Hi folks, Over the weekend, Nathaniel Smith and I put together a proposal to allow security sensitive APIs (os.urandom, random.SystemRandom and the new secrets module) to throw BlockingIOError if the operating system's random number generator isn't ready. We think this approach provides all the desired security guarantees, while being relatively straightforward for affected system integrators to diagnose and appropriately resolve if they're currently using these APIs in a context where Linux is currently feeding them potentially predictable random values. Rendered: https://www.python.org/dev/peps/pep-0522/ GitHub: https://github.com/python/peps/blob/master/pep-0522.txt The "Additional Background" section is mainly for the sake of folks that haven't been following any of the previous discussions, but also provides the reasoning for why we don't consider retaining consistency with "man urandom" to be a useful design goal (any more than the builtin open tries to retain consistency with "man open") Cheers, Nick. ================= PEP: 522 Title: Allow BlockingIOError in security sensitive APIs on Linux Version: $Revision$ Last-Modified: $Date$ Author: Nick Coghlan <ncoghlan@gmail.com>, Nathaniel J. Smith <njs@pobox.com> Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 16 June 2016 Python-Version: 3.6 Abstract ======== A number of APIs in the standard library that return random values nominally suitable for use in security sensitive operations currently have an obscure Linux-specific failure mode that allows them to return values that are not, in fact, suitable for such operations. This PEP proposes changing such failures in Python 3.6 from the current silent, hard to detect, and hard to debug, errors to easily detected and debugged errors by raising ``BlockingIOError`` with a suitable error message, allowing developers the opportunity to unambiguously specify their preferred approach for handling the situation. The APIs affected by this change would be: * ``os.urandom`` * ``random.SystemRandom`` * the new ``secrets`` module added by PEP 506 The new exception would potentially be encountered in the following situations: * Python code calling these APIs during Linux system initialization * Python code running on improperly initialized Linux systems (e.g. embedded hardware without adequate sources of entropy to seed the system random number generator, or Linux VMs that aren't configured to accept entropy from the VM host) CPython interpreter initialization and ``random`` module initialization would also be updated to gracefully fall back to alternative seeding options if the system random number generator is not ready. Proposal ======== Changing ``os.urandom()`` on Linux ---------------------------------- This PEP proposes that in Python 3.6+, ``os.urandom()`` be updated to call the new Linux ``getrandom()`` syscall in non-blocking mode if available and raise ``BlockingIOError: system random number generator is not ready`` if the kernel reports that the call would block. This behaviour will then propagate through to higher level standard library APIs that depend on ``os.urandom`` (specifically ``random.SystemRandom`` and the new ``secrets`` module introduced by PEP 506). In all cases, as soon as a call to one of these security sensitive APIs succeeds, all future calls to these APIs in that process will succeed (once the operating system random number generator is ready after system boot, it remains ready). Related changes --------------- Currently, SipHash initialization and ``random`` module initialization both gather random bytes using the same code that underlies ``os.urandom``. This PEP proposes to modify these so that in situations where ``os.urandom`` would raise a ``BlockingIOError``, they automatically fall back on potentially more predictable sources of randomness (and in the SipHash case, print a warning message to ``stderr`` indicating that that particular Python process should not be used to process untrusted data). To transparently accommodate a potential future where Linux adopts the same "potentially blocking during system initialization" ``/dev/urandom`` behaviour used by other \*nix systems, this fallback source of randomness will *not* be the ``/dev/urandom`` device. Limitations on scope -------------------- No changes are proposed for Windows or Mac OS X systems, as neither of those platforms provides any mechanism to run Python code before the operating system random number generator has been initialized. Mac OS X goes so far as to kernel panic and abort the boot process if it can't properly initialize the random number generator (although Apple's restrictions on the supported hardware platforms make that exceedingly unlikely in practice). Similarly, no changes are proposed for other \*nix systems where ``os.urandom()`` will currently block waiting for the system random number generator to be initialized, rather than returning values that are potentially unsuitable for use in security