99 lines
4.6 KiB
ReStructuredText
99 lines
4.6 KiB
ReStructuredText
=================================
|
|
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);
|
|
- 'd-modsig': the digest of the event without the appended modsig;
|
|
- 'n-ng': the name of the event, without size limitations;
|
|
- 'sig': the file signature;
|
|
- 'modsig' the appended file signature;
|
|
- 'buf': the buffer data that was used to generate the hash without size limitations;
|
|
|
|
|
|
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``;
|
|
- "ima-buf": its format is ``d-ng|n-ng|buf``;
|
|
- "ima-modsig": its format is ``d-ng|n-ng|sig|d-modsig|modsig``;
|
|
|
|
|
|
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=``.
|