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