docs: place all our markdown docs in rough categories

[thirdparty/systemd.git] / docs / RANDOM_SEEDS.md

---
title: Random Seeds
category: Concepts
---

# Random Seeds

systemd can help in a number of ways with providing reliable, high quality
random numbers from early boot on.

## Linux Kernel Entropy Pool

Today's computer systems require random number generators for numerous
cryptographic and other purposes. On Linux systems, the kernel's entropy pool
is typically used as high-quality source of random numbers. The kernel's
entropy pool combines various entropy inputs together, mixes them and provides
an API to userspace as well as to internal kernel subsystems to retrieve
it. This entropy pool needs to be initialized with a minimal level of entropy
before it can provide high quality, cryptographic random numbers to
applications. Until the entropy pool is fully initialized application requests
for high-quality random numbers cannot be fulfilled.

The Linux kernel provides three relevant userspace APIs to request random data
from the kernel's entropy pool:

* The [`getrandom()`](http://man7.org/linux/man-pages/man2/getrandom.2.html)
  system call with its `flags` parameter set to 0. If invoked the calling
  program will synchronously block until the random pool is fully initialized
  and the requested bytes can be provided.

* The `getrandom()` system call with its `flags` parameter set to
  `GRND_NONBLOCK`. If invoked the request for random bytes will fail if the
  pool is not initialized yet.

* Reading from the
  [`/dev/urandom`](http://man7.org/linux/man-pages/man4/urandom.4.html)
  pseudo-device will always return random bytes immediately, even if the pool
  is not initialized. The provided random bytes will be of low quality in this
  case however. Moreover the kernel will log about all programs using this
  interface in this state, and which thus potentially rely on an uninitialized
  entropy pool.

(Strictly speaking there are more APIs, for example `/dev/random`, but these
should not be used by almost any application and hence aren't mentioned here.)

Note that the time it takes to initialize the random pool may differ between
systems. If local hardware random number generators are available,
initialization is likely quick, but particularly in embedded and virtualized
environments available entropy is small and thus random pool initialization
might take a long time (up to tens of minutes!).

Modern hardware tends to come with a number of hardware random number
generators (hwrng), that may be used to relatively quickly fill up the entropy
pool. Specifically:

* All recent Intel and AMD CPUs provide the CPU opcode
  [RDRAND](https://en.wikipedia.org/wiki/RdRand) to acquire random bytes. Linux
  includes random bytes generated this way in its entropy pool, but didn't use
  to credit entropy for it (i.e. data from this source wasn't considered good
  enough to consider the entropy pool properly filled even though it was
  used). This has changed recently however, and most big distributions have
  turned on the `CONFIG_RANDOM_TRUST_CPU=y` kernel compile time option. This
  means systems with CPUs supporting this opcode will be able to very quickly
  reach the "pool filled" state.

* The TPM security chip that is available on all modern desktop systems has a
  hwrng. It is also fed into the entropy pool, but generally not credited
  entropy. You may use `rng_core.default_quality=1000` on the kernel command
  line to change that, but note that this is a global setting affect all
  hwrngs. (Yeah, that's weird.)

* Many Intel and AMD chipsets have hwrng chips. Their Linux drivers usually
  don't credit entropy. (But there's `rng_core.default_quality=1000`, see
  above.)

* Various embedded boards have hwrng chips. Some drivers automatically credit
  entropy, others do not. Some WiFi chips appear to have hwrng sources too, and
  they usually do not credit entropy for them.

* `virtio-rng` is used in virtualized environments and retrieves random data
  from the VM host. It credits full entropy.

* The EFI firmware typically provides a RNG API. When transitioning from UEFI
  to kernel mode Linux will query some random data through it, and feed it into
  the pool, but not credit entropy to it. What kind of random source is behind
  the EFI RNG API is often not entirely clear, but it hopefully is some kind of
  hardware source.

If neither of these are available (in fact, even if they are), Linux generates
entropy from various non-hwrng sources in various subsystems, all of which
ultimately are rooted in IRQ noise, a very "slow" source of entropy, in
particular in virtualized environments.

