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

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Message-ID: <968319ed-ae4b-02fe-41c4-06799e940d94@linux.ibm.com>
Date:   Wed, 23 Mar 2022 10:30:32 +0100
From:   Christian Borntraeger <borntraeger@...ux.ibm.com>
To:     Janosch Frank <frankja@...ux.ibm.com>,
        Janis Schoetterl-Glausch <scgl@...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 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.

> 
>>>
>>>> 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
>>>
>>
>