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SubjectRe: [PATCH RFC v1 0/2] VM fork detection for RNG
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On 24.02.22 11:43, Daniel P. Berrangé wrote:
> On Thu, Feb 24, 2022 at 09:53:59AM +0100, Alexander Graf wrote:
>> Hey Jason,
>>
>> On 23.02.22 14:12, Jason A. Donenfeld wrote:
>>> This small series picks up work from Amazon that seems to have stalled
>>> out later year around this time: listening for the vmgenid ACPI
>>> notification, and using it to "do something." Last year, that something
>>> involved a complicated userspace mmap chardev, which seems frought with
>>> difficulty. This year, I have something much simpler in mind: simply
>>> using those ACPI notifications to tell the RNG to reinitialize safely,
>>> so we don't repeat random numbers in cloned, forked, or rolled-back VM
>>> instances.
>>>
>>> This series consists of two patches. The first is a rather
>>> straightforward addition to random.c, which I feel fine about. The
>>> second patch is the reason this is just an RFC: it's a cleanup of the
>>> ACPI driver from last year, and I don't really have much experience
>>> writing, testing, debugging, or maintaining these types of drivers.
>>> Ideally this thread would yield somebody saying, "I see the intent of
>>> this; I'm happy to take over ownership of this part." That way, I can
>>> focus on the RNG part, and whoever steps up for the paravirt ACPI part
>>> can focus on that.
>>>
>>> As a final note, this series intentionally does _not_ focus on
>>> notification of these events to userspace or to other kernel consumers.
>>> Since these VM fork detection events first need to hit the RNG, we can
>>> later talk about what sorts of notifications or mmap'd counters the RNG
>>> should be making accessible to elsewhere. But that's a different sort of
>>> project and ties into a lot of more complicated concerns beyond this
>>> more basic patchset. So hopefully we can keep the discussion rather
>>> focused here to this ACPI business.
>>
>> The main problem with VMGenID is that it is inherently racy. There will
>> always be a (short) amount of time where the ACPI notification is not
>> processed, but the VM could use its RNG to for example establish TLS
>> connections.
>>
>> Hence we as the next step proposed a multi-stage quiesce/resume mechanism
>> where the system is aware that it is going into suspend - can block network
>> connections for example - and only returns to a fully functional state after
>> an unquiesce phase:
>>
>> https://github.com/systemd/systemd/issues/20222
> The downside of course is precisely that the guest now needs to be aware
> and involved every single time a snapshot is taken.
>
> Currently with virt the act of taking a snapshot can often remain invisible
> to the VM with no functional effect on the guest OS or its workload, and
> the host OS knows it can complete a snapshot in a specific timeframe. That
> said, this transparency to the VM is precisely the cause of the race
> condition described.
>
> With guest involvement to quiesce the bulk of activity for time period,
> there is more likely to be a negative impact on the guest workload. The
> guest admin likely needs to be more explicit about exactly when in time
> it is reasonable to take a snapshot to mitigate the impact.
>
> The host OS snapshot operations are also now dependant on co-operation
> of a guest OS that has to be considered to be potentially malicious, or
> at least crashed/non-responsive. The guest OS also needs a way to receive
> the triggers for snapshot capture and restore, most likely via an extension
> to something like the QEMU guest agent or an equivalent for othuer
> hypervisors.


What you describe sounds almost exactly like pressing a power button on
modern systems. You don't just kill the power line, you press a button
and wait for the guest to acknowledge that it's ready.

Maybe the real answer to all of this is S3: Suspend to RAM. You press
the suspend button, the guest can prepare for sleep (quiesce!) and the
next time you run, it can check whether VMGenID changed and act accordingly.


> Despite the above, I'm not against the idea of co-operative involvement
> of the guest OS in the acts of taking & restoring snapshots. I can't
> see any other proposals so far that can reliably eliminate the races
> in the general case, from the kernel right upto user applications.
> So I think it is neccessary to have guest cooperative snapshotting.
>
>> What exact use case do you have in mind for the RNG/VMGenID update? Can you
>> think of situations where the race is not an actual concern?
> Lets assume we do take the approach described in that systemd bug and
> have a co-operative snapshot process. If the hypervisor does the right
> thing and guest owners install the right things, they'll have a race
> free solution that works well in normal operation. That's good.
>
>
> Realistically though, it is never going to be universally and reliably
> put into practice. So what is our attitude to cases where the preferred
> solution isn't availble and/or operative ?
>
>
> There are going to be users who continue to build their guest disk images
> without the QEMU guest agent (or equivalent for whatever hypervisor they
> run on) installed because they don't know any better. Or where the guest
> agent is mis-configured or fails to starts or some other scenario that
> prevents the quiesce working as desired. The host mgmt could refuse to
> take a snapshot in these cases. More likely is that they are just
> going to go ahead and do a snapshot anyway because lack of guest agent
> is a very common scenario today and users want their snapshots.
>
>
> There are going to be virt management apps / hypervisors that don't
> support talking to any guest agent across their snapshot operation
> in the first place, so systemd gets no way to trigger the required
> quiesce dance on snapshot, but they likely have VMGenID support
> implemented already.
>
>
> IOW, I could view VMGenID triggered fork detection integrated with
> the kernel RNG as providing a backup line of defence that is going
> to "just work", albeit with the known race. It isn't as good as the
> guest co-operative snapshot approach, because it only tries to solve
> the one specific targetted problem of updating the kernel RNG.
>
> Is it still better than doing nothing at all though, for the scenario
> where guest co-operative snapshot is unavailable ?
>
> If it is better than nothing, is it then compelling enough to justify
> the maint cost of the code added to the kernel ?


I'm tempted to say "If it also exposes the VMGenID via sysfs so that you
can actually check whether you were cloned, probably yes."


Alex





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