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SubjectRe: [PATCH 1/2] s390/vfio-ap: r/w lock for PQAP interception handler function pointer
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On 19.08.21 01:25, Halil Pasic wrote:
> On Wed, 18 Aug 2021 19:03:33 +0200
> Christian Borntraeger <borntraeger@de.ibm.com> wrote:
>
>> On 19.07.21 21:35, 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>
>>> ---
>>> 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 9b4473f76e56..f18849d259e6 100644
>>> --- a/arch/s390/include/asm/kvm_host.h
>>> +++ b/arch/s390/include/asm/kvm_host.h
>>> @@ -798,14 +798,12 @@ 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 b655a7d82bf0..a08f242a9f27 100644
>>> --- a/arch/s390/kvm/kvm-s390.c
>>> +++ b/arch/s390/kvm/kvm-s390.c
>>> @@ -2630,6 +2630,7 @@ 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) { <--- 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".

Right, I missed that.

Then
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>

>
> 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.

Right we can probably do better, but I would rather have the functional
fix in now.

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

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