Developer Summit 2007 minutes

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Below are the minutes from the developer summit that followed the 2007 SELinux Symposium.

SELinux developer summit / March 16 2007

Contents

[edit] Purpose

The purpose of the developer summit was to provide a forum for focused, detailed technical discussion of ongoing development and future plans for SELinux among core SELinux developers and contributors.


[edit] Attendees

  • James Antill, Red Hat
  • Joshua Brindle, Tresys / Gentoo
  • Dave Caplan, Tresys
  • Jim Carter, NSA
  • Darrel Goeddel, TCS
  • Chad Hanson, TCS
  • Trent Jaeger, Penn State Univ
  • KaiGai Kohei, NEC / JSELUG
  • Peter Loscocco, NSA
  • Karl MacMillan, Red Hat
  • Paul Moore, HP
  • James Morris, Red Hat
  • Eric Paris, Red Hat
  • Chris PeBenito, Tresys / Gentoo
  • Chad Sellers, Tresys
  • Stephen Smalley, NSA
  • Brian Sniffen, MITRE
  • Manoj Srivastava, Debian
  • Chris Vance, SPARTA
  • Eamon Walsh, NSA
  • Dan Walsh, Red Hat

[edit] Agenda

  • Opening Session
  • Welcome and introductions.
  • Take any final suggestions for topic or schedule changes.
  • Discuss general topics.
    • Central project site / pan-distribution support.
    • Dealing with different user groups and their needs.
  • Policy Session
    • Policy language/toolchain.
    • Policy tools/infrastructure.
    • Policy configuration.
  • Userland Session
    • Securing the desktop.
    • Improved userland integration.
    • Userspace object managers.
    • Embedded support.
    • MCS
    • User interface / label translation.
  • Networking Session
    • Labeled networking.
    • SECMARK.

[edit] General Topics

The summit began with an hour-long opening session to introduce the participants, take any final suggestions for topic or schedule changes, and discuss several general topics. The main topics that were covered during the initial session included a central project site for SELinux and dealing with different user groups and their needs.


[edit] Central Project Site/Pan-Distribution Support

The first topic of discussion was on the issue of having a central project site for all SELinux-related information and improving pan-distribution support. At present, information and code for SELinux is distributed among a disparate collection of web sites and repositories (nsa.gov/selinux, selinux.sourceforge.net, selinuxnews.org, oss.tresys.com, tresys.com/selinux, kernel.org, distribution-specific resources), with no single jumping off point for everything related to SELinux. It was agreed that a centralized wiki that could be directly edited by core SELinux developers and distribution maintainers would be helpful, but that there was no need to centralize all of the SELinux content and repositories. James Morris may set this up eventually, but is also open to someone else doing so sooner. It would require a site that could provide accounts for all core participants to enable direct editing.

Related to this topic was the topic of handling the SELinux userland patches, which have migrated into the per-distribution packages and have no selinux upstream repository presently. It was agreed that extracting the remaining patches from the distribution-specific packages, re-basing them against some upstream baseline, and keeping them in a form that could be consumed by all the different distributions would be very difficult and not maintainable long term. Instead, the suggestion was made to simply provide a definitive list per distribution of the remaining patches and where they can be found via the wiki, and to foster greater communication among the distributions about the patches and future changes to them, as well as continuing to strive towards upstream acceptance for the remaining patches where feasible. This also requires encouraging package maintainers to notify the right people when making changes to the selinux userland patches in their packages. Dan Walsh took the action of generating a list of SELinux userland modifications in Fedora.

During the discussion of the userland patches, it was noted that the selinux patches have become very intertwined in Fedora with MLS/MCS-specific functionality and with audit functionality, so the question was raised as to the portability of those patches to other distributions. At present, MLS/MCS functionality is not of interest to the Gentoo selinux maintainers but is of interest to the Debian selinux maintainers (which are also enabling MCS by default), and audit userland support is not yet in Debian and is not up-to-date in Gentoo.


