Re: [PATCH v4 5/7] s390: ap: implement PAPQ AQIC interception in kernel

Pierre Morel
2019-03-01T09:35:48Z

On 28/02/2019 21:20, Christian Borntraeger wrote:
>
>
> On 22.02.2019 16:29, Pierre Morel wrote:
>> We register the AP PQAP instruction hook during the open
>> of the mediated device. And unregister it on release.
>>
>> In the AP PQAP instruction hook, if we receive a demand to
>> enable IRQs,
>> - we retrieve the vfio_ap_queue based on the APQN we receive
>> in REG1,
>> - we retrieve the page of the guest address, (NIB), from
>> register REG2
>> - we the mediated device to use the VFIO pinning infratrsucture
>> to pin the page of the guest address,
>> - we retrieve the pointer to KVM to register the guest ISC
>> and retrieve the host ISC
>> - finaly we activate GISA
>>
>> If we receive a demand to disable IRQs,
>> - we deactivate GISA
>> - unregister from the GIB
>> - unping the NIB
>>
>> Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
>> ---
>> arch/s390/include/asm/kvm_host.h | 1 +
>> drivers/s390/crypto/ap_bus.h | 1 +
>> drivers/s390/crypto/vfio_ap_ops.c | 199 +++++++++++++++++++++++++++++++++-
>> drivers/s390/crypto/vfio_ap_private.h | 1 +
>> 4 files changed, 199 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
>> index 49cc8b0..5f3bb8c 100644
>> --- a/arch/s390/include/asm/kvm_host.h
>> +++ b/arch/s390/include/asm/kvm_host.h
>> @@ -720,6 +720,7 @@ struct kvm_s390_cpu_model {
>> struct kvm_s390_crypto {
>> struct kvm_s390_crypto_cb *crycb;
>> int (*pqap_hook)(struct kvm_vcpu *vcpu);
>> + void *vfio_private;
>> __u32 crycbd;
>> __u8 aes_kw;
>> __u8 dea_kw;
>> diff --git a/drivers/s390/crypto/ap_bus.h b/drivers/s390/crypto/ap_bus.h
>> index bfc66e4..323f2aa 100644
>> --- a/drivers/s390/crypto/ap_bus.h
>> +++ b/drivers/s390/crypto/ap_bus.h
>> @@ -43,6 +43,7 @@ static inline int ap_test_bit(unsigned int *ptr, unsigned int nr)
>> #define AP_RESPONSE_BUSY 0x05
>> #define AP_RESPONSE_INVALID_ADDRESS 0x06
>> #define AP_RESPONSE_OTHERWISE_CHANGED 0x07
>> +#define AP_RESPONSE_INVALID_GISA 0x08
>> #define AP_RESPONSE_Q_FULL 0x10
>> #define AP_RESPONSE_NO_PENDING_REPLY 0x10
>> #define AP_RESPONSE_INDEX_TOO_BIG 0x11
>> diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
>> index 1b5130a..0196065 100644
>> --- a/drivers/s390/crypto/vfio_ap_ops.c
>> +++ b/drivers/s390/crypto/vfio_ap_ops.c
>> @@ -43,7 +43,7 @@ struct vfio_ap_queue *vfio_ap_get_queue(int apqn, struct list_head *l)
>> return NULL;
>> }
>>
>> -static int vfio_ap_mdev_reset_queue(struct vfio_ap_queue *q)
>> +int vfio_ap_mdev_reset_queue(struct vfio_ap_queue *q)
>> {
>> struct ap_queue_status status;
>> int retry = 20;
>> @@ -75,6 +75,27 @@ static int vfio_ap_mdev_reset_queue(struct vfio_ap_queue *q)
>> return -EBUSY;
>> }
>>
>> +/**
>> + * vfio_ap_free_irq:
>> + * @q: The vfio_ap_queue
>> + *
>> + * Unpin the guest NIB
>> + * Unregister the ISC from the GIB alert
>> + * Clear the vfio_ap_queue intern fields
>> + */
>> +static void vfio_ap_free_irq(struct vfio_ap_queue *q)
>> +{
>> + if (!q)
>> + return;
>> + if (q->g_pfn)
>> + vfio_unpin_pages(mdev_dev(q->matrix_mdev->mdev), &q->g_pfn, 1);
>> + if (q->isc)
>> + kvm_s390_gisc_unregister(q->matrix_mdev->kvm, q->isc);
>> + q->nib = 0;
>> + q->isc = 0;
>> + q->g_pfn = 0;
>> +}
>
> Pierre, unless there is some magic, I think we need to free the gisa stuff before kvm exit.
>
> Imagine a malicious userspace that setups everything fine, but then closes all kvm file
> descriptors but not the vfio file descriptor. This might result in random access to the
> memory that contained the gisa potentially resulting in random memory overwrites.
>
> the problem is that kvm_destroy_vm calls kvm_arch_destroy_vm(kvm) before it calls
> kvm_destroy_devices(kvm); So we already free the gisa before we do the unregister call.
>
> What about calling kvm_get_kvm/put from some of the callbacks in the right places.
>
> Debugging random memory overwrites is a PITA, so we either should document why I cannot
> happen (even with malicious userspace) or simply fix the refcounting.
>

OK, understood.
I think we can do something simple by using kvm_get/kvm_put, as you
suggested, in the vfio KVM notifier to ensure the order of the calls and
also unregister the GISA at this moment.
I will investigate in this direction.

Thanks
Pierre

--
Pierre Morel
Linux/KVM/QEMU in Böblingen - Germany