## `systemd`'s Use of Random Numbers

systemd is responsible for bringing up the OS. It generally runs as the first
userspace process the kernel invokes. Because of that it runs at a time where
the entropy pool is typically not yet initialized, and thus requests to acquire
random bytes will either be delayed, will fail or result in a noisy kernel log
message (see above).

Various other components run during early boot that require random bytes. For
example, initial RAM disks nowadays communicate with encrypted networks or
access encrypted storage which might need random numbers. systemd itself
requires random numbers as well, including for the following uses:

* systemd assigns 'invocation' UUIDs to all services it invokes that uniquely
  identify each invocation. This is useful retain a global handle on a specific
  service invocation and relate it to other data. For example, log data
  collected by the journal usually includes the invocation UUID and thus the
  runtime context the service manager maintains can be neatly matched up with
  the log data a specific service invocation generated. systemd also
  initializes `/etc/machine-id` with a randomized UUID. (systemd also makes use
  of the randomized "boot id" the kernel exposes in
  `/proc/sys/kernel/random/boot_id`). These UUIDs are exclusively Type 4 UUIDs,
  i.e. randomly generated ones.

* systemd maintains various hash tables internally. In order to harden them
  against [collision
  attacks](https://rt.perl.org/Public/Bug/Display.html?CSRF_Token=165691af9ddaa95f653402f1b68de728)
  they are seeded with random numbers.

* At various places systemd needs random bytes for temporary file name
  generation, UID allocation randomization, and similar.

* systemd-resolved and systemd-networkd use random number generators to harden
  the protocols they implement against packet forgery.

* systemd-udevd and systemd-nspawn can generate randomized MAC addresses for
  network devices.

Note that these cases generally do not require a cryptographic-grade random
number generator, as most of these utilize random numbers to minimize risk of
collision and not to generate secret key material. However, they usually do
require "medium-grade" random data. For example: systemd's hash-maps are
reseeded if they grow beyond certain thresholds (and thus collisions are more
likely). This means they are generally fine with low-quality (even constant)
random numbers initially as long as they get better with time, so that
collision attacks are eventually thwarted as better, non-guessable seeds are
acquired.

## Keeping `systemd'`s Demand on the Kernel Entropy Pool Minimal

Since most of systemd's own use of random numbers do not require
cryptographic-grade RNGs, it tries to avoid reading entropy from the kernel
entropy pool if possible. If it succeeds this has the benefit that there's no
need to delay the early boot process until entropy is available, and noisy
kernel log messages about early reading from `/dev/urandom` are avoided
too. Specifically:

1. When generating [Type 4
   UUIDs](https://en.wikipedia.org/wiki/Universally_unique_identifier#Version_4_\(random\)),
   systemd tries to use Intel's and AMD's RDRAND CPU opcode directly, if
   available. While some doubt the quality and trustworthiness of the entropy
   provided by these opcodes, they should be good enough for generating UUIDs,
   if not key material (though, as mentioned, today's big distributions opted
   to trust it for that too, now, see above — but we are not going to make that
   decision for you, and for anything key material related will only use the
   kernel's entropy pool). If RDRAND is not available or doesn't work, it will
   use synchronous `getrandom()` as fallback, and `/dev/urandom` on old kernels
   where that system call doesn't exist yet. This means on non-Intel/AMD
   systems UUID generation will block on kernel entropy initialization.

2. For seeding hash tables, and all the other similar purposes systemd first
   tries RDRAND, and if that's not available will try to use asynchronous
   `getrandom()` (if the kernel doesn't support this system call,
   `/dev/urandom` is used). This may fail too in case the pool is not
   initialized yet, in which case it will fall back to glibc's internal rand()
   calls, i.e. weak pseudo-random numbers. This should make sure we use good
   random bytes if we can, but neither delay boot nor trigger noisy kernel log
   messages during early boot for these use-cases.

## `systemd`'s Support for Filling the Kernel Entropy Pool

systemd has various provisions to ensure the kernel entropy is filled during
boot, in order to ensure the entropy pool is filled up quickly.

