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Date:   Tue, 12 Jan 2021 14:15:58 +0200
From:   Adrian Catangiu <>
To:     <>, <>,
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CC:     <>, <>, <>,
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        <>, Adrian Catangiu <>
Subject: [PATCH v4 0/2] System Generation ID driver and VMGENID backend

This feature is aimed at virtualized or containerized environments
where VM or container snapshotting duplicates memory state, which is a
challenge for applications that want to generate unique data such as
request IDs, UUIDs, and cryptographic nonces.

The patch set introduces a mechanism that provides a userspace
interface for applications and libraries to be made aware of uniqueness
breaking events such as VM or container snapshotting, and allow them to
react and adapt to such events.

Solving the uniqueness problem strongly enough for cryptographic
purposes requires a mechanism which can deterministically reseed
userspace PRNGs with new entropy at restore time. This mechanism must
also support the high-throughput and low-latency use-cases that led
programmers to pick a userspace PRNG in the first place; be usable by
both application code and libraries; allow transparent retrofitting
behind existing popular PRNG interfaces without changing application
code; it must be efficient, especially on snapshot restore; and be
simple enough for wide adoption.

The first patch in the set implements a device driver which exposes a
read-only device /dev/sysgenid to userspace, which contains a
monotonically increasing u32 generation counter. Libraries and
applications are expected to open() the device, and then call read()
which blocks until the SysGenId changes. Following an update, read()
calls no longer block until the application acknowledges the new
SysGenId by write()ing it back to the device. Non-blocking read() calls
return EAGAIN when there is no new SysGenId available. Alternatively,
libraries can mmap() the device to get a single shared page which
contains the latest SysGenId at offset 0.

SysGenId also supports a notification mechanism exposed as two IOCTLs
on the device. SYSGENID_GET_OUTDATED_WATCHERS immediately returns the
number of file descriptors to the device that were open during the last
SysGenId change but have not yet acknowledged the new id.
SYSGENID_WAIT_WATCHERS blocks until there are no open file handles on
the device which haven’t acknowledged the new id. These two interfaces
are intended for serverless and container control planes, which want to
confirm that all application code has detected and reacted to the new
SysGenId before sending an invoke to the newly-restored sandbox.

The second patch in the set adds a VmGenId driver which makes use of
the ACPI vmgenid device to drive SysGenId and to reseed kernel entropy
on VM snapshots.


v3 -> v4:

  - split functionality in two separate kernel modules: 
    1. drivers/misc/sysgenid.c which provides the generic userspace
       interface and mechanisms
    2. drivers/virt/vmgenid.c as VMGENID acpi device driver that seeds
       kernel entropy and acts as a driving backend for the generic
  - renamed /dev/vmgenid -> /dev/sysgenid
  - renamed uapi header file vmgenid.h -> sysgenid.h
  - renamed ioctls VMGENID_* -> SYSGENID_*
  - added ‘min_gen’ parameter to SYSGENID_FORCE_GEN_UPDATE ioctl
  - fixed races in documentation examples
  - various style nits
  - rebased on top of linus latest

v2 -> v3:

  - separate the core driver logic and interface, from the ACPI device.
    The ACPI vmgenid device is now one possible backend.
  - fix issue when timeout=0 in VMGENID_WAIT_WATCHERS
  - add locking to avoid races between fs ops handlers and hw irq
    driven generation updates
  - change VMGENID_WAIT_WATCHERS ioctl so if the current caller is
    outdated or a generation change happens while waiting (thus making
    current caller outdated), the ioctl returns -EINTR to signal the
    user to handle event and retry. Fixes blocking on oneself.
  - add VMGENID_FORCE_GEN_UPDATE ioctl conditioned by
    CAP_CHECKPOINT_RESTORE capability, through which software can force
    generation bump.

v1 -> v2:

  - expose to userspace a monotonically increasing u32 Vm Gen Counter
    instead of the hw VmGen UUID
  - since the hw/hypervisor-provided 128-bit UUID is not public
    anymore, add it to the kernel RNG as device randomness
  - insert driver page containing Vm Gen Counter in the user vma in
    the driver's mmap handler instead of using a fault handler
  - turn driver into a misc device driver to auto-create /dev/vmgenid
  - change ioctl arg to avoid leaking kernel structs to userspace
  - update documentation
  - various nits
  - rebase on top of linus latest

Adrian Catangiu (2):
  drivers/misc: sysgenid: add system generation id driver
  drivers/virt: vmgenid: add vm generation id driver

 Documentation/misc-devices/sysgenid.rst | 240 +++++++++++++++++++++++++
 Documentation/virt/vmgenid.rst          |  34 ++++
 drivers/misc/Kconfig                    |  16 ++
 drivers/misc/Makefile                   |   1 +
 drivers/misc/sysgenid.c                 | 298 ++++++++++++++++++++++++++++++++
 drivers/virt/Kconfig                    |  14 ++
 drivers/virt/Makefile                   |   1 +
 drivers/virt/vmgenid.c                  | 153 ++++++++++++++++
 include/uapi/linux/sysgenid.h           |  18 ++
 9 files changed, 775 insertions(+)
 create mode 100644 Documentation/misc-devices/sysgenid.rst
 create mode 100644 Documentation/virt/vmgenid.rst
 create mode 100644 drivers/misc/sysgenid.c
 create mode 100644 drivers/virt/vmgenid.c
 create mode 100644 include/uapi/linux/sysgenid.h


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