linux-kernel - Re: [PATCH] KVM: s390: Fix lockdep issue in vm memop

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Date:   Wed, 23 Mar 2022 10:45:46 +0100
From:   Christian Borntraeger <borntraeger@...ux.ibm.com>
To:     Janis Schoetterl-Glausch <scgl@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Sven Schnelle <svens@...ux.ibm.com>, kvm@...r.kernel.org,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: s390: Fix lockdep issue in vm memop



Am 23.03.22 um 10:39 schrieb Janis Schoetterl-Glausch:
> On 3/23/22 10:30, Christian Borntraeger wrote:
>>
>>
>> Am 23.03.22 um 09:57 schrieb Janosch Frank:
>>> On 3/23/22 09:52, Janis Schoetterl-Glausch wrote:
>>>> On 3/23/22 08:58, Janosch Frank wrote:
>>>>> On 3/22/22 16:32, Janis Schoetterl-Glausch wrote:
>>>>>> Issuing a memop on a protected vm does not make sense,
>>>>>
>>>>> Issuing a vm memop on a protected vm...
>>>>>
>>>>> The cpu memop still makes sense, no?
>>>>
>>>> The vcpu memop does hold the vcpu->lock, so no lockdep issue.
>>>> If you issue a vcpu memop while enabling protected virtualization,
>>>> the memop might find that the vcpu is not protected, while other vcpus
>>>> might already be, but I don't think there's a way to create secure memory
>>>> concurrent with the memop.
>>>
>>> I just wanted you to make this a bit more specific since we now have vm and vcpu memops. vm memops don't make sense for pv guests but vcpu ones are needed to access the sida.
>>
>> Right, I think changing the commit messages
>> - Issuing a memop on a protected vm does not make sense
>> + Issuing a vm memop on a protected vm does not make sense
>>
>> does make sense.
> 
> Ok, want me to send a v2?

I can fixup when applying. Done and queued for kvm.
>>
>>>
>>>>>
>>>>>> neither is the memory readable/writable, nor does it make sense to check
>>>>>> storage keys. This is why the ioctl will return -EINVAL when it detects
>>>>>> the vm to be protected. However, in order to ensure that the vm cannot
>>>>>> become protected during the memop, the kvm->lock would need to be taken
>>>>>> for the duration of the ioctl. This is also required because
>>>>>> kvm_s390_pv_is_protected asserts that the lock must be held.
>>>>>> Instead, don't try to prevent this. If user space enables secure
>>>>>> execution concurrently with a memop it must accecpt the possibility of
>>>>>> the memop failing.
>>>>>> Still check if the vm is currently protected, but without locking and
>>>>>> consider it a heuristic.
>>>>>>
>>>>>> Fixes: ef11c9463ae0 ("KVM: s390: Add vm IOCTL for key checked guest absolute memory access")
>>>>>> Signed-off-by: Janis Schoetterl-Glausch <scgl@...ux.ibm.com>
>>>>>
>>>>> Makes sense to me.
>>>>>
>>>>> Reviewed-by: Janosch Frank <frankja@...ux.ibm.com>
>>>>>
>>>>>> ---
>>>>>>     arch/s390/kvm/kvm-s390.c | 11 ++++++++++-
>>>>>>     1 file changed, 10 insertions(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>>>>>> index ca96f84db2cc..53adbe86a68f 100644
>>>>>> --- a/arch/s390/kvm/kvm-s390.c
>>>>>> +++ b/arch/s390/kvm/kvm-s390.c
>>>>>> @@ -2385,7 +2385,16 @@ static int kvm_s390_vm_mem_op(struct kvm *kvm, struct kvm_s390_mem_op *mop)
>>>>>>             return -EINVAL;
>>>>>>         if (mop->size > MEM_OP_MAX_SIZE)
>>>>>>             return -E2BIG;
>>>>>> -    if (kvm_s390_pv_is_protected(kvm))
>>>>>> +    /*
>>>>>> +     * This is technically a heuristic only, if the kvm->lock is not
>>>>>> +     * taken, it is not guaranteed that the vm is/remains non-protected.
>>>>>> +     * This is ok from a kernel perspective, wrongdoing is detected
>>>>>> +     * on the access, -EFAULT is returned and the vm may crash the
>>>>>> +     * next time it accesses the memory in question.
>>>>>> +     * There is no sane usecase to do switching and a memop on two
>>>>>> +     * different CPUs at the same time.
>>>>>> +     */
>>>>>> +    if (kvm_s390_pv_get_handle(kvm))
>>>>>>             return -EINVAL;
>>>>>>         if (mop->flags & KVM_S390_MEMOP_F_SKEY_PROTECTION) {
>>>>>>             if (access_key_invalid(mop->key))
>>>>>>
>>>>>> base-commit: c9b8fecddb5bb4b67e351bbaeaa648a6f7456912
>>>>>
>>>>
>>>
>