[Python-checkins] peps: PEP 466: switch to more selective backports
nick.coghlan
python-checkins at python.org
Wed Mar 26 12:46:52 CET 2014
http://hg.python.org/peps/rev/8527f6e2beb0
changeset: 5430:8527f6e2beb0
user: Nick Coghlan <ncoghlan at gmail.com>
date: Wed Mar 26 21:46:42 2014 +1000
summary:
PEP 466: switch to more selective backports
files:
pep-0466.txt | 268 ++++++++++++++++++--------------------
1 files changed, 125 insertions(+), 143 deletions(-)
diff --git a/pep-0466.txt b/pep-0466.txt
--- a/pep-0466.txt
+++ b/pep-0466.txt
@@ -7,7 +7,7 @@
Type: Informational
Content-Type: text/x-rst
Created: 23-Mar-2014
-Post-History: 23-Mar-2014
+Post-History: 23-Mar-2014, 24-Mar-2014, 25-Mar-2014, 26-Mar-2014
Abstract
@@ -22,8 +22,8 @@
feature release cycle, which is still applicable to the Python 3 series.
However, the age of the standard library in Python 2 has now reached a point
where it is sufficiently far behind the state of the art in network security
-protocols for it to be causing real problems in commercial use cases
-where upgrading to Python 3 in the near term may not be practical.
+protocols for it to be causing real problems in use cases where upgrading to
+Python 3 in the near term may not be feasible.
In recognition of the additional practical considerations that have arisen
during the 4+ year maintenance cycle for Python 2.7, this PEP allows
@@ -31,91 +31,74 @@
overall security of the internet to be updated in line with the
corresponding Python 3 feature releases.
-Specifically, the exception will apply to:
-
-* the ``ssl`` module
-* the ``hashlib`` module
-* the ``hmac`` module
-* the components of the ``random`` and ``os`` modules that the above
- modules rely on for cryptographic randomness
-* the version of OpenSSL bundled with the binary installers for Windows
- and Mac OS X
-
-Changes to these modules will still need to undergo normal backwards
-compatibility assessments to ensure their default behaviour remains
-consistent with earlier Python 2.7 releases, but otherwise they will be
-kept entirely aligned with the latest feature release of their Python 3
-counterparts. This is designed to make it easier to implement secure
-networked software in Python, even for software that currently needs to
-remain compatible with the Python 2 series (which includes a lot of
-network infrastructure software).
+Specifically, the exception allows a critical set of network security
+related features to be backported from Python 3.4 to the upcoming Python
+2.7.7 maintenance release.
While this PEP does not make any changes to the core development team's
handling of security-fix-only branches that are no longer in active
maintenance, it *does* recommend that commercial redistributors providing
-extended support periods for the Python standard library either adopt a
-similar approach to ensuring that the secure networking infrastructure
-keeps pace with the evolution of the internet, or else explicitly
-disclaim support for the use of older versions in roles that involve
-connecting directly to the public internet.
+extended support periods for the Python standard library either backport
+these features to their supported versions, or else explicitly disclaim
+support for the use of older versions in roles that involve connecting
+directly to the public internet.
Exemption Policy
================
-Under this policy, the following network security related modules are
-granted a blanket exemption to the normal restriction against adding new
-features in Python 2.7 maintenance releases, for the purpose of keeping
-their APIs aligned with their counterparts in the latest feature release
-of Python 3:
+Under this policy, the following features SHOULD be backported from Python
+3.4 to the upcoming Python 2.7.7 maintenance release:
-* the ``ssl`` module
-* the ``hashlib`` module
-* the ``hmac`` module
+* in the ``os`` module:
-Under this exemption, these modules are updated to provide identical
-functionality to their Python 3 counterparts after every new Python 3
-feature release. The default behaviour of the backported modules will be
-adjusted as appropriate to meet the backwards compatibility requirements
-of a Python 2.7 maintenance release.
+ * persistent file descriptor for ``os.urandom()``.
+
+* in the ``hmac`` module:
+
+ * constant time comparison function (``hmac.compare_digest()``).
+
+* in the ``hashlib`` module:
+
+ * password hashing function (``hashlib.pbkdf2_hmac()``).
+ * details of hash algorithm availability (``hashlib.algorithms_guaranteed``
+ and ``hashlib.algorithms_guaranteed``).
+
+* in the ``ssl`` module:
+
+ * this module is almost entirely synchronised with its Python 3
+ counterpart, bringing TLSv2, SSLContext manipulation, Server Name
+ Identification, access to platform certificate stores, standard
+ library support for peer hostname validation and more to the Python 2
+ series.
