PEP 524 – Make os.urandom() blocking on Linux
- Victor Stinner <vstinner at python.org>
- Standards Track
Table of Contents
- The bug
- Use Cases
- Fix system urandom
- Denial-of-service when reading random
- Examples using os.getrandom()
os.urandom() to block on Linux 3.17 and newer until the OS
urandom is initialized to increase the security.
Add also a new
os.getrandom() function (for Linux and Solaris) to be
able to choose how to handle when
os.urandom() is going to block on
Python 3.5.0 was enhanced to use the new
introduced in Linux 3.17 and Solaris 11.3. The problem is that users
started to complain that Python 3.5 blocks at startup on Linux in
virtual machines and embedded devices: see issues #25420 and #26839.
getrandom(0) blocks until the kernel initialized urandom
with 128 bits of entropy. The issue #25420 describes a Linux build
platform blocking at
import random. The issue #26839 describes a
short Python script used to compute a MD5 hash, systemd-cron, script
called very early in the init process. The system initialization blocks
on this script which blocks on
getrandom(0) to initialize Python.
The Python initialization requires random bytes to implement a counter-measure against the hash denial-of-service (hash DoS), see:
random module creates an instance of
random._inst. On Python 3.5, random.Random
constructor reads 2500 bytes from
os.urandom() to seed a Mersenne
Twister RNG (random number generator).
Other platforms may be affected by this bug, but in practice, only Linux systems use Python scripts to initialize the system.
Status in Python 3.5.2
Python 3.5.2 behaves like Python 2.7 and Python 3.4. If the system
urandom is not initialized, the startup does not block, but
os.urandom() can return low-quality entropy (even it is not easily
The following use cases are used to help to choose the right compromise between security and practicability.
Use Case 1: init script
Use a Python 3 script to initialize the system, like systemd-cron. If the script blocks, the system initialize is stuck too. The issue #26839 is a good example of this use case.
Use case 1.1: No secret needed
If the init script doesn’t have to generate any secure secret, this use case is already handled correctly in Python 3.5.2: Python startup doesn’t block on system urandom anymore.
Use case 1.2: Secure secret required
If the init script has to generate a secure secret, there is no safe solution.
Falling back to weak entropy is not acceptable, it would reduce the security of the program.
Python cannot produce itself secure entropy, it can only wait until system urandom is initialized. But in this use case, the whole system initialization is blocked by this script, so the system fails to boot.
The real answer is that the system initialization must not be blocked by such script. It is ok to start the script very early at system initialization, but the script may blocked a few seconds until it is able to generate the secret.
Reminder: in some cases, the initialization of the system urandom never occurs and so programs waiting for system urandom blocks forever.
Use Case 2: Web server
Run a Python 3 web server serving web pages using HTTP and HTTPS protocols. The server is started as soon as possible.
The first target of the hash DoS attack was web server: it’s important that the hash secret cannot be easily guessed by an attacker.
If serving a web page needs a secret to create a cookie, create an encryption key, …, the secret must be created with good entropy: again, it must be hard to guess the secret.
A web server requires security. If a choice must be made between security and running the server with weak entropy, security is more important. If there is no good entropy: the server must block or fail with an error.
The question is if it makes sense to start a web server on a host before system urandom is initialized.
The issues #25420 and #26839 are restricted to the Python startup, not to generate a secret before the system urandom is initialized.
Fix system urandom
Load entropy from disk at boot
Collecting entropy can take up to several minutes. To accelerate the system initialization, operating systems store entropy on disk at shutdown, and then reload entropy from disk at the boot.
If a system collects enough entropy at least once, the system urandom will be initialized quickly, as soon as the entropy is reloaded from disk.
Virtual machines don’t have a direct access to the hardware and so have less sources of entropy than bare metal. A solution is to add a virtio-rng device to pass entropy from the host to the virtual machine.
A solution for embedded devices is to plug an hardware RNG.
For example, Raspberry Pi have an hardware RNG but it’s not used by default. See: Hardware RNG on Raspberry Pi.
Denial-of-service when reading random
Don’t use /dev/random but /dev/urandom
/dev/random device should only used for very specific use cases.
/dev/random on Linux is likely to block. Users don’t
like when an application blocks longer than 5 seconds to generate a
secret. It is only expected for specific cases like generating
explicitly an encryption key.
