Re: [PATCH 1/5] KVM: Make the maximum number of user memslots a per-VM thing

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Paolo Bonzini <pbonzini@redhat.com> writes:

> On 28/01/21 11:48, Maciej S. Szmigiero wrote:
>>>
>>> VMMs (especially big ones like QEMU) are complex and e.g. each driver
>>> can cause memory regions (-> memslots in KVM) to change. With this
>>> feature it becomes possible to set a limit upfront (based on VM
>>> configuration) so it'll be more obvious when it's hit.
>>>
>> 
>> I see: it's a kind of a "big switch", so every VMM doesn't have to be
>> modified or audited.
>> Thanks for the explanation.
>
> Not really, it's the opposite: the VMM needs to opt into a smaller 
> number of memslots.
>
> I don't know... I understand it would be defense in depth, however 
> between dynamic allocation of memslots arrays and GFP_KERNEL_ACCOUNT, it 
> seems to be a bit of a solution in search of a problem.  For now I 
> applied patches 1-2-5.

An alternative with a new module parameter was also suggested, that
would make it possible to protect against buggy/malicious VMMs but
again, the attack is not any different from just creating many
VMs. Module parameter will most like end up being unused 99,9% of the
time (if not 100). I don't seem to have a strong opinion.

-- 
Vitaly