[cap-talk] Capabilities and the NCSC Trusted Computer Security Evaluation Criteria (TCSEC)

Valerio Bellizzomi devbox at selnet.org
Sat Nov 4 16:37:33 CST 2006


On 01/11/2006, at 18.33, Jed at Webstart wrote:

>At 02:54 PM 10/30/2006, Bill Tulloh wrote:
>>I've been trying to trace the history of capability-based approaches
>>in the context of the emergence of the Trusted Computer Systems
>>Evaluation Criteria (the Orange Book)...
>
>As Bill mentioned I scanned in the following document:
>
>Traditional Capability-Based Systems:
>An Analysis of Their Ability to Meet the
>Trusted Computer Security Evaluation Criteria
>
>into:  http://www.webstart.com/jed/papers/P-1935/
>
>It was large enough that to make the scanning practical
>I had to divide it into roughly 10 page sections.  I already
>noticed that I put one page in twice (31).  If you take a look
>at that document, please let me know if you notice any
>other problems with the scanning - especially omissions.
>Of course the scan was of an already poor quality
>photocopy, but I believe everything significant in the
>original paper can be read, though some of the OCR'ed
>text is broken, especially in places where the photocopy
>wasn't too good.
>
>Here is my overall summary of this document.  These authors
>absolutely trash (!) the object/capability approach to computer
>systems, believing that it makes difficult or impossible just
>about every important aspect of their Trusted Computer Security
>Evaluation Criteria.
>
>At a high level (more details below) I would say that their view is
>that capabilities are designed to allow easy sharing of permissions
>(direct communication of permission tokens) and that this facility
>makes difficult to impossible nearly all the security features that
>they believe are important for blocking inappropriate sharing of
>permissions and auditing appropriate sharing of permissions.
>
>There is one quote it paper that I've sadly lost that says that above
>nearly perfectly, but I think the quotes below taken as a whole make
>their position quite clear - despite their show of academic non bias
>and thorough review of capability systems.
>
>Still staying at a high level I would say their position stems from
>two basic factors:
>
>1. They don't appreciate and/or understand the fact that blocking
>sharing of permissions between collaborating communicators
>(sometimes referred to as "conspiring" communicators) is
>simply not possible - with all the blocking and auditing
>that goes along with that truth.
>
>2.  They don't appreciate that by facilitating communication
>of POLA permissions that systems end up being more secure
>rather than less secure - contrary to their TCSEC.
>
>I also don't believe they understand just how easy and convenient
>revocation through a membrane can be.  I also believe that the
>implementations that they considered (perhaps any implementations)
>didn't have effective support for the sorts of revocation that they feel
>to be important.  For example, run a program, give it some permissions,
>the program ends, revoke all permissions that it was given directly or
>that it received indirectly.  Of course in a capability system one can
>extend that paradigm to requests on remote services.  I think that a
>discussion of what one might call "temporary" capabilities vs.
>permanent capabilities might be worthwhile on this list.
>
>However, for now I'll comment in more details on the NCSC TCSEC
>document (incidentally, the document referred to as Donnelley80
>is an earlier version of :
>
>http://www.webstart.com/jed/papers/Managing-Domains/
>
>).  I believe the concerns of the authors about capability systems
>are pretty clear, e.g.:
>
>pp. 3 "merely possessing the capability ... is sufficient proof
>that access should be granted.  Note that, if a capability is
>ever stolen or given away, this protection mechanism can result
>in other subjects accessing the object without incurring a
>protection violation.  Thus, special care must be taken to
>prevent the use of capabilities that are stolen while migrating
>on off-line storage or among different sites of a network
>[Nessett82, Donnelley80]"


Comparison of "capabilities" with "badges" is apropos, a stolen badge
doesn't incur in security violation, but it allows an unauthorized person
to open the door. It is then a question of detecting the theft ASAP, so
that the badge can be disabled (i.e. the capability revoked).