[edit] Dealing with Different User Groups/Needs

The second main topic of discussion was how to deal with different user groups (e.g. government vs. corporate, embedded vs. general purpose) in a manner that satisfies their needs but doesn't lead to trying to impose the same solution on everyone or to fragmentation of SELinux into specialized variants. Dave Caplan noted that most of Tresys' work for government customers has not required the use of the MLS support at all, as Type Enforcement has met the real need more directly and effectively. Karl MacMillan suggested that the only case where the MLS support provided value was when the customer needed thousands of categories. In the common case, the call for MLS was simply driven by existing (paper) policy and procedures, customers used to legacy systems with certain functionality, and the existing (niche) market for "trusted" operating systems. Nonetheless, it was acknowledged that the MLS support would need to be retained long term to support such users, and that MLS support in SELinux can serve as an enabler for introducing TE to the existing MLS user base.

The need for a general API for certain label manipulation functions to support the small set of policy-aware applications was then discussed, such as an interface to compute a label that dominates a set of input labels or an interface to order a set of labels. Such an API could be provided by libsepol based on a policy file or by libselinux using new selinuxfs nodes to query the active kernel policy, although in some cases the detailed label logic is hidden in the context translation library (as when using the MITRE label encodings library). It was agreed that SELinux should provide general APIs for such functions to applications as needed, seeking to encapsulate as much as possible (e.g. acting on entire contexts rather than just the MLS field, even though the rest of the context may not presently have any real ordering).

If the MLS support is to be sustained long term, it needs to be attractive to ordinary users. Labeled printing was cited as an example of functionality that could be useful to non-government users, although the current labeled printing support doesn't (yet) support preserving the document label. Desktop integration was cited as another example where MLS and/or MCS style functionality could potentially be more useful due to more human-meaningful labels. However, the inherent limitation of MLS/MCS to confidentiality (not integrity) and its inflexible model was noted by Karl MacMillan as a fundamental impediment to widespread use, and TE could provide the same benefits for labeled printing and desktop operations if types are translated to more meaningful labels (using the existing context translation facilities already employed for MLS) and made more directly accessible to end users. In the end, it wasn't clear that MLS support could be made attractive outside of specialized users due to its inherent limitations.

One of the aspects of SELinux in general that has been attractive to users is labeling data based on what it is, not who can access it (unlike ACLs). Users are also interested in support for roles for administrative decomposition, although in practice RBAC is often applied at the application level rather than the OS level.

There was some discussion of what MLS offers that TE doesn't do well today. In addition to supporting large numbers of categories (and their combinations), the implicit support for hierarchy and ranges in MLS and its more direct link back to the user was cited as possible advantages. It was noted that it should be possible to build higher level tools and languages for expressing hierarchical relationships among types more easily, mapping them down to the low level access matrix. However, particularly with increasing number of roles, the number of types and their relationships tend to explode, suggesting that we may ultimately need/want more compact ways of expressing related types even in the low level representation. One simple example would be easily expressing that a new type should have all of the permissions of an existing type plus a specific list of additional permissions. Prototyping such support in a higher level representation first and then later assessing whether it should be migrated into the userspace policy module format or even down into the kernel representation was viewed as the best approach for exploring these ideas.


[edit] Policy Session

The remainder of the morning was spent in the policy session. This session included discussions on the policy language and toolchain, the policy tools and infrastructure, and the policy configuration.


[edit] Policy Language/Toolchain

This topic began with discussion of the proposed new libsepol policy representation aka the policy IR that had been posted to the list as an RFC. There was agreement that such an IR would facilitate a number of improvements to the language and toolchain and was desirable. Karl MacMillan requested help in completing the implementation in a timely manner, and there will be a followup meeting among Karl, Joshua, and Stephen to discuss the current code and approach. The basic steps are to finish the parser, remove the module format support from the policydb, generate the policydb from the tree representation, provide convertors to and from the tree representation for compatibility, and introduce the new binary on-disk format. The serialization code developed for policy management server should largely be unaffected, although it may wish to take advantage of the string compression support as an optimization. A branch will be created for joint development on sourceforge.

Language support for reference policy interface is still planned, but is blocked on the completion of the new policy representation. Automatic generation of require statements and deprecation of require statements altogether is also still planned. There was a concern about being able to unambiguously distinguish types from attributes, although that should be discernible at expand time.

