Re: [PATCH RFC v1 0/2] VM fork detection for RNG

Alexander Graf
2022-02-24T11:36:28Z

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

Amazon Development Center Germany GmbH
Krausenstr. 38
10117 Berlin
Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss
Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B
Sitz: Berlin
Ust-ID: DE 289 237 879