>
>pp. 13 "Whenever capabilities migrate on removable media, or among 
>the network hosts,
>their integrity must be maintained. Since capabilities in this 
>environment are out of the
>control of their creating system, the only possible approach to 
>integrity maintenance is the
>detection and rejection of capabilities whose integrity has been 
>violated. The option of not
>allowing capabilities to migrate is somewhat simplistic and, in many 
>cases, impractical
>[Lampson76]. Administrative controls, while helpful and necessary, 
>are based on the
>skills, care, and integrity of administrative personnel. Total 
>reliance on such controls are
>inadequate and impractical wherever capabilities migrate on off-line
>storage.
>In general, to help detect whether any migrated object (e.g., a
>capability) is
>changed, an encryption scheme can be used [Gligor79b]. This method 
>encrypts both the
>external representation of the object and a signature that may be 
>computed from the external
>representation when an object is placed on the removable media. When 
>the type manager
>reads an object from the removable media, it decrypts the external 
>representation, computes
>a new signature, then decrypts the returned signature and compares 
>signatures. If the
>signatures are the same, then the data is assumed to be correct. If 
>not, the data is assumed
>to be modified and the object is not restored. A similar scheme, 
>which works for
>capabilities but not for other types of objects, has also been 
>proposed in [Chaum78].
>Encryption-based mechanisms have also been proposed in [Nessett82,
>Donnelley80]
>to prevent the use of stolen capabilities. Capability theft becomes 
>important whenever
>capabilities are allowed to migrate on off-line storage or among the 
>sites of a distributed
>system. Their re-introduction and use within the system by an 
>unintended user must be
>prevented [Nessett82, Donnelley80]."
>
>pp. 14 "It must be noted that, regardless of the type of capability 
>system, the access
>authorization mechanism relies on (1) the capability integrity 
>mechanism, (2) the capability
>mapping mechanism, and (3) the ability of the processors and/or the 
>operating system
>software to protect their internal registers, data structures and 
>capabilities from
>unauthorized user access. (Such mechanisms must also be extended to 
>prevent the theft of
>capabilities wherever capabilities migrate across the sites of a 
>distributed system).
>Otherwise, the access authorization mechanism may be circumvented by
>users."
>
>Then we start to get to the crux of their argument:
>______________________________________________________________
>pp. 26 "2.3.1 Security Policies
>
>In traditional capability systems, capabilities are freely copyable 
>and can be placed
>in any storage object In the tagged memory approach, capabilities can 
>be intermixed with
>the data of storage object and in the partition approach, 
>capabilities can be stored in the C-list
>of the storage objects. This flexibility, although advantageous in 
>many ways, causes
>several important security policy and object management problems. In 
>fact, these problems
>are so great that traditional capability systems are unable to 
>support any mandatory security
>policies without extensions or basic modifications. (More information 
>is provided in
>Section 3.L4 - mandatory access control in the TCSEC.) Therefore, 
>without extensions or
>basic modifications, traditional capability systems are limited to 
>the support of discretionary
>security policies.
>
>Discretionary security policies are based on the notion that access 
>privileges for an
>object may only be changed by the owner (creator) of that object 
>[Saltzer75]. The three
>important components of such policies are: (1) the distribution, (2) 
>the review, and (3) the
>revocation of access privileges. Note that the discretionary policies 
>may allow an owner to
>entrust the authority of further distribution and/or revocation of 
>privileges to other parties at
>his discretion.
>
>     (1) Distribution of access privileges
>
>In capability systems, if a capability for an object with the 
>appropriate privileges can
>be presented, no further validation is necessary. In this scenario, 
>the identity of the subject
>requesting access is not needed for access checking. Since possession 
>of a capability is all
>that is needed to access an object, subjects and their surrogates 
>jealously guard capabilities.
>However, there are several ways that capabilities can be leaked to 
>outsiders. The first is
>through the domain call. Since capabilities for the input parameters 
>are copied and passed
>from the calling domain to the called domain, the called domain can 
>surreptitiously save the
>input capabilities in a storage segment that only it can access. At a 
>later time, the called
>domain is invoked by its owner and passes back the saved capability. 
>This Trojan Horse
>attack is an example of how easily capabilities can be propagated 
>without the knowledge of
>the capability owner. [Note that both in the partitioned approach and 
>in the tagged
>approach, capabilities can be passed from domain to domain as 
>parameters using the
>parameter passing mechanism. No storage of the passed capabilities in 
>shared, long-lived
>objects is necessary to distribute the capability.]
>
>Another method for acquiring a capability is to have it stored into a 
>segment that is
>shared with another user. For example, if one user has read access to 
>a segment that
>another user has write access to, anything written in that segment, 
>including capabilities,
>can be read and then used by the first user.
>
>Since capabilities can be leaked to outsiders, owner-controlled 
>distribution of access
>privileges is not possible in traditional capability systems. 
>However, it is possible to
>modify such systems so they can support owner-controlled distribution of
>access
>privileges. Such modifications are discussed in Section 3.4.
>
>Instead of owner-controlled distribution, traditional capability 
>systems support