1. When systemd's PID 1 detects it runs in a virtualized environment providing
   the `virtio-rng` interface it will load the necessary kernel modules to make
   use of it during earliest boot, if possible — much earlier than regular
   kernel module loading done by `systemd-udevd.service`. This should ensure
   that in VM environments the entropy pool is quickly filled, even before
   systemd invokes the first service process — as long as the VM environment
   provides virtualized RNG hardware (and VM environments really should!).

2. The
   [`systemd-random-seed.service`](https://www.freedesktop.org/software/systemd/man/systemd-random-seed.service.html)
   system service will load a random seed from `/var/lib/systemd/random-seed`
   into the kernel entropy pool. By default it does not credit entropy for it
   though, since the seed is — more often than not — not reset when 'golden'
   master images of an OS are created, and thus replicated into every
   installation. If OS image builders carefully reset the random seed file
   before generating the image it should be safe to credit entropy, which can
   be enabled by setting the `$SYSTEMD_RANDOM_SEED_CREDIT` environment variable
   for the service to `1` (or even `force`, see man page). Note however, that
   this service typically runs relatively late during early boot: long after
   the initial RAM disk (`initrd`) completed, and after the `/var/` file system
   became writable. This is usually too late for many applications, it is hence
   not advised to rely exclusively on this functionality to seed the kernel's
   entropy pool. Also note that this service synchronously waits until the
   kernel's entropy pool is initialized before completing start-up. It may thus
   be used by other services as synchronization point to order against, if they
   require an initialized entropy pool to operate correctly.

3. The
   [`systemd-boot`](https://www.freedesktop.org/software/systemd/man/systemd-boot.html)
   EFI boot loader included in systemd is able to maintain and provide a random
   seed stored in the EFI System Partition (ESP) to the booted OS, which allows
   booting up with a fully initialized entropy pool from earliest boot
   on. During installation of the boot loader (or when invoking [`bootctl
   random-seed`](https://www.freedesktop.org/software/systemd/man/bootctl.html#random-seed))
   a seed file with an initial seed is placed in a file `/loader/random-seed`
   in the ESP. In addition, an identically sized randomized EFI variable called
   the the 'system token' is set, which is written to the machine's firmware
   NVRAM. During boot, when `systemd-boot` finds both the random seed file and
   the system token they are combined and hashed with SHA256 (in counter mode,
   to generate sufficient data), to generate a new random seed file to store in
   the ESP as well as a random seed to pass to the OS kernel. The new random
   seed file for the ESP is then written to the ESP, ensuring this is completed
   before the OS is invoked. Very early during initialization PID 1 will read
   the random seed provided in the EFI variable and credit it fully to the
   kernel's entropy pool.

   This mechanism is able to safely provide an initialized entropy pool already
   in the `initrd` and guarantees that different seeds are passed from the boot
   loader to the OS on every boot (in a way that does not allow regeneration of
   an old seed file from a new seed file). Moreover, when an OS image is
   replicated between multiple images and the random seed is not reset, this
   will still result in different random seeds being passed to the OS, as the
   per-machine 'system token' is specific to the physical host, and not
   included in OS disk images. If the 'system token' is properly initialized
   and kept sufficiently secret it should not be possible to regenerate the
   entropy pool of different machines, even if this seed is the only source of
   entropy.

   Note that the writes to the ESP needed to maintain the random seed should be
   minimal. The size of the random seed file is directly derived from the Linux
   kernel's entropy pool size, which defaults to 512 bytes. This means updating
   the random seed in the ESP should be doable safely with a single sector
   write (since hard-disk sectors typically happen to be 512 bytes long, too),
   which should be safe even with FAT file system drivers built into
   low-quality EFI firmwares.

