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

Alexander Graf
2022-02-24T08:54:21Z

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

Looking at the issue again, it seems like we completely missed to follow
up with a PR to implement that functionality :(.

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?

Alex

>
> Cc: dwmw@amazon.co.uk
> Cc: acatan@amazon.com
> Cc: graf@amazon.com
> Cc: colmmacc@amazon.com
> Cc: sblbir@amazon.com
> Cc: raduweis@amazon.com
> Cc: jannh@google.com
> Cc: gregkh@linuxfoundation.org
> Cc: tytso@mit.edu
>
> Jason A. Donenfeld (2):
> random: add mechanism for VM forks to reinitialize crng
> drivers/virt: add vmgenid driver for reinitializing RNG
>
> drivers/char/random.c | 58 ++++++++++++++++++
> drivers/virt/Kconfig | 8 +++
> drivers/virt/Makefile | 1 +
> drivers/virt/vmgenid.c | 133 +++++++++++++++++++++++++++++++++++++++++
> include/linux/random.h | 1 +
> 5 files changed, 201 insertions(+)
> create mode 100644 drivers/virt/vmgenid.c
>
> --
> 2.35.1
>

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