A concern about being able to ship a single policy package (.pp) file that can support multiple base policies was raised. In the past, the notion of fat policy packages was considered but rejected. The problem will be partly addressed through link-time expansion of interfaces, but there will still be dependencies on particulars of the base policies (e.g. MLS vs. non-MLS, particular types defined by a given policy, etc).

On the package management front, the plan in Fedora is still to install policy packages from %post and restorecon afterward. The view is that restorecon is needed after install regardless due to files created from %post scriptlets. There are still issues to be resolved with the installer, such as generating a full file contexts configuration during build for use by the installer. We also need to resolve what other files need to be installed by software packages beyond the .pp file, such as the interface files (possibly eliminated by introduction of link-time expansion of interfaces) and help/documentation files. There was some debate as to whether such files belongs in the .pp file itself or as separate auxiliary files. It was also noted that package managers still have no way to transparently support policy upgrades where the decomposition of policy changes, such as moving a policy module in or out of the base. Although semodule does support such transactions (e.g. semodule -r x.pp -i base.pp), there is no way for the package manager to automatically determine the right incantation presently. Finally, removing of types still presents a problem for per-package policy


[edit] Policy Tools/Infrastructure

This topic began with discussion of policy generation and development tools, with a call to eliminate and avoid duplication among the tools efforts, ensure that we are working toward a common goal, and making sure that we are reaching the user. There was a question as to whether the use of python by Madison/SEPolgen and other policy tools (like Polgen or semanage) is an impediment to integration with other tools efforts and whether the use of a scripting language for such tools is a security concern for SELinux.

SLIDE is presently limited to using Madison/SEPolgen via helper programs as jython lacks adequate support. SETools has no problems with the use of python. The larger concern seems to be with the stability of the python language. However, there are no plans to move away from python.

The security concerns can be addressed by providing a binary wrapper for the python scripts that performs adequate sanitization and ensures that the proper script is invoked. Such a wrapper has already been created for the CLIP work from the upstream python, and should be integrated into policycoreutils and used as the gate to our python scripts.

Tools for generating new types from existing ones with small deltas are needed, e.g. creating a new cgi domain that is identical to the base one except for adding or removing a small set of permissions, and then applying that domain to a particular script. Similarly for firefox, in order to easily construct separate firefox domains for e.g. internal vs. external access.

Discussion then moved into policy management tools. Dan Walsh asked whether we should be moving all object context definitions (e.g. port contexts) out of the base policy and into semanage as Brickwall has done to enable full customizability, rewriting policy to use attributes more. This will require introducing support into libsemanage for system definitions of these object contexts separate from local definitions or modules (e.g. ports vs. ports.local), as has been done for some objects. There is a concern that rewriting the policy in terms of attributes to such an extent could harm understandability.

Management of the home directory labeling was noted a crucial problem for enabling strict policy and user roles. Home directories are the most common place for labeling problems to manifest, and relabeling entire home directories is too expensive. Joshua noted that we need a general facility for per-user types that covers all objects, not just user home directories as in genhomedircon. This is to be taken up on list.

Dan noted that allowing setting of specific booleans for a less privileged user role is currently impossible because libsemanage regenerates the entire policy and resets all booleans, even for non-persistent boolean changes. Some investigation into what is happening presently and whether libsemanage needs to be refactored is required.

Dan also noted that non-modular configuration files such as the default_contexts file and default_type files pose a problem for fully modular policy and adding roles. We need to support specification and manipulation of such data via semanage or in the .pp files or in the policy module sources.

The policy management server (PMS) RFC release was discussed. One of the immediate potential applications of the server, delegation of access to booleans, isn't presently supportable since the current boolean controls are based on file-level labeling and access control. Karl recommended just emulating the file-based checks in the server. The PMS patches will be coordinated with the new policy representation patches.

[editorial note: Moved some discussion notes down to the Policy Configuration section as they fit under it more cleanly.]

It was agreed that front-end tools for policy could be included in the "upstream" selinux. system-config-selinux needs to be cleaned up and converted over to executing helper programs rather than directly using python bindings to ensure that the security relevant functionality runs in its own domain. Management of local policy modules should be integrated into such tools to help users with preserving the results of prior modules. Integration with Madison/SEPolgen is planned.