   As a special restriction: in virtualized environments PID 1 will refrain
   from using this mechanism, for safety reasons. This is because on VM
   environments the EFI variable space and the disk space is generally not
   maintained physically separate (for example, `qemu` in EFI mode stores the
   variables in the ESP itself). The robustness towards sloppy OS image
   generation is the main purpose of maintaining the 'system token' however,
   and if the EFI variable storage is not kept physically separate from the OS
   image there's no point in it. That said, OS builders that know that they are
   not going to replicate the built image on multiple systems may opt to turn
   off the 'system token' concept by setting `random-seed-mode always` in the
   ESP's
   [`/loader/loader.conf`](https://www.freedesktop.org/software/systemd/man/loader.conf.html)
   file. If done, `systemd-boot` will use the random seed file even if no
   system token is found in EFI variables.

With the three mechanisms described above it should be possible to provide
early-boot entropy in most cases. Specifically:

1. On EFI systems, `systemd-boot`'s random seed logic should make sure good
   entropy is available during earliest boot — as long as `systemd-boot` is
   used as boot loader, and outside of virtualized environments.

2. On virtualized systems, the early `virtio-rng` hookup should ensure entropy
   is available early on — as long as the VM environment provides virtualized
   RNG devices, which they really should all do in 2019. Complain to your
   hosting provider if they don't.

3. On Intel/AMD systems systemd's own reliance on the kernel entropy pool is
   minimal (as RDRAND is used on those for UUID generation). This only works if
   the CPU has RDRAND of course, which most physical CPUs do (but I hear many
   virtualized CPUs do not. Pity.)

4. In all other cases, `systemd-random-seed.service` will help a bit, but — as
   mentioned — is too late to help with early boot.

This primarily leaves two kind of systems in the cold:

1. Some embedded systems. Many embedded chipsets have hwrng functionality these
   days. Consider using them while crediting
   entropy. (i.e. `rng_core.default_quality=1000` on the kernel command line is
   your friend). Or accept that the system might take a bit longer to
   boot. Alternatively, consider implementing a solution similar to
   systemd-boot's random seed concept in your platform's boot loader.

2. Virtualized environments that lack both virtio-rng and RDRAND. Tough
   luck. Talk to your hosting provider, and ask them to fix this.

3. Also note: if you deploy an image without any random seed and/or without
   installing any 'system token' in an EFI variable, as described above, this
   means that on the first boot no seed can be passed to the OS
   either. However, as the boot completes (with entropy acquired elsewhere),
   systemd will automatically install both a random seed in the GPT and a
   'system token' in the EFI variable space, so that any future boots will have
   entropy from earliest boot on — all provided `systemd-boot` is used.

## Frequently Asked Questions

1. *Why don't you just use getrandom()? That's all you need!*

   Did you read any of the above? getrandom() is hooked to the kernel entropy
   pool, and during early boot it's not going to be filled yet, very likely. We
   do use it in many cases, but not in all. Please read the above again!

2. *Why don't you use
   [getentropy()](http://man7.org/linux/man-pages/man3/getentropy.3.html)? That's
   all you need!*

   Same story. That call is just a different name for `getrandom()` with
   `flags` set to zero, and some additional limitations, and thus it also needs
   the kernel's entropy pool to be initialized, which is the whole problem we
   are trying to address here.

3. *Why don't you generate your UUIDs with
   [`uuidd`](http://man7.org/linux/man-pages/man8/uuidd.8.html)? That's all you
   need!*

   First of all, that's a system service, i.e. something that runs as "payload"
   of systemd, long after systemd is already up and hence can't provide us
   UUIDs during earliest boot yet. Don't forget: to assign the invocation UUID
   for the `uuidd.service` start we already need a UUID that the service is
   supposed to provide us. More importantly though, `uuidd` needs state/a random
   seed/a MAC address/host ID to operate, all of which are not available during
   early boot.

4. *Why don't you generate your UUIDs with `/proc/sys/kernel/random/uuid`?
   That's all you need!*

   This is just a different, more limited interface to `/dev/urandom`. It gains
   us nothing.

