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authorKees Cook <keescook@chromium.org>2017-05-13 04:51:39 -0700
committerJonathan Corbet <corbet@lwn.net>2017-05-18 10:30:14 -0600
commit7b05b116845bafe8fd19cccb3719333b5f5fe3a2 (patch)
treeca57a87a5eca7b6968c6e4ff6da12ce4393578ee /Documentation/security/IMA-templates.txt
parent40fde647ccb0ae8c11d256d271e24d385eed595b (diff)
doc: ReSTify IMA-templates.txt
Adjust IMA-templates.txt for ReST markup and add to the index for security/, under the Kernel API Documentation. Cc: Mimi Zohar <zohar@linux.vnet.ibm.com> Signed-off-by: Kees Cook <keescook@chromium.org> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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- IMA Template Management Mechanism
-
-
-==== INTRODUCTION ====
-
-The original 'ima' template is fixed length, containing the filedata hash
-and pathname. The filedata hash is limited to 20 bytes (md5/sha1).
-The pathname is a null terminated string, limited to 255 characters.
-To overcome these limitations and to add additional file metadata, it is
-necessary to extend the current version of IMA by defining additional
-templates. For example, information that could be possibly reported are
-the inode UID/GID or the LSM labels either of the inode and of the process
-that is accessing it.
-
-However, the main problem to introduce this feature is that, each time
-a new template is defined, the functions that generate and display
-the measurements list would include the code for handling a new format
-and, thus, would significantly grow over the time.
-
-The proposed solution solves this problem by separating the template
-management from the remaining IMA code. The core of this solution is the
-definition of two new data structures: a template descriptor, to determine
-which information should be included in the measurement list; a template
-field, to generate and display data of a given type.
-
-Managing templates with these structures is very simple. To support
-a new data type, developers define the field identifier and implement
-two functions, init() and show(), respectively to generate and display
-measurement entries. Defining a new template descriptor requires
-specifying the template format (a string of field identifiers separated
-by the '|' character) through the 'ima_template_fmt' kernel command line
-parameter. At boot time, IMA initializes the chosen template descriptor
-by translating the format into an array of template fields structures taken
-from the set of the supported ones.
-
-After the initialization step, IMA will call ima_alloc_init_template()
-(new function defined within the patches for the new template management
-mechanism) to generate a new measurement entry by using the template
-descriptor chosen through the kernel configuration or through the newly
-introduced 'ima_template' and 'ima_template_fmt' kernel command line parameters.
-It is during this phase that the advantages of the new architecture are
-clearly shown: the latter function will not contain specific code to handle
-a given template but, instead, it simply calls the init() method of the template
-fields associated to the chosen template descriptor and store the result
-(pointer to allocated data and data length) in the measurement entry structure.
-
-The same mechanism is employed to display measurements entries.
-The functions ima[_ascii]_measurements_show() retrieve, for each entry,
-the template descriptor used to produce that entry and call the show()
-method for each item of the array of template fields structures.
-
-
-
-==== SUPPORTED TEMPLATE FIELDS AND DESCRIPTORS ====
-
-In the following, there is the list of supported template fields
-('<identifier>': description), that can be used to define new template
-descriptors by adding their identifier to the format string
-(support for more data types will be added later):
-
- - 'd': the digest of the event (i.e. the digest of a measured file),
- calculated with the SHA1 or MD5 hash algorithm;
- - 'n': the name of the event (i.e. the file name), with size up to 255 bytes;
- - 'd-ng': the digest of the event, calculated with an arbitrary hash
- algorithm (field format: [<hash algo>:]digest, where the digest
- prefix is shown only if the hash algorithm is not SHA1 or MD5);
- - 'n-ng': the name of the event, without size limitations;
- - 'sig': the file signature.
-
-
-Below, there is the list of defined template descriptors:
- - "ima": its format is 'd|n';
- - "ima-ng" (default): its format is 'd-ng|n-ng';
- - "ima-sig": its format is 'd-ng|n-ng|sig'.
-
-
-
-==== USE ====
-
-To specify the template descriptor to be used to generate measurement entries,
-currently the following methods are supported:
-
- - select a template descriptor among those supported in the kernel
- configuration ('ima-ng' is the default choice);
- - specify a template descriptor name from the kernel command line through
- the 'ima_template=' parameter;
- - register a new template descriptor with custom format through the kernel
- command line parameter 'ima_template_fmt='.