On 12/21/2017 01:12 PM, Christian Borntraeger wrote:
>
>
> On 12/21/2017 01:03 PM, David Hildenbrand wrote:
>> On 21.12.2017 12:57, David Hildenbrand wrote:
>>> On 21.12.2017 10:04, Christian Borntraeger wrote:
>>>> This is targetted as minimal fixups. Claudio is reworking this
>>>> to handle this better in general (e.g. memory wholes).
>>>>
>>>> I will wait for a review for patch2 (which is new) and then spin
>>>> a pull request as we now have 2 patches for master
>>>>
>>>> Christian Borntraeger (2):
>>>> KVM: s390: fix cmma migration for multiple memory slots
>>>> KVM: s390: prevent buffer overrun on memory hotplug during migration
>>>>
>>>> arch/s390/kvm/kvm-s390.c | 9 +++++----
>>>> arch/s390/kvm/priv.c | 2 +-
>>>> 2 files changed, 6 insertions(+), 5 deletions(-)
>>>>
>>>
>>> [waking up from vacation mode]
>>>
>>> Sorry to say, but I guess there is more.
>>>
>>> do_essa() can race with
>>> kvm_s390_vm_set_migration(KVM_S390_VM_MIGRATION_STOP).
>>>
>>> CPU0: handle_essa()
>>> -> vcpu->kvm->arch.migration_state == true
>>> -> do_essa()
>>> CPU1: kvm_s390_vm_set_migration(KVM_S390_VM_MIGRATION_STOP)
>>> -> vcpu->kvm->arch.migration_state = false;
>>> -> vfree(mgs->pgste_bitmap)
>>> CPU0: test_and_set_bit(gfn, ms->pgste_bitmap)
>>> -> access to freed data structure
>>>
>>> The kvm_s390_sync_request_broadcast(kvm, KVM_REQ_STOP_MIGRATION) will
>>> only make sure that all CPUs have left SIE mode, not that the request
>>> has been processed.
>>>
>>> Does that make sense?
>>>
>>> [going back to vacation mode]
>>>
>>
>> FWIW, doesn't apply the same thing to kvm_s390_get_cmma_bits(), too? I
>> can't find locking while quickly glimpsing over it. But I might be wrong.
>
> If there is a bug in a piece of code it is certainly a good idea to look deeper....
> I think we can prevent both bugs by something like
>
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index 8951ad4d33be..6ee12afced5a 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -841,6 +841,7 @@ static int kvm_s390_vm_stop_migration(struct kvm *kvm)
>
> if (kvm->arch.use_cmma) {
> kvm_s390_sync_request_broadcast(kvm, KVM_REQ_STOP_MIGRATION);
> + synchronize_srcu(&kvm->srcu);
> vfree(mgs->pgste_bitmap);
> }
> kfree(mgs);
>
>
> Need to have a look.
It needs a bit more changes for the get_cmma case.
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