+ * the only ``ssl`` module features *not* backported under this policy are
+ the ``ssl.RAND_*`` functions that provide access to OpenSSL's random
+ number generation capabilities - use ``os.urandom()`` instead.
As part of this policy, permission is also granted to upgrade to newer
feature releases of OpenSSL when preparing the binary installers
for new maintenance releases of Python 2.7.
-Note that the ``sha`` and ``md5`` modules are not covered by this policy,
-but have been deprecated in favour of ``hashlib`` since Python 2.5 and have
-been removed entirely in the Python 3 series.
-
-In addition to the above blanket exemption, a conditional exemption is
-granted for these modules that may include some network security related
-features:
-
-* the ``os`` module (specifically ``os.urandom``)
-* the ``random`` module
-
-This more limited exemption for these modules requires that the *specific*
-enhancement being proposed for backporting needs to be justified as being
-network security related. This is generally restricted to cases where the
-feature in question is needed by an update to one of the modules covered
-by the blanket exemption.
-
Backwards Compatibility Considerations
======================================
-This PEP does *not* grant Python 2.7 any general exemptions to the usual
-backwards compatibility policy for maintenance releases. Instead, by
-explicitly encouraging the use of feature based checks and explicitly
-opting in to less secure configurations, it is designed to make it easier
+As in the Python 3 series, the backported ``ssl.create_default_context()``
+API is granted a backwards compatibility exemption that permits the
+protocol, options, cipher and other settings of the created SSL context to
+be made
+
+This PEP does *not* grant any exemptions to the usual backwards
+compatibility policy for maintenance releases. Instead, by explicitly
+encouraging the use of feature based checks, it is designed to make it easier
to write more secure cross-version compatible Python software, while still
limiting the risk of breaking currently working software when upgrading to
a new Python 2.7 maintenance release.
-In *all* cases where this policy is applied to backport enhancements to
-Python 2.7 maintenance releases, it MUST be possible to write cross-version
+In all cases where this policy allows new features to be backported to
+the Python 2.7 release series, it is possible to write cross-version
compatible code that operates by "feature detection" (for example, checking
-for particular attributes in the module), without needing to explicitly check
+for particular attributes in a module), without needing to explicitly check
the Python version.
It is then up to library and framework code to provide an appropriate warning
@@ -124,16 +107,17 @@
security feature is unavailable, most software SHOULD instead emit a warning
and continue operating using a slightly degraded security configuration.
-Affected APIs SHOULD be designed to allow library and application code to
-perform the following actions after detecting the presence of a relevant
+The backported APIs allow library and application code to perform the
+following actions after detecting the presence of a relevant
network security related feature:
* explicitly opt in to more secure settings (to allow the use of enhanced
- security features in older maintenance releases of Python)
+ security features in older maintenance releases of Python with less
+ secure default behaviour)
* explicitly opt in to less secure settings (to allow the use of newer Python
feature releases in lower security environments)
* determine the default setting for the feature (this MAY require explicit
- Python version checks to determine the Python feature release, but MUST
+ Python version checks to determine the Python feature release, but DOES
NOT require checking for a specific maintenance release)
Security related changes to other modules (such as higher level networking
@@ -187,12 +171,12 @@
Backporting security related fixes and enhancements to earlier versions is
a common service for commercial redistributors to offer to their customers.
-This policy represents an explicit invitation to contribute some of those
-changes back to the upstream Python community in cases where they are likely
-to have a broad impact that helps improve the security of the internet as a
-whole, with the assurance that the existing core development team not only
-won't object to such contributions, but will actively encourage their
-incorporation into the next Python 2.7 maintenance release.
+This policy represents an explicit invitation to implement those changes
+in the core development tree in cases where they are likely to have a broad
+impact that helps improve the security of the internet as a whole, with the
+assurance that the existing core development team not only won't object to
+such contributions, but will actively encourage their incorporation into
+the next Python 2.7 maintenance release.
Documentation
@@ -201,7 +185,7 @@
All modules covered by this policy MUST include a "Security Considerations"
section in their documentation in order to take advantage of this policy.
-In addition to any other module specific contents, this section MUST
+In addition to any other module specific contents, this section SHOULD
enumerate key security enhancements and fixes (with CVE identifiers where
applicable), along with the feature and maintenance releases that first
included them.
@@ -232,8 +216,8 @@
libraries, frameworks and applications can still test their handling of the
legacy security infrastructure correctly (either failing or degrading
gracefully, depending on the security sensitivity of the software), even
-after the latest Python 2.7 maintenance release has been synchronised with
-a new Python 3 feature release for the modules covered by this policy.
+after the features covered in this policy have been backported to the
+Python 2.7 series.