sensitive applications. While other \*nix systems that offer a non-blocking API for requesting random numbers suitable for use in security sensitive applications could potentially receive a similar update to the one proposed for Linux in this PEP, such changes are out of scope for this particular proposal. Python's behaviour on older Linux systems that do not offer the new ``getrandom()`` syscall will also remain unchanged. Rationale ========= Raising ``BlockingIOError`` in ``os.urandom()`` on Linux -------------------------------------------------------- For several years now, the security community's guidance has been to use ``os.urandom()`` (or the ``random.SystemRandom()`` wrapper) when implementing security sensitive operations in Python. To help improve API discoverability and make it clearer that secrecy and simulation are not the same problem (even though they both involve random numbers), PEP 506 collected several of the one line recipes based on the lower level ``os.urandom()`` API into a new ``secrets`` module. However, this guidance has also come with a longstanding caveat: developers writing security sensitive software at least for Linux, and potentially for some other \*BSD systems, may need to wait until the operating system's random number generator is ready before relying on it for security sensitive operations. This generally only occurs if ``os.urandom()`` is read very early in the system initialization process, or on systems with few sources of available entropy (e.g. some kinds of virtualized or embedded systems), but unfortunately the exact conditions that trigger this are difficult to predict, and when it occurs then there is no direct way for userspace to tell it has happened without querying operating system specific interfaces. On \*BSD systems (if the particular \*BSD variant allows the problem to occur at all), encountering this situation means ``os.urandom()`` will either block waiting for the system random number generator to be ready (the associated symptom would be for the affected script to pause unexpectedly on the first call to ``os.urandom()``) or else will behave the same way as it does on Linux. On Linux, in Python versions up to and including Python 3.4, and in Python 3.5 maintenance versions following Python 3.5.2, there's no clear indicator to developers that their software may not be working as expected when run early in the Linux boot process, or on hardware without good sources of entropy to seed the operating system's random number generator: due to the behaviour of the underlying ``/dev/urandom`` device, ``os.urandom()`` on Linux returns a result either way, and it takes extensive statistical analysis to show that a security vulnerability exists. By contrast, if ``BlockingIOError`` is raised in those situations, then developers using Python 3.6+ can easily choose their desired behaviour: 1. Loop until the call succeeds (security sensitive) 2. Switch to using the random module (non-security sensitive) 3. Switch to reading ``/dev/urandom`` directly (non-security sensitive) Issuing a warning for potentially predictable internal hash initialization -------------------------------------------------------------------------- The challenge for internal hash initialization is that it might be very important to initialize SipHash with a reliably unpredictable random seed (for processes that are exposed to potentially hostile input) or it might be totally unimportant (for processes that never have to deal with untrusted data). The Python runtime has no way to know which case a given invocation involves, which means that if we allow SipHash initialization to block or error out, then our intended security enhancement may break code that is already safe and working fine, which is unacceptable -- especially since we are reasonably confident that most Python invocations that might run during Linux system initialization fall into this category (exposure to untrusted input tends to involve network access, which typically isn't brought up until after the system random number generator is initialized). However, at the same time, since Python has no way to know whether any given invocation needs to handle untrusted data, when the default SipHash initialization fails this *might* indicate a genuine security problem, which should not be allowed to pass silently. Accordingly, if internal hash initialization needs to fall back to a potentially predictable seed due to the system random number generator not being ready, it will also emit a warning message on ``stderr`` to say that the system random number generator is not available and that processing potentially hostile untrusted data should be avoided. Allowing potentially predictable ``random`` module initialization ----------------------------------------------------------------- Other than for ``random.SystemRandom`` (which is a relatively thin wrapper around ``os.urandom``), the ``random`` module has never made any guarantees