[edit] Policy Configuration

This topic began with discussion of the hierarchical apache policy RFC was discussed. The hierarchical type system provides a way for a higher level administrator to define a broad container type with a maximal set of permissions to the rest of the system that he is willing to delegate to a subordinate administrator, and then allow the subordinate administrator to subdivide that container type into any number of specific types, and the hierarchical apache policy is a rewrite of the apache policy with no functional changes that provides such a decomposition. Applications of the approach could include delegation to application administrators, representation of a meta-policy for a virtualized platform with multiple policies being enforced by the hypervisor and individual guest OSes, enabling users to craft more limited policies for user cgis bounded by the general cgi policy, etc.

Several concerns about the practical applicability of the hierarchical type scheme were raised, such as whether the abstract container types would effectively grow to effectively being unconfined, whether policy writers can effectively predefine the right set of container types for end administrators, and whether end administrators can make effective use of such a mechanism. Karl argued that real administrators would either require subordinates to pass policy changes through them for review and committing or only use very coarse-grained delegation (e.g. at the machine/VM level). He also expressed concern that the meta-policy will quickly grow to being as complex as normal policy if we go beyond very coarse-grained container types, reducing management of meta-policy to the same level as regular policy.

The question was raised as to whether there would be any objection to merging the hierarchical apache policy into reference policy, as it should be functionally equivalent to the original apache policy, in order to retain a worked example of how to use the type hierarchy for users. The only concern was compatibility due to the current practice of tying the hierarchy to the type names. Although type aliases are defined in the hierarchical apache policy to preserve compatibility, the kernel will always use the primary type names when reporting the contexts to users via kernel interfaces and audit logs, so the new hierarchical type names would be user-visible. As specific type names have become well-known to users, particularly for dealing with apache policy and other targeted domains, we no longer have the freedom to arbitrarily change them even with type aliases. However, if the hierarchy can be defined explicitly separate from the naming, there is no fundamental objection to merging the hierarchical apache policy. Language support for such hierarchy definition needs to be done.

Discussion then moved on to improving the security of the policy. Dan argued that we need to move away from high level interfaces in reference policy because they foster poor policy writing - users will always pick the most abstract interface that meets their needs, and thus the most permissive. When users had to always write policy manually, such high level interfaces were required, but with tools like SEPolgen that can generate interface calls based on need and loadable module support, we can migrate back to using the lowest level interfaces and building up from the bottom. Dan was also concerned that the hierarchical type system would lead to misuse of the abstract container types by users in the same manner. We need to ensure that reference policy is providing the right set of primitive interfaces and that tools favor least privilege. Dan suggested that we need some lower level primitives than apache_template and user_template. Dan also argued that we can begin tightening reference policy significantly since users now have the tools they need to generate their own local policy easily, and get the system into a secure-by-default state instead of shipping a relatively open policy and requiring users to manually tighten it via booleans or other mechanisms.

Plans for merging of strict and targeted policy were discussed, using presence of the unconfined module as the selector. unconfined_u, _r, _t would be added by the module. A boolean/tunable may be needed for other unconfined domains under targeted. Handling of user home directory labeling is still being worked out; the current idea is to introduce a new user role and types for confined users under targeted so that no change is required for existing unconfined users.

The last topic under the policy session was on how to improve policy QA to minimize regressions. A large part of the problem is simply getting packagers to test with SELinux prior to releasing new or updated packages. It was noted that Red Hat has a QA process for RHEL that checks for the presence of AVC messages, mislabeled files, devices left in device_t, processes left in initrc_t, etc. It would be beneficial to have something similar for Fedora and other distributions.


[edit] Userland Session

The afternoon began with a session on userland issues. This session included discussions on securing the desktop, improved userland integration, userspace object managers, embedded support, Multi-Category Security (MCS), user interface, and label translation issues.