When the system has no available entropy, choosing between blocking until entropy is available or falling back on lower quality entropy is a matter of compromise between security and practicability. The choice depends on the use case.
/dev/urandom is secure, it should be used instead of
/dev/random. See Myths about /dev/urandom by Thomas Hühn: “Fact:
/dev/urandom is the preferred source of cryptographic randomness on
getrandom(size, 0) can block forever on Linux
The origin of the Python issue #26839 is the Debian bug
report #822431: in fact,
getrandom(size, 0) blocks forever on the virtual machine. The system
succeeded to boot because systemd killed the blocked process after 90
Solutions like Load entropy from disk at boot reduces the risk of this bug.
On Linux, reading the
/dev/urandom can return “weak” entropy before
urandom is fully initialized, before the kernel collected 128 bits of
entropy. Linux 3.17 adds a new
getrandom() syscall which allows to
block until urandom is initialized.
On Python 3.5.2, os.urandom() uses the
getrandom(size, GRND_NONBLOCK), but falls back on reading the
Security experts promotes
os.urandom() to generate cryptographic
keys because it is implemented with a Cryptographically secure
pseudo-random number generator (CSPRNG).
By the way,
os.urandom() is preferred over
This PEP proposes to modify os.urandom() to use
blocking mode to not return weak entropy, but also ensure that Python
will not block at startup.
Make os.urandom() blocking on Linux
All changes described in this section are specific to the Linux platform.
- Modify os.urandom() to block until system urandom is initialized:
_PyOS_URandom()) is modified to always call
getrandom(size, 0)(blocking mode) on Linux and Solaris.
- Add a new private
_PyOS_URandom_Nonblocking()function: try to call
getrandom(size, GRND_NONBLOCK)on Linux and Solaris, but falls back on reading
/dev/urandomif it fails with
- Initialize hash secret from non-blocking system urandom:
_PyRandom_Init()is modified to call
random.Randomconstructor now uses non-blocking system urandom: it is modified to use internally the new
_PyOS_URandom_Nonblocking()function to seed the RNG.
Add a new os.getrandom() function
os.getrandom(size, flags=0) function is added: use
getrandom() syscall on Linux and
getrandom() C function on
The function comes with 2 new flags:
os.GRND_RANDOM: read bytes from
/dev/randomrather than reading
os.GRND_NONBLOCK: raise a BlockingIOError if
os.getrandom() is a thin wrapper on the
syscall/C function and so inherit of its behaviour. For example, on
Linux, it can return less bytes than requested if the syscall is
interrupted by a signal.
Examples using os.getrandom()
Example of a portable non-blocking RNG function: try to get random bytes from the OS urandom, or fallback on the random module.
def best_effort_rng(size): # getrandom() is only available on Linux and Solaris if not hasattr(os, 'getrandom'): return os.urandom(size) result = bytearray() try: # need a loop because getrandom() can return less bytes than # requested for different reasons while size: data = os.getrandom(size, os.GRND_NONBLOCK) result += data size -= len(data) except BlockingIOError: # OS urandom is not initialized yet: # fallback on the Python random module data = bytes(random.randrange(256) for byte in range(size)) result += data return bytes(result)
This function can block in theory on a platform where
os.getrandom() is not available but
os.urandom() can block.
Example of function waiting timeout seconds until the OS urandom is initialized on Linux or Solaris:
def wait_for_system_rng(timeout, interval=1.0): if not hasattr(os, 'getrandom'): return deadline = time.monotonic() + timeout while True: try: os.getrandom(1, os.GRND_NONBLOCK) except BlockingIOError: pass else: return if time.monotonic() > deadline: raise Exception('OS urandom not initialized after %s seconds' % timeout) time.sleep(interval)
This function is not portable. For example,
os.urandom() can block
on FreeBSD in theory, at the early stage of the system initialization.
Create a best-effort RNG
Simpler example to create a non-blocking RNG on Linux: choose between
Random.Random depending if
getrandom(size) would block.
def create_nonblocking_random(): if not hasattr(os, 'getrandom'): return random.Random() try: os.getrandom(1, os.GRND_NONBLOCK) except BlockingIOError: return random.Random() else: return random.SystemRandom()
This function is not portable. For example,
can block on FreeBSD in theory, at the early stage of the system
Leave os.urandom() unchanged, add os.getrandom()
os.urandom() remains unchanged: never block, but it can return weak entropy if system urandom is not initialized yet.