that the numbers it generates are suitable for use in security sensitive operations, so the use of the system random number generator to seed the default Mersenne Twister instance is mainly beneficial as a harm mitigation measure for code that is using the ``random`` module inappropriately. Since a single call to ``os.urandom()`` is cheap once the system random number generator has been initialized it makes sense to retain that as the default behaviour, but there's no need to issue a warning when falling back to a potentially more predictable alternative when necessary (in such cases, a warning will typically already have been issued as part of interpreter startup, as the only way for the call when importing the random module to fail without the implicit call during interpreter startup also failing if for the latter to have been skipped by entirely disabling the hash randomization mechanism). Backwards Compatibility Impact Assessment ========================================= Similar to PEP 476, this is a proposal to turn a previously silent security failure into a noisy exception that requires the application developer to make an explicit decision regarding the behaviour they desire. As no changes are proposed for operating systems other than Linux, ``os.urandom()`` retains its existing behaviour as a nominally blocking API that is non-blocking in practice due to the difficulty of scheduling Python code to run before the operating system random number generator is ready. We believe it may be possible to encounter problems akin to those described in this PEP on at least some \*BSD variants, but nobody has explicitly demonstrated that. On Mac OS X and Windows, it appears to be straight up impossible to even try to run a Python interpreter that early in the boot process. On Linux, ``os.urandom()`` retains its status as a guaranteed non-blocking API. However, the means of achieving that status changes in the specific case of the operating system random number generator not being ready for use in security sensitive operations: historically it would return potentially predictable random data, with this PEP it would change to raise ``BlockingIOError``. Developers of affected applications would then be required to make one of the following changes to gain forward compatibility with Python 3.6, based on the kind of application they're developing. Unaffected Applications ----------------------- The following kinds of applications would be entirely unaffected by the change, regardless of whether or not they perform security sensitive operations: - applications that don't support Linux - applications that are only run on desktops or conventional servers - applications that are only run after the system RNG is ready Applications in this category simply won't encounter the new exception, so it will be reasonable for developers to wait and see if they receive Python 3.6 compatibility bugs related to the new runtime behaviour, rather than attempting to pre-emptively determine whether or not they're affected. Affected security sensitive applications ---------------------------------------- Security sensitive applications would need to either change their system configuration so the application is only started after the operating system random number generator is ready for security sensitive operations, or else change their code to busy loop until the operating system is ready:: def blocking_urandom(num_bytes): while True: try: return os.urandom(num_bytes) except BlockingIOError: pass Affected non-security sensitive applications -------------------------------------------- Non-security sensitive applications that don't want to assume access to ``/dev/urandom`` (or assume a non-blocking implementation of that device) can be updated to use the ``random`` module as a fallback option:: def pseudorandom_fallback(num_bytes): try: return os.urandom(num_bytes) except BlockingIOError: random.getrandbits(num_bytes*8).to_bytes(num_bytes, "little") Depending on the application, it may also be appropriate to skip accessing ``os.urandom`` at all, and instead rely solely on the ``random`` module. Affected Linux specific non-security sensitive applications ----------------------------------------------------------- Non-security sensitive applications that don't need to worry about cross platform compatibility and are willing to assume that ``/dev/urandom`` on Linux will always retain its current behaviour can be updated to access ``/dev/urandom`` directly:: def dev_urandom(num_bytes): with open("/dev/urandom", "rb") as f: return f.read(num_bytes) However, pursuing this option has the downside of contributing to ensuring that the default behaviour of Linux at the operating system level can never be changed. Additional Background ===================== Why propose this now? --------------------- The main reason is because the Python 3.5.0 release switched to using the new Linux ``getrandom()`` syscall when available in order to