>"holder"-controlled distribution; that is, they support a policy in 
>which any holder of an
>access privilege can freely distribute the privilege to anybody else. 
>This attribute has been
>considered a major advantage of capability systems because the 
>contrasting owner controlled
>policies must rely on a single distribution authority, potentially 
>making a system
>somewhat inflexible [SaItzer75]. However, due to access review and
>revocation
>requirements, it is often desirable to constrain the holder - 
>controlled distribution policy so
>that the owner of an object can delegate the right to distribute 
>access privileges to only a
>subset of all users that hold the capability. The delegation of 
>access privilege distribution
>cannot be supported in traditional capability-based systems. However, the
>same
>modifications that are necessary for the support of owner-controlled 
>distribution can also
>support the delegation of access privilege distribution to other 
>entities whom an owner
>trusts."
>__________________________
>
>Notice that they refer to delegation as "leaking".  You can see from 
>the above that
>they feel that the "Security Policies" that can be supported by 
>capabilities are
>completely inadequate.
>
>Then it gets worse:
>
>pp. 27  "(2) Access Review
>Access review is the mechanism which allows an owner of an object to 
>discover the
>identity of all users that have access to that object. The 
>holder-controlled distribution policy
>supported by traditional capability-based systems makes access review 
>in a capability
>system a very difficult procedure because all objects that can be 
>read by a subject must be
>examined for internal capabilities that also have the read privilege 
>enabled. Finding all
>capabilities that one subject can access requires searching the 
>transitive closure of all objects
>that can be read by that user. This is illustrated by an example in 
>Section 3.1. When a
>capability for an extended-type object is encountered, there is some 
>uncertainty about which
>capabilities the holder of the extended-type capability can actually 
>access. This depends on
>the function of the domain. Thus, access review in a capability 
>system providing type
>extension is possibly inaccurate. Note that even the partitioned 
>memory approach cannot
>avoid an extensive search since a potentially very large number of 
>object C-lists may be
>stored anywhere on disk space and, thus, all the disk space needs to 
>be searched. In
>systems with very large address spaces such searches are 
>prohibitively expensive
>regardless of whether tagging or memory partitioning is used for 
>capability implementation.
>
>If a traditional capability-based system is modified to support only 
>owner-controlled
>distribution of access privileges, then efficient access review by 
>object owners becomes
>possible. For example,..."
>
>and the related on:
>
>pp. 28 "In [Rede1174a,b, Gligor76,79c], it is shown that traditional 
>capability-based
>systems cannot support any of the review and revocation policies 
>suggested above.
>Modifications to capability systems that are necessary to support the 
>above policies are
>presented in Section 3.4."
>
>
>By the time they get to reviewing the NCSC Criteria they clearly feel
that
>the rout is on.  The "Access Review" section is particularly damning,
e.g.
>
>pp. 35 "Such a review mechanism for a traditional capability system 
>assumes that all
>capabilities that allow direct or indirect access to an object can be 
>found This assumption
>requires a form of "reverse" transitive closure, which starts with an 
>object and discovers all
>o~ects containing capabilities for that object, repeating the process 
>for all newly formed
>objects. However, neither the TCB nor any other operating system 
>module can rely on
>extensive searches of the system storage, particularly in systems 
>with virtual memory.
>(Experience with traditional garbage collectors, which also require 
>extensive memory-
>searches, supports this contention [BIShop77].) This procedure is 
>further complicated by
>sealed capabilities, which must be unsealed during the search to 
>determine what capabilities
>are sealed inside.
>
>and it goes on:
>
>pp. 37 "3.1.1. 5 Impact of the Controlled Distribution of Access
Privileges
>Requirement on Traditional Capability-Based Systems
>
>The interpretation of the controlled distribution of access 
>privileges requires the
>precise definition of the "authorized users" that can distribute 
>capabilities. In a traditional
>capability-based system, any holder of a capability is an authorized 
>distributor of access
>privileges.The capability and its access privileges are distributed 
>by merely making a copy
>of the capability (see Section 2.3.1). Thus, such systems may not 
>satisfy the required
>policy of user authorization. However, in some other capability-based 
>systems, the
>distribution of capability (and access privileges) is restricted in 
>some ways and, thus, the
>authorization policy supported by such systems differ from those 
>supported by traditional
>systems[Neumann75,77, Wu1f74]. (These modifications are discussed in 
>Section 3.4.)
>To determine whether a capability system can satisfy a given policy 
>of controlled
>distribution of access privileges, the authorization policy for 
>capability propagation must be
>explicitly stated or defined a model."
>
>By the time it gets to "Mandatory Access Control" the situation is 
>already hopeless, e.g.:
>
>pp. 39 "3.1.4 Mandatory Access Control
>
>Mandatory access control ensures that the subjects doing data accesses
have
>sufficient authorization for the data. Any secure system must support 
>this concept."
>
>and then without going into the gory details of one Mandatory Access 
>Control example here:
>
>"Although this problem also appears in non-capability-based systems, such
>as
>MULTICS, SCOMP and UNIX, when mandatory access controls are implemented,
>the
>problem can be solved in a simple way. By contrast, this problem 
>cannot be solved in
>traditional capability-based system without elimination of the 
>reference count interface -an
