linux/linux-5.4.31/Documentation/security/IMA-templates.rst

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IMA Template Management Mechanism
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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=``.