avoid consuming a file descriptor [1]_, and this had the side effect of making the following operations block waiting for the system random number generator to be ready: * ``os.urandom`` (and APIs that depend on it) * importing the ``random`` module * initializing the randomized hash algorithm used by some builtin types While the first of those behaviours is arguably desirable (and consistent with ``os.urandom``'s existing behaviour on other operating systems), the latter two behaviours are unnecessary and undesirable, and the last one is now known to cause a system level deadlock when attempting to run Python scripts during the Linux init process with Python 3.5.0 or 3.5.1 [2]_, while the second one can cause problems when using virtual machines without robust entropy sources configured [3]_. Since decoupling these behaviours in CPython will involve a number of implementation changes more appropriate for a feature release than a maintenance release, the relatively simple resolution applied in Python 3.5.2 was to revert all three of them to a behaviour similar to that of previous Python versions: if the new Linux syscall indicates it will block, then Python 3.5.2 will implicitly fall back on reading ``/dev/urandom`` directly [4]_. However, this bug report *also* resulted in a range of proposals to add *new* APIs like ``os.getrandom()`` [5]_, ``os.urandom_block()`` [6]_, ``os.pseudorandom()`` and ``os.cryptorandom()`` [7]_, or adding new optional parameters to ``os.urandom()`` itself [8]_, and then attempting to educate users on when they should call those APIs instead of just using a plain ``os.urandom()`` call. These proposals represent dramatic overreactions, as the question of reliably obtaining random numbers suitable for security sensitive work on Linux is a relatively obscure problem of interest mainly to operating system developers and embedded systems programmers, that in no way justifies cluttering up the Python standard library's cross-platform APIs with new Linux-specific concerns. This is especially so with the ``secrets`` module already being added as the "use this and don't worry about the low level details" option for developers writing security sensitive software that for some reason can't rely on even higher level domain specific APIs (like web frameworks) and also don't need to worry about Python versions prior to Python 3.6. That said, it's also the case that low cost ARM devices are becoming increasingly prevalent, with a lot of them running Linux, and a lot of folks writing Python applications that run on those devices. That creates an opportunity to take an obscure security problem that currently requires a lot of knowledge about Linux boot processes and provably unpredictable random number generation to diagnose and resolve, and instead turn it into a relatively mundane and easy-to-find-in-an-internet-search runtime exception. The cross-platform behaviour of ``os.urandom()`` ------------------------------------------------ On operating systems other than Linux, ``os.urandom()`` may already block waiting for the operating system's random number generator to be ready. This will happen at most once in the lifetime of the process, and the call is subsequently guaranteed to be non-blocking. Linux is unique in that, even when the operating system's random number generator doesn't consider itself ready for use in security sensitive operations, reading from the ``/dev/urandom`` device will return random values based on the entropy it has available. This behaviour is potentially problematic, so Linux 3.17 added a new ``getrandom()`` syscall that (amongst other benefits) allows callers to either block waiting for the random number generator to be ready, or else request an error return if the random number generator is not ready. Notably, the new API does *not* support the old behaviour of returning data that is not suitable for security sensitive use cases. Versions of Python prior up to and including Python 3.4 access the Linux ``/dev/urandom`` device directly. Python 3.5.0 and 3.5.1 called ``getrandom()`` in blocking mode in order to avoid the use of a file descriptor to access ``/dev/urandom``. While there were no specific problems reported due to ``os.urandom()`` blocking in user code, there *were* problems due to CPython implicitly invoking the blocking behaviour during interpreter startup and when importing the ``random`` module. Rather than trying to decouple SipHash initialization from the ``os.urandom()`` implementation, Python 3.5.2 switched to calling ``getrandom()`` in non-blocking mode, and falling back to reading from ``/dev/urandom`` if the syscall indicates it will block. As a result of the above, ``os.urandom()`` in all Python versions up to and including Python 3.5 propagate the behaviour of the underling ``/dev/urandom`` device to Python code. Problems with the behaviour of ``/dev/urandom`` on Linux -------------------------------------------------------- The Python ``os`` module has largely co-evolved with Linux APIs, so having ``os`` module functions closely follow the behaviour of their Linux operating system level counterparts when running on Linux is typically considered to be a desirable feature. However, ``/dev/urandom`` represents a case where the current behaviour is acknowledged to be problematic, but fixing it unilaterally at the kernel level has been shown to prevent some Linux distributions from booting (at least in part due to components like Python currently using it for non-security-sensitive purposes early in the system initialization process). As an analogy, consider the following two functions:: def generate_example_password(): """Generates passwords solely for use in code examples""" return generate_unpredictable_password() def generate_actual_password(): """Generates actual passwords for use in real applications""" return generate_unpredictable_password() If you think of an operating system's random number generator as a method for generating unpredictable, secret passwords, then you can think of Linux's ``/dev/urandom`` as being implemented like:: # Oversimplified artist's conception of the kernel code # implementing /dev/urandom def generate_unpredictable_password(): if system_rng_is_ready: return use_system_rng_to_generate_password() else: # we can't make an unpredictable password; silently return a # potentially predictable one instead: return "p4ssw0rd" In this scenario, the author of ``generate_example_password`` is fine - even if ``"p4ssw0rd"`` shows up a bit more often than they expect, it's only used in examples anyway. However, the author of ``generate_actual_password`` has a problem - how do they prove that their calls to ``generate_unpredictable_password`` never follow the path that returns a predictable answer? In real life it's slightly more complicated than this, because there might be some level of system entropy available -- so the fallback might be more like ``return random.choice(["p4ssword", "passw0rd", "p4ssw0rd"])`` or something even more variable and hence only statistically predictable with better odds than the author of ``generate_actual_password`` was expecting. This doesn't really make things more provably secure, though; mostly it just means that if you try to catch the problem in the obvious way -- ``if returned_password == "p4ssw0rd": raise UhOh`` -- then it doesn't work, because ``returned_password`` might instead be ``p4ssword`` or even ``pa55word``, or just an arbitrary 64 bit sequence selected from fewer than 2**64 possibilities. So this rough sketch does give the right general idea of the consequences of the "more predictable than expected" fallback behaviour, even though it's thoroughly unfair to the Linux kernel team's efforts to mitigate the practical consequences of this problem without resorting to breaking backwards compatibility. This design is generally agreed to be a bad idea. As far as we can tell, there are no use cases whatsoever in which this is the behavior you actually want. It has led to the use of insecure ``ssh`` keys on real systems, and many \*nix-like systems (including at least Mac OS X, OpenBSD, and FreeBSD) have modified their ``/dev/urandom`` implementations so that they never return predictable outputs, either by making reads block in this case, or by simply refusing to run any userspace programs until the system RNG has been initialized. Unfortunately, Linux has so far been unable to follow suit, because it's been empirically determined that enabling the blocking behavior causes some currently extant distributions to fail to boot. Instead, the new ``getrandom()`` syscall was introduced, making it *possible* for userspace applications to access the system random number generator safely, without introducing hard to debug deadlock problems into the system initialization processes of existing Linux distros. Consequences of ``getrandom()`` availability for Python ------------------------------------------------------- Prior to the introduction of the ``getrandom()`` syscall, it simply wasn't feasible to access the Linux system random number generator in a provably safe way, so we were forced to settle for reading from ``/dev/urandom`` as the best available option. However, with ``getrandom()`` insisting on raising an error or blocking rather than returning predictable data, as well as having other advantages, it is now the recommended method for accessing the kernel RNG on Linux, with reading ``/dev/urandom`` directly relegated to "legacy" status. This moves Linux into the same category as other operating systems like Windows, which doesn't provide a ``/dev/urandom`` device at all: the best available option for implementing ``os.urandom()`` is no longer simply reading bytes from the ``/dev/urandom`` device. This means that what used to be somebody else's problem (the Linux kernel development team's) is now Python's problem -- given a way to detect that the system RNG is not initialized, we