Re: propagating vmgenid outward and upward

From: Alexander Graf
Date: Thu Mar 10 2022 - 06:18:43 EST


On 09.03.22 23:02, Jason A. Donenfeld wrote:
Hi Alex,

On Wed, Mar 9, 2022 at 3:10 AM Alexander Graf <graf@xxxxxxxxxx> wrote:
The vmgenid driver basically works, though it is racy, because that ACPI
notification can arrive after the system is already running again. This

I believe enough people already pointed out that this assumption is
incorrect. The thing that is racy about VMGenID is the interrupt based
notification.
I'm having a hard time figuring out what's different between your
statement and mine. I said that the race is due to the notification.
You said that the race is due to the notification. What subtle thing
am I missing here that would lead you to say that my assumption is
incorrect? Or did you just misread?


The subtle difference is that you don't need to rely on the notification to learn about the world switch. If you instead read VMGenID explicitly without waiting for the notification, you're guaranteed to always know whether you were cloned. That means the actual VMGenID interface is not always racy. Just the notification part is.

So you *can* build a race-free VMGenID mechanism. You just can't build a race-free *event based* VMGenID mechanism if you want to allow cloning at arbitrary points in time.



The actual identifier is updated before the VM resumes
from its clone operation, so if you match on that you will know whether
you are in a new or old world. And that is enough to create
transactions: Save the identifier before a "crypto transaction",
validate before you finish, if they don't match, abort, reseed and replay.
Right. But more than just transactions, it's useful to preventing key
reuse vulnerabilities, in which case, you store the current identifier
just before an ephemeral key is generated, and then subsequently check
to see that the identifier hasn't changed before transmitting anything
related to that key.

If you follow the logic at the beginning of the mail, you can create
something race free if you consume the hardware VMGenID counter. You can
not make it race free if you rely on the interrupt mechanism.
Yes, as mentioned and discussed in depth before. However, your use of
the word "counter" is problematic. Vmgenid is not a counter. It's a
unique identifier. That means you can't compare it with a single word
comparison but have to compare all of the 16 bytes. That seems
potentially expensive. It's for that reason that I suggested
augmenting the vmgenid spec with an additional word-sized _counter_
that could be mapped into the kernels and into userspaces.


I think Michael's benchmarks show quite well that it's not all that expensive. We're talking 2x 64bit compares within the same cache line. You already realized that we don't need them to be atomic - just properly barriered.

So what if we created a vsyscall that takes a buffer of "up to 16 bytes". If we later realize that an additional page with a 4 byte counter is a viable performance optimization, we can work with MS to add that to the spec. But the user space interface would stay identical.



So following that train of thought, if you expose the hardware VMGenID
to user space, you could allow user space to act race free based on
VMGenID. That means consumers of user space RNGs could validate whether
the ID is identical between the beginning of the crypto operation and
the end.
Right.

However, there are more complicated cases as well. What do you do with
Samba for example? It needs to generate a new SID after the clone.
That's a super heavy operation. Do you want to have smbd constantly poll
on the VMGenID just to see whether it needs to kick off some
administrative actions?
Were it a single word-sized integer, mapped into memory, that wouldn't
be much of a problem at all. It could constantly read this before and
after every operation. The problem is that it's 16 bytes and
understandably applications don't want to deal with that clunkiness.


I don't think applications should be in the business of mapping arbitrary locations of the vdso space to match on them. We need to build an interface that is fast and flexible enough so they can just say "here is a buffer, tell me if the ID changed". That's practically all you need - on init you run that once. Later on, you invoke it every time between finishing a crypto operation and putting it on the wire.



In that case, all we would need from the kernel is an easily readable
GenID that changes
Actually, no, you need even less than that. All that's required is a
sysfs/procfs file that can be poll()'d on. It doesn't need to have any
content. When poll() returns readable, the VM has been forked. Then
userspace rngs and other things like that can call getrandom() to
receive a fresh value to mix into whatever their operation is. Since
all we're talking about here is _event notification_, all we need is
that event, which is what poll() provides.

I'm also not a super big fan of putting all that logic into systemd. It
means applications need to create their own notification mechanisms to
pass that cloning notification into actual processes. Don't we have any
mechanism that applications and libraries could use to natively get an
event when the GenID changes?
Yes. poll() can do this. For the purposes of discussion, I've posted
an implementation of this idea here:
https://lore.kernel.org/lkml/20220309215907.77526-1-Jason@xxxxxxxxx/

What I'm sort of leaning toward is doing something like that patch,
and then later if vmgenid ever grows an additional word-sized counter,
moving to explore the race-free approach. Given the amount of
programming required to actually implement the race-free approach
(transactions and careful study of each case), the poll() file
approach might be a medium-grade compromise for the time being.
Evidently that's what Microsoft decided too.


I agree on the slightly racy compromise and that it's a step into the right direction. Doing this is a no brainer IMHO and I like the proc based poll approach.

I have an additional problem you might have an idea for with the poll based path. In addition to the clone notification, I'd need to know at which point everyone who was listening to a clone notification is finished acting up it. If I spawn a tiny VM to do "work", I want to know when it's safe to hand requests into it. How do I find out when that point in time is?

As far as the race-free approach goes, I wouldn't get hung up on 4 byte vs 16 byte UUID to match against. Outside of FUD that this might potentially have performance impact (4 byte reads will have impact too!), there's nothing that would keep us from implementing that interface in addition to the poll today.

I'm happy to see all of these things evolve incrementally though. We can start with the poll interface and then later implement a vsyscall that allows transactions in hot paths.


Alex




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