On the desktop front, XACE/XSELinux work is progressing well and on target for upstreaming. There is some uncertainty as to the optimal window manager / desktop environment for continuing the work. metacity / GNOME would be advantageous from a user base / technology transfer point of view, but there are concerns with the size, complexity, level of interactions among components, and vibrancy of the development community (seemingly little activity on public lists). Dan Walsh recommended talking to the head of the desktop team at Red Hat, and will provide a POC to Eamon and Jim. With continued migration over to D-BUS, things should be well positioned for SELinux to control.

Another concern for the desktop work is the lack of robust error handling in X clients, primarily due to Xlib. As a result, permission denials have destructive effects on applications.

Handling cut-and-paste is still planned, but there are some problems with the existing mechanism, and a number of applications implement their own custom mechanisms for cut-and-paste that might also need to be addressed. Some in the X community have proposed starting over fresh with a new approach to cut-and-paste functionality.

Related to the desktop, Karl mentioned the ConsoleKit and PolicyKit work on freedesktop. They are creating their own infrastructure to support simple policies over actions like the ability to mount usb drives. Karl has tried to encourage them to leverage SELinux, but the problem is that they need to support some form of access control on all distributions, so they can't bind to SELinux alone. We need a LSM-like framework for ConsoleKit and/or PolicyKit to enable them to achieve their goals (e.g. via an ACL module) without precluding proper integration with SELinux (e.g. via a Flask/SELinux module). Eamon and Jim will try to talk with them further, although they can't scale to handle all such applications.

It was also discussed whether something like the userspace security server could enable broader use of SELinux access controls in userspace without requiring full use of SELinux underneath. However, one would still need some underlying support for e.g. getpeercon functionality, and one would still need SELinux policy and tools for the application policies, so it isn't clear that such an approach is useful.

Discussion then turned to userspace object managers, in particular the SEPostgreSQL work by KaiGai Kohei. KaiGai discussed issues he had encountered in his work, such as the need to create a custom userspace AVC for performance reasons (avoid indirection through AVC SID table when the object manager needs to directly deal with contexts e.g. for persistent labeling), to enable sharing of the AVC (via shared memory) and the netlink thread among all PostgreSQL instances. He also discussed the need to deal with contexts invalidated by a policy reload, as the userspace AVC presently has no way to detect such contexts easily (security_compute_av just returns EINVAL) or to remap them to an unlabeled context (as the kernel does). It was agreed that the userspace AVC in libselinux should be extended to better support such functionality, and discussion will follow up on the list.

KaiGai still plans to work on converting to dynamic lookup of classes and permissions for SEPostgreSQL, using the libselinux interfaces that presently just use the generated tables internally. The file format issues were also discussed, but James Antill of Red Hat noted that PostgreSQL file format changes are not uncommon and are not a fundamental obstacle.

Next KaiGai presented work done by JSELUG on SEBusybox and SEDiet. The SEBusybox work is proceeding well. There was a proposed solution to busybox domain transitions using dynamic context transitions, but this approach has been discarded in favor of the wrapper binary or decomposition approach due to its lack of real separation.

SEDiet is a new activity by JSELUG to work on reducing the resource requirements of SELinux to facilitate embedded use. libsepol and policy are of particular concern. They have some patches to reduce the need for libsepol, which they should submit for discussion on list. One point of uncertainty was what was meant by removing libsepol, as libsepol (or some subset of it) is required for loading policy, and if using shared libraries, it shouldn't require more than one instance of the library code. Karl also noted that libsepol is expected to get smaller through the new policy representation. It is also likely that libsepol can be reduced through eliminating legacy functionality (e.g. see prior RFC on Dropping setlocaldefs support from list), although that raises a question for the embedded SELinux community as to whether they still need such support for handling local booleans without needing libsemanage. Discussion should follow up on list.

The SEDiet effort also proposed a couple ways of reducing policy size, both of which leveraged greater use of type attributes. The first approach (aggregation of macros) replaced multiple macro calls with a series of typeattribute declarations followed by a single macro call on the attribute. The preferred approach there is to instead perform the typeattribute statement in the macro/interface, and continue to call the macro/interface on each type in the policy. Such an approach has already been leveraged in refpolicy for unconfined and could be applied to other frequently used interfaces, such as the one for using shared libraries.