Handling lower security environments with low risk tolerance
@@ -258,16 +242,16 @@
Evolution of this Policy
========================
-The key requirement for a module to be considered for inclusion in this
-policy (whether under a blanket or conditional exemption) is that it must
-have security implications *beyond* the specific application that is written
-in Python and the system that application is running on. Thus the focus on
-network security protocols and related cryptographic infrastructure - Python
-is a popular choice for the development of web services and clients, and
-thus the capabilities of widely used Python versions have implications for
-the security design of other services that may themselves be using newer
-versions of Python or other development languages, but need to interoperate
-with clients or servers written using older versions of Python.
+The key requirement for a feature to be considered for inclusion in this
+policy is that it must have security implications *beyond* the specific
+application that is written in Python and the system that application is
+running on. Thus the focus on network security protocols, password storage
+and related cryptographic infrastructure - Python is a popular choice for
+the development of web services and clients, and thus the capabilities of
+widely used Python versions have implications for the security design of
+other services that may themselves be using newer versions of Python or
+other development languages, but need to interoperate with clients or
+servers written using older versions of Python.
The intent behind this requirement is to minimise any impact that the
introduction of this policy may have on the stability and compatibility of
@@ -280,15 +264,6 @@
Motivation and Rationale
========================
-This PEP can be seen as a more targeted version of the "faster standard
-library release cycle" proposals discussed in PEP 407 and PEP 413,
-focusing specifically on those areas which have implications beyond the
-Python community.
-
-
-Background
-----------
-
The creation of this PEP was prompted primarily by the aging SSL support in
the Python 2 series. As of March 2014, the Python 2.7 SSL module is
approaching four years of age, and the SSL support in the still popular
@@ -315,8 +290,8 @@
shipped with the corresponding Python feature release.
With increased popularity comes increased responsibility, and this policy
-aims to acknowledge the fact that Python's popularity and adoption has now
-reached a level where some of our design and policy decisions have
+aims to acknowledge the fact that Python's popularity and adoption is at a
+sufficiently high level that some of our design and policy decisions have
significant implications beyond the Python development community.
As one example, the Python 2 ``ssl`` module does not support the Server
@@ -371,11 +346,10 @@
*series*. The individual RHEL 6.x point releases within the series then
receive a wide variety of new features, including security enhancements,
all while meeting strict backwards compatibility guarantees for existing
-software. Subscribers *are* able to continue using a given point release
-after the next point release has become available, but a corresponding
-add-on subscription for `Extended Update Support
-<https://access.redhat.com/site/support/policy/updates/errata/#Extended_Update_Support>`__
-is needed to cover the additional backporting work involved.
+software. The policy described in this PEP brings our approach to long term
+maintenance more into line with this precedent - we retain our strict
+backwards compatibility requirements, but slightly relax the restrictions
+against adding new features.
To date, downstream redistributors have respected our upstream policy of
"no new features in Python maintenance releases". This PEP explicitly
@@ -385,8 +359,8 @@
Enterprise Linux and its downstream derivatives.
-Alternative: advise developers of networked software to migrate to Python 3
----------------------------------------------------------------------------
+Rejected alternative: just advise developers to migrate to Python 3
+-------------------------------------------------------------------
This alternative represents the status quo. Unfortunately, it has proven
to be unworkable in practice, as the backwards compatibility implications
@@ -428,7 +402,7 @@
The OpenStack migration to Python 3 is also still in its infancy, and even
though that's a project with an extensive and relatively robust automated
test suite, it's still large enough that it is going to take quite some time
-to migrate.
+to migrate fully to a Python 2/3 compatible code base.
And that's just three of the highest profile open source projects that
make heavy use of Python. Given the likely existence of large amounts of
@@ -455,8 +429,8 @@
my preferred approach to addressing the situation.
-Alternative: create and release Python 2.8
-------------------------------------------
+Rejected alternative: create and release Python 2.8
+---------------------------------------------------
With sufficient corporate support, it likely *would* be possible to create
and release Python 2.8 (it's highly unlikely such a project would garner
@@ -485,8 +459,8 @@
addressed.
-Alternative: distribute the security enhancements via PyPI
-----------------------------------------------------------
+Rejected alternative: distribute the security enhancements via PyPI
+-------------------------------------------------------------------
While this initially appears to be an attractive and easier to manage
approach, it actually suffers from several significant problems.
@@ -537,8 +511,8 @@
security infrastructure, which most of them are unlikely to do.
-Alternative: provide a "legacy SSL infrastructure" branch
----------------------------------------------------------
+Rejected variant: provide a "legacy SSL infrastructure" branch
+--------------------------------------------------------------
Earlier versions of this PEP included the concept of a ``2.7-legacy-ssl``
branch that preserved the exact feature set of the Python 2.7.6 network
@@ -559,28 +533,14 @@
refer to Python 2.7.6 and earlier releases as "Python 2.7 with Legacy SSL".