have to choose how to handle this situation whenever we try to use the system RNG. It could simply block, as was somewhat inadvertently implemented in 3.5.0:: # artist's impression of the CPython 3.5.0-3.5.1 behavior def generate_unpredictable_bytes_or_block(num_bytes): while not system_rng_is_ready: wait return unpredictable_bytes(num_bytes) Or it could raise an error, as this PEP proposes (in *some* cases):: # artist's impression of the behavior proposed in this PEP def generate_unpredictable_bytes_or_raise(num_bytes): if system_rng_is_ready: return unpredictable_bytes(num_bytes) else: raise BlockingIOError Or it could explicitly emulate the ``/dev/urandom`` fallback behavior, as was implemented in 3.5.2rc1 and is expected to remain for the rest of the 3.5.x cycle:: # artist's impression of the CPython 3.5.2rc1+ behavior def generate_unpredictable_bytes_or_maybe_not(num_bytes): if system_rng_is_ready: return unpredictable_bytes(num_bytes) else: return (b"p4ssw0rd" * (num_bytes // 8 + 1))[:num_bytes] (And the same caveats apply to this sketch as applied to the ``generate_unpredictable_password`` sketch of ``/dev/urandom`` above.) There are five places where CPython and the standard library attempt to use the operating system's random number generator, and thus five places where this decision has to be made: * initializing the SipHash used to protect ``str.__hash__`` and friends against DoS attacks (called unconditionally at startup) * initializing the ``random`` module (called when ``random`` is imported) * servicing user calls to the ``os.urandom`` public API * the higher level ``random.SystemRandom`` public API * the new ``secrets`` module public API added by PEP 506 Currently, these five places all use the same underlying code, and thus make this decision in the same way. This whole problem was first noticed because 3.5.0 switched that underlying code to the ``generate_unpredictable_bytes_or_block`` behavior, and it turns out that there are some rare cases where Linux boot scripts attempted to run a Python program as part of system initialization, the Python startup sequence blocked while trying to initialize SipHash, and then this triggered a deadlock because the system stopped doing anything -- including gathering new entropy -- until the Python script was forcibly terminated by an external timer. This is particularly unfortunate since the scripts in question never processed untrusted input, so there was no need for SipHash to be initialized with provably unpredictable random data in the first place. This motivated the change in 3.5.2rc1 to emulate the old ``/dev/urandom`` behavior in all cases (by calling ``getrandom()`` in non-blocking mode, and then falling back to reading ``/dev/urandom`` if the syscall indicates that the ``/dev/urandom`` pool is not yet fully initialized.) A similar problem was found due to the ``random`` module calling ``os.urandom`` as a side-effect of import in order to seed the default global ``random.Random()`` instance. We have not received any specific complaints regarding direct calls to ``os.urandom()`` or ``random.SystemRandom()`` blocking with 3.5.0 or 3.5.1 - only problem reports due to the implicit blocking on interpreter startup and as a side-effect of importing the random module. Accordingly, this PEP proposes providing consistent shared behaviour for the latter three cases (ensuring that their behaviour is unequivocally suitable for all security sensitive operations), while updating the first two cases to account for that behavioural change. This approach should mean that the vast majority of Python users never need to even be aware that this change was made, while those few whom it affects will receive an exception at runtime that they can look up online and find suitable guidance on addressing. References ========== .. [1] os.urandom() should use Linux 3.17 getrandom() syscall (http://bugs.python.org/issue22181) .. [2] Python 3.5 running on Linux kernel 3.17+ can block at startup or on importing the random module on getrandom() (http://bugs.python.org/issue26839) .. [3] "import random" blocks on entropy collection on Linux with low entropy (http://bugs.python.org/issue25420) .. [4] os.urandom() doesn't block on Linux anymore (https://hg.python.org/cpython/rev/9de508dc4837) .. [5] Proposal to add os.getrandom() (http://bugs.python.org/issue26839#msg267803) .. [6] Add os.urandom_block() (http://bugs.python.org/issue27250) .. [7] Add random.cryptorandom() and random.pseudorandom, deprecate os.urandom() (http://bugs.python.org/issue27279) .. [8] Always use getrandom() in os.random() on Linux and add block=False parameter to os.urandom() (http://bugs.python.org/issue27266) For additional background details beyond those captured in this PEP, also see Victor Stinner's summary at http://haypo-notes.readthedocs.io/pep_random.html Copyright ========= This document has been placed into the public domain. -- Nick Coghlan | ncoghlan@gmail.com | Brisbane, Australia