Only add the new
os.getrandom() function (wrapper to the
getrandom() syscall/C function).
secrets.token_bytes() function should be used to write portable
The problem with this change is that it expects that users understand
well security and know well each platforms. Python has the tradition of
hiding “implementation details”. For example,
os.urandom() is not a
thin wrapper to the
/dev/urandom device: it uses
CryptGenRandom() on Windows, it uses
getentropy() on OpenBSD, it
getrandom() on Linux and Solaris or falls back on reading
/dev/urandom. Python already uses the best available system RNG
depending on the platform.
This PEP does not change the API:
random.SystemRandom) for all other usages
Raise BlockingIOError in os.urandom()
PEP 522: Allow BlockingIOError in security sensitive APIs on Linux.
Python should not decide for the developer how to handle The bug:
raising immediately a
os.urandom() is going to
block allows developers to choose how to handle this case:
- catch the exception and falls back to a non-secure entropy source:
/dev/urandomon Linux, use the Python
randommodule (which is not secure at all), use time, use process identifier, etc.
- don’t catch the error, the whole program fails with this fatal exception
More generally, the exception helps to notify when sometimes goes wrong.
The application can emit a warning when it starts to wait for
For the use case 2 (web server), falling back on non-secure entropy is
not acceptable. The application must handle
os.urandom() until it completes. Example:
def secret(n=16): try: return os.urandom(n) except BlockingIOError: pass print("Wait for system urandom initialization: move your " "mouse, use your keyboard, use your disk, ...") while 1: # Avoid busy-loop: sleep 1 ms time.sleep(0.001) try: return os.urandom(n) except BlockingIOError: pass
For correctness, all applications which must generate a secure secret
must be modified to handle
BlockingIOError even if The bug is
The case of applications using
os.urandom() but don’t really require
security is not well defined. Maybe these applications should not use
os.urandom() at the first place, but always the non-blocking
random module. If
os.urandom() is used for security, we are back
to the use case 2 described above: Use Case 2: Web server. If a
developer doesn’t want to drop
os.urandom(), the code should be
def almost_secret(n=16): try: return os.urandom(n) except BlockingIOError: return bytes(random.randrange(256) for byte in range(n))
The question is if The bug is common enough to require that so many applications have to be modified.
Another simpler choice is to refuse to start before the system urandom is initialized:
def secret(n=16): try: return os.urandom(n) except BlockingIOError: print("Fatal error: the system urandom is not initialized") print("Wait a bit, and rerun the program later.") sys.exit(1)
Compared to Python 2.7, Python 3.4 and Python 3.5.2 where os.urandom() never blocks nor raise an exception on Linux, such behaviour change can be seen as a major regression.
Add an optional block parameter to os.urandom()
See the issue #27250: Add os.urandom_block().
Add an optional block parameter to os.urandom(). The default value may
True (block by default) or
The first technical issue is to implement
all platforms. Only Linux 3.17 (and newer) and Solaris 11.3 (and newer)
have a well defined non-blocking API (
As Raise BlockingIOError in os.urandom(), it doesn’t seem worth it to make the API more complex for a theoretical (or at least very rare) use case.
As Leave os.urandom() unchanged, add os.getrandom(), the problem is that it makes the API more complex and so more error-prone.
The PEP was accepted on 2016-08-08 by Guido van Rossum.
Operating system random functions
os.urandom() uses the following functions:
- OpenBSD: getentropy() (OpenBSD 5.6)
- Linux: getrandom() (Linux 3.17) – see also A system call for random numbers: getrandom()
- Solaris: getentropy(), getrandom() (both need Solaris 11.3)
- UNIX, BSD: /dev/urandom, /dev/random
- Windows: CryptGenRandom() (Windows XP)
On Linux, commands to get the status of
/dev/random (results are
number of bytes):
$ cat /proc/sys/kernel/random/entropy_avail 2850 $ cat /proc/sys/kernel/random/poolsize 4096
Why using os.urandom()?
os.urandom() is implemented in the kernel, it doesn’t have
issues of user-space RNG. For example, it is much harder to get its
state. It is usually built on a CSPRNG, so even if its state is
“stolen”, it is hard to compute previously generated numbers. The kernel
has a good knowledge of entropy sources and feed regularly the entropy
That’s also why
os.urandom() is preferred over
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Last modified: 2022-01-21 11:03:51 GMT