The second approach (replacing allow rules with typeattributes) proposed converting series of allow rules with the same accesses to a series of typeattribute statements with a single allow rule, possibly using a tool that would analyze and rewrite binary policy automatically. The consensus was that we should instead focus on applying such improvements at the source level in reference policy, except possibly for certain specialized optimizations that cannot be done effectively in the sources, such as optimization of allow rules with type set exclusions (e.g. { a -b -c }) and rearranging the types to put the most frequently used ones near the beginning in an effort to shrink the ebitmaps in the binary representation.

There are also a number of improvements that can still be applied to libsemanage for space optimization, such as removing files after installation and incorporating bzip2 support. These have been discussed on list but need to be brought to completion.

Discussion then moved onto Multi-Category Security (MCS). Points of agreement here include the desire to improve general SELinux userland integration, make TE more accessible and usable to end users, and keep the MLS engine viable for its user community. Whether or not MCS will or will not ultimately prove useful remains uncertain at this point, but we are unlikely to know for sure until we have greater userland integration. Hence, it was suggested that we leave MCS intact, continue to work toward better SELinux userland integration (in general, not limited to MCS), and let the users decide. A concern was raised however about a lack of a common user experience for SELinux since not all distributions enable the MLS engine or use MCS. On the one hand, this helps ensure that both MLS and non-MLS code paths are being tested by users, but on the other hand, it yields a different user interface, could create confusion, and could yield applications that work in one environment but not another. One option here would be to take the next logical step for the MLS engine, having already moved from a compile-time option to a runtime option, and make it always enabled in SELinux

It was also suggested that MCS integration would serve as a proof of concept for integration for MLS later (although there are issues with such an approach, particularly in the discretionary MCS model, as what works for MCS may very well break under MLS) and as a stepping stone to greater functionality. Some believe that MLS/MCS labels are inherently more understandable to end users and transferable across systems than TE types, while others believe that TE types can and should be made understandable and transferable.

Related to this discussion, it was noted that the user interface necessarily exposes policy-specific knowledge, such as what types mean (e.g. httpd_sys_content_t). We need tools and conventions for how a tool or user can find out what types are accessible, what they mean, etc. Discussion also touched on the abstract label translation work prototyped at Tresys for crossing administrative domains, and on the need for a policy-driven translation service for SELinux, used by everything that imports or exports labeled data.


[edit] Networking Session

The last session focused on networking issues. This session included discussions on improving the labeled networking support and enabling use of SECMARK.

The first topic was labeled networking improvements. These fell into two categories: labeling of network buffers over loopback, and handling packet forwarding. On loopback, the basic need is to propagate the security label with the sk_buff. In CIPSO, this is effectively faked by putting the label in the payload/data, but the same approach doesn't work for labeled IPSEC. Attempts were made to enable labeled IPSEC over loopback, but various problems arose, including with the IKE daemon. Paul Moore suggested adding code in the loopback driver to add another pseudo header on top of the ethernet header to convey the information. Pete suggested creating a fake IP security option for loopback. James Morris suggested hijacking the MAC address. Paul Moore will follow up.

For forwarding, Paul Moore suggested introducing a mechanism that would be exclusive of SECMARK, using preroute, postroute, and forwarding hooks. The forwarding hook can look at the label on the packet and should know where it is going, so it should be able to apply a check at that time.

On enabling use of SECMARK, a number of concerns were raised with the usability of SECMARK today, including impact on users who disable iptables, bad interactions with management tools and user customizations, understandability, synchronization of updates with policy, and possible performance implications. Red Hat would like a boolean for every domain to switch between allowing all network traffic and only allowing secmark-labeled traffic, although there was some question as to whether that was truly desirable vs. the existing compat_net global. netfilter_contexts is presently built from refpolicy and shipped in policy packages (.pp files), including libsemanage support, yet completely unused today. The short term plan is to stop generating and shipping netfilter_contexts (possibly deprecating it and removing it altogether if it cannot be integrated) and simply allow users who wish to use secmark to manually enable it and manage it using the usual iptables tools. James Morris will follow up on list with some ideas for enabling more effective use of secmark.

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