5. *Why don't you use [`rngd`](https://github.com/nhorman/rng-tools),
   [`haveged`](http://www.issihosts.com/haveged/),
   [`egd`](http://egd.sourceforge.net/)? That's all you need!*

   Like `uuidd` above these are system services, hence come too late for our
   use-case. In addition much of what `rngd` provides appears to be equivalent
   to `CONFIG_RANDOM_TRUST_CPU=y` or `rng_core.default_quality=1000`, except
   being more complex and involving userspace. These services partly measure
   system behavior (such as scheduling effects) which the kernel either
   already feeds into its pool anyway (and thus shouldn't be fed into it a
   second time, crediting entropy for it a second time) or is at least
   something the kernel could much better do on its own. Hence, if what these
   daemons do is still desirable today, this would be much better implemented
   in kernel (which would be very welcome of course, but wouldn't really help
   us here in our specific problem, see above).

6. *Why don't you use [`arc4random()`](https://man.openbsd.org/arc4random.3)?
   That's all you need!*

   This doesn't solve the issue, since it requires a nonce to start from, and
   it gets that from `getrandom()`, and thus we have to wait for random pool
   initialization the same way as calling `getrandom()`
   directly. `arc4random()` is nothing more than optimization, in fact it
   implements similar algorithms that the kernel entropy pool implements
   anyway, hence besides being able to provide random bytes with higher
   throughput there's little it gets us over just using `getrandom()`. Also,
   it's not supported by glibc. And as long as that's the case we are not keen
   on using it, as we'd have to maintain that on our own, and we don't want to
   maintain our own cryptographic primitives if we don't have to. Since
   systemd's uses are not performance relevant (besides the pool initialization
   delay, which this doesn't solve), there's hence little benefit for us to
   call these functions. That said, if glibc learns these APIs one day, we'll
   certainly make use of them where appropriate.

7. *This is boring: NetBSD had [boot loader entropy seed
   support](https://netbsd.gw.com/cgi-bin/man-cgi?boot+8) since ages!*

   Yes, NetBSD has that, and the above is inspired by that (note though: this
   article is about a lot more than that). NetBSD's support is not really safe,
   since it neither updates the random seed before using it, nor has any
   safeguards against replicating the same disk image with its random seed on
   multiple machines (which the 'system token' mentioned above is supposed to
   address). This means reuse of the same random seed by the boot loader is
   much more likely.

8. *Why does PID 1 upload the boot loader provided random seed into kernel
   instead of kernel doing that on its own?*

   That's a good question. Ideally the kernel would do that on its own, and we
   wouldn't have to involve userspace in this.

9. *What about non-EFI?*

   The boot loader random seed logic described above uses EFI variables to pass
   the seed from the boot loader to the OS. Other systems might have similar
   functionality though, and it shouldn't be too hard to implement something
   similar for them. Ideally, we'd have an official way to pass such a seed as
   part of the `struct boot_params` from the boot loader to the kernel, but
   this is currently not available.

10. *I use a different boot loader than `systemd-boot`, I'd like to use boot
    loader random seeds too!*

    Well, consider just switching to `systemd-boot`, it's worth it. See
    [systemd-boot(7)](https://www.freedesktop.org/software/systemd/man/systemd-boot.html)
    for an introduction why. That said, any boot loader can re-implement the
    logic described above, and can pass a random seed that systemd as PID 1
    will then upload into the kernel's entropy pool. For details see the [Boot
    Loader Interface](https://systemd.io/BOOT_LOADER_INTERFACE) documentation.

11. *Why not pass the boot loader random seed via kernel command line instead
    of as EFI variable?*

    The kernel command line is accessible to unprivileged processes via
    `/proc/cmdline`. It's not desirable if unprivileged processes can use this
    information to possibly gain too much information about the current state
    of the kernel's entropy pool.

12. *Why doesn't `systemd-boot` rewrite the 'system token' too each time
    when updating the random seed file stored in the ESP?*

    The system token is stored as persistent EFI variable, i.e. in some form of
    NVRAM. These memory chips tend be of low quality in many machines, and
    hence we shouldn't write them too often. Writing them once during
    installation should generally be OK, but rewriting them on every single
    boot would probably wear the chip out too much, and we shouldn't risk that.