>unacceptable solution owing to the practical necessity of such a
>mechanism."
>
>They even get some digs in related to Covert Channels, e.g. the summary:
>
>pp. 57 "The three channel examples given in this section are not by 
>any means a complete
>list of channels that are unique to capability-based systems. Only by 
>exhaustive analysis of
>a concrete system implementation can such a listing be attempted."
>
>They even dig in regarding what they refer to as "Trusted Facility 
>Management", e.g.:
>
>pp. 58  "The discovery of lost objects is a very complex task in
>traditional
>capability-based systems.
>
>An important difference between capability-based systems and other
systems
>models is the ability to store and distribute capabilities for the 
>long term without any TCB
>software intervention. With this ability, applications 
>programmers/users may manage their
>object storage and access individually without much system 
>intervention. An important
>consequence of providing this ability is that capability-based system 
>functions, such as
>storage reclamation and garbage collection, which are traditionally 
>performed either
>automatically or by operators, become more complex because of the 
>need to discover all
>outstanding capabilities. Garbage collection in a traditional 
>capability system is an
>extremely time consuming task [Bishop77]. Unless additional 
>mechanisms are provided
>for guiding the "directed" search for capabilities on secondary 
>storage, exhaustive searches
>may render the system useless for long periods of time. Note that 
>directed searches of
>system storage also require "reverse" transitive closures to be taken 
>just as for capability
>use recovery.
>
>Similarly, user requests to the system operators or system 
>administrators for the
>recovery of specific capabilities that are accidentally erased (lost 
>objects) may require
>extensive memory searches and administrative system interfaces that 
>are unnecessary in
>other systems."
>
>
>They give a summary of what they view as the basic problems when they
>discuss
>potential modifications to "traditional" capability systems.  These 
>are essentially
>efforts to turn them into identity based systems.  Here are the four
>general
>properties that cause problems:
>
>pp. 59  "The following four general properties of traditional 
>capability-based systems cause
>security problems:
>
>(1) Capabilities can be stored by any domain freely, without TCB software
>intervention, in any segment of the storage system (uncontrolled
>propagation
>property);
>
>(2) Access authorization by capabilities does not require the identity of
>the
>accessor, namely that of domains, processes, or users (ticket-based
>authorization property);
>
>(3) The access privileges, objects, and address space available to an 
>accessor (i.e.,
>domain, process, or user) can only be determined by taking the transitive
>closure of all capabilities available to that accessor (transitive 
>closure property);
>
>(4) The identity of all accessors of any given object can only be
>determined by
>finding all capabilities that allow accesses to that object by direct 
>or indirect
>addressing (reverse transitive-closure property).
>
>The above properties of traditional capability-based systems cause 
>difficulties in the
>implementation of discretionary and mandatory access control 
>policies, audit, and trusted
>facility management (viz., Sections 3.1 - 3.3 above). Also, these 
>properties cause
>revocation of access to become difficult, or impossible, to perform. 
>Revocation has also
>been considered an important property of discretionary policy
[Redell74b]."
>
>With all this there is no doubt about their conclusions, e.g.:
>
>pp. 71 "...traditional capability-based systems prevent the 
>implementation of security
>policy and accountability as required by the TCSEC, and make some 
>aspects of trusted
>facility management and recovery more difficult than those of other
>systems."
>
>and:
>
>"Descriptor-based systems are superior to
>traditional capability-based systems in support of DoD policies and 
>accountability."
>
>
> From that time forward you can be sure that every effort was made by the
>DoD community to quash any development of capability systems, hence the
>continued use of the disastrous ambient authority model that we have
>in the market leading systems today.
>
>
>I believe this document and a few others like it during this time
>were essential in basically killing the object/capability approach
>to computer security.  I also believe that any effort we make to
>reverse this trend will have to answer the objections in this
>and similar documents.
>
>--Jed http://www.webstart.com/jed/ 


But this is an old paper, I believe the "traditional" capability-based
system they talk of are also old, it is possible that the *new*
"traditional" capability-based systems are different from the old ones.
Anyway I think that the TCSEC requirement 2,

"Requirement 2 - MARKING - Access control labels must be associated with
objects. In order to control access to information stored in a computer,
according to the rules of a mandatory security policy, it must be possible
to mark every object with a label that reliably identifies the object's
sensitivity level (e.g., classification), and/or the modes of access
accorded those subjects who may potentially access the object."

gives the impression that such "labels" are similar to the philosophy of
capabilities, except that capabilities identify the object itself (not its
sensitivity level) plus an access mode.
A badge is similar to a label too. If a badge is similar to a capability,
a thief can open doors with a stolen badge, but he cannot pass an human
security control that verifies the badge visually, bacause a badge
identifies also the person to who it is given by a photo and the name of
the person.
So, is it possible that the "marking" could be used as an element of a way
out of TCSEC, and as an argument to "rebirth" the object/capability
approach to computer security?


val





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