_pqap, because we know that as long as the module is there
> the callback will sit at the same address and won't go away. And
> we do the try_module_get() to ensure that the module stays loaded.

I'll take your word that is true. If so, then I think I can replace
the way we get the matrix_mdev in the pqap handler which
can be done under the matrix_dev->lock. I can write a function
to get the matrix from the list of mdevs in matrix_dev. If the
matrix_mdev has been removed, then we can bail out of the
handle_pqap function. Maybe the RCU can be used after all.

>
>
>> +	apqn = vcpu->run->s.regs.gprs[0] & 0xffff;
>>   
>>   	/*
>>   	 * If the KVM pointer is in the process of being set, wait until the
>> @@ -322,9 +328,10 @@ static int handle_pqap(struct kvm_vcpu *vcpu)
>>   		qstatus = vfio_ap_irq_disable(q);
>>   
>>   out_unlock:
>> +	mutex_unlock(&matrix_dev->lock);
>> +set_status:
>>   	memcpy(&vcpu->run->s.regs.gprs[1], &qstatus, sizeof(qstatus));
>>   	vcpu->run->s.regs.gprs[1] >>= 32;
>> -	mutex_unlock(&matrix_dev->lock);
>>   	return 0;
>>   }
>>   
>> @@ -353,8 +360,6 @@ 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;
> I guess the member of struct ap_matrix_mdev is still around, it will
> remain all zero. Is this somehow intentional?
>
>
>>   	mutex_lock(&matrix_dev->lock);
>>   	list_add(&matrix_mdev->node, &matrix_dev->mdev_list);
>>   	mutex_unlock(&matrix_dev->lock);
>> @@ -1085,6 +1090,22 @@ static const struct attribute_group *vfio_ap_mdev_attr_groups[] = {
>>   	NULL
>>   };
>>   
>> +static int vfio_ap_mdev_set_pqap_hook(struct ap_matrix_mdev *matrix_mdev,
>> +				       struct kvm *kvm)
>> +{
>> +	struct kvm_s390_module_hook *pqap_hook;
>> +
>> +	pqap_hook = kmalloc(sizeof(*kvm->arch.crypto.pqap_hook), GFP_KERNEL);
> What is the extra allocation supposed to buy us?
>
>> +	if (!pqap_hook)
>> +		return -ENOMEM;
>> +	pqap_hook->data = matrix_mdev;
>> +	pqap_hook->hook = handle_pqap;
>> +	pqap_hook->owner = THIS_MODULE;
>> +	rcu_assign_pointer(kvm->arch.crypto.pqap_hook, pqap_hook);
>> +
>> +	return 0;
>> +}
>> +
>>   /**
>>    * vfio_ap_mdev_set_kvm
>>    *
>> @@ -1107,6 +1128,7 @@ static const struct attribute_group *vfio_ap_mdev_attr_groups[] = {
>>   static int vfio_ap_mdev_set_kvm(struct ap_matrix_mdev *matrix_mdev,
>>   				struct kvm *kvm)
>>   {
>> +	int ret;
>>   	struct ap_matrix_mdev *m;
>>   
>>   	if (kvm->arch.crypto.crycbd) {
>> @@ -1115,6 +1137,10 @@ static int vfio_ap_mdev_set_kvm(struct ap_matrix_mdev *matrix_mdev,
>>   				return -EPERM;
>>   		}
>>   
>> +		ret = vfio_ap_mdev_set_pqap_hook(matrix_mdev, kvm);
>> +		if (ret)
>> +			return ret;
>> +
>>   		kvm_get_kvm(kvm);
>>   		matrix_mdev->kvm_busy = true;
>>   		mutex_unlock(&matrix_dev->lock);
>> @@ -1123,7 +1149,6 @@ static int vfio_ap_mdev_set_kvm(struct ap_matrix_mdev *matrix_mdev,
>>   					  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);
>> @@ -1161,6 +1186,13 @@ static int vfio_ap_mdev_iommu_notifier(struct notifier_block *nb,
>>   	return NOTIFY_DONE;
>>   }
>>   
>> +static void vfio_ap_mdev_unset_pqap_hook(struct kvm *kvm)
>> +{
>> +	rcu_assign_pointer(kvm->arch.crypto.pqap_hook, NULL);
>> +	synchronize_rcu();
>> +	kfree(kvm->arch.crypto.pqap_hook);
>> +}
>> +
>>   /**
>>    * vfio_ap_mdev_unset_kvm
>>    *
>> @@ -1189,11 +1221,11 @@ static void vfio_ap_mdev_unset_kvm(struct ap_matrix_mdev *matrix_mdev)
>>   
>>   	if (matrix_mdev->kvm) {
>>   		matrix_mdev->kvm_busy = true;
>> +		vfio_ap_mdev_unset_pqap_hook(matrix_mdev->kvm);
>>   		mutex_unlock(&matrix_dev->lock);
>>   		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;