-Alternative: selectively backport particular APIs
--------------------------------------------------
+Rejected variant: synchronise particular modules entirely with Python 3
+-----------------------------------------------------------------------
-An instinctively minimalist reaction to this proposal is to only backport
-particular APIs in the affected modules that are judged to be "security
-critical". However, this ends up providing a worse end user experience,
-as well as a worse developer experience.
+Earlier versions of this PEP suggested synchronising the ``hmac``,
+``hashlib`` and ``ssl`` modules entirely with their Python 3 counterparts.
-For end users, the selective backporting approach means learning not only
-the legacy Python 2.7 API and the current Python 3 APIs, but also the
-hybrid API created by the selective backporting process. It is much
-simpler to just learn the APIs of the original Python 2.7 feature release
-and the relevant Python 3 features releases, without trying to define a new
-hybrid API that only exists in later Python 2.7 maintenance branches.
-
-For developers, the main benefit of the "just align the available
-functionality with Python 3" approach is that it reduces the amount of
-design work needing when updating Python 2.7 with new security features.
-The "feature or fix?" and "security related or not?" debates are both
-handled in advance by this policy, leaving only the matter of ensuring
-that backwards compatibility is preserved as needed by adjusting the
-default behaviour in the Python 2.7 backport when appropriate.
+This approach proved too vague to build a compelling case for the exception,
+and has thus been replaced by the current more explicit proposal.
Open Questions
@@ -601,10 +561,8 @@
as support for handling any more specific security issues affecting these
modules.
-* I believe I've addressed all the technical and scope questions I had, or
- others raised. That just leaves the question of "Do we agree this plan
- actually makes sense, given the constraints on downstream redistributors
- that would also like to see this problem solved?" :)
+* Did I miss anything important in the switch to a more restrictive
+ proposal?
Disclosure of Interest
@@ -620,7 +578,7 @@
Acknowledgements
================
-Thanks to Christian Heimes for his recent efforts on greatly improving
+Thanks to Christian Heimes and other for their efforts in greatly improving
Python's SSL support in the Python 3 series, and a variety of members of
the Python community for helping me to better understand the implications
of the default settings we provide in our SSL modules, and the impact that
@@ -628,20 +586,44 @@
(Python 2.7) or even 2008 (Python 2.6) potentially has for the security
of the web as a whole.
-Christian and Donald Stufft also provided valuable feedback on a preliminary
+Thanks to Donald Stufft and Alex Gaynor for identifying a more limited set
+of essential security features that allowed the proposal to be made more
+fine-grained than backporting entire modules from Python 3.4 [7,8]_.
+
+Christian and Donald also provided valuable feedback on a preliminary
draft of this proposal.
-Thanks also to participants in the python-dev mailing list threads [1,2,5]_
+Thanks also to participants in the python-dev mailing list threads
+[1,2,5,6]_
References
==========
-.. [1] https://mail.python.org/pipermail/python-dev/2014-March/133334.html
-.. [2] https://mail.python.org/pipermail/python-dev/2014-March/133389.html
-.. [3] https://mail.python.org/pipermail/python-dev/2014-March/133438.html
-.. [4] https://mail.python.org/pipermail/python-dev/2014-March/133347.html
-.. [5] https://mail.python.org/pipermail/python-dev/2014-March/133442.html
+.. [1] PEP 466 discussion (round 1)
+ (https://mail.python.org/pipermail/python-dev/2014-March/133334.html)
+
+.. [2] PEP 466 discussion (round 2)
+ (https://mail.python.org/pipermail/python-dev/2014-March/133389.html)
+
+.. [3] Marc-Andre Lemburg's OpenSSL feedback for Windows
+ (https://mail.python.org/pipermail/python-dev/2014-March/133438.html)
+
+.. [4] Ned Deily's OpenSSL feedback for Mac OS X
+ (https://mail.python.org/pipermail/python-dev/2014-March/133347.html)
+
+.. [5] PEP 466 discussion (round 3)
+ (https://mail.python.org/pipermail/python-dev/2014-March/133442.html)
+
+.. [6] PEP 466 discussion (round 4)
+ (https://mail.python.org/pipermail/python-dev/2014-March/133472.html)
+
+.. [7] Donald Stufft's recommended set of backported features
+ (https://mail.python.org/pipermail/python-dev/2014-March/133500.html)
+
+.. [8] Alex Gaynor's recommended set of backported features
+ (https://mail.python.org/pipermail/python-dev/2014-March/133503.html)
+
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