2IMA Template Management Mechanism
   9The original ``ima`` template is fixed length, containing the filedata hash
  10and pathname. The filedata hash is limited to 20 bytes (md5/sha1).
  11The pathname is a null terminated string, limited to 255 characters.
  12To overcome these limitations and to add additional file metadata, it is
  13necessary to extend the current version of IMA by defining additional
  14templates. For example, information that could be possibly reported are
  15the inode UID/GID or the LSM labels either of the inode and of the process
  16that is accessing it.
  18However, the main problem to introduce this feature is that, each time
  19a new template is defined, the functions that generate and display
  20the measurements list would include the code for handling a new format
  21and, thus, would significantly grow over the time.
  23The proposed solution solves this problem by separating the template
  24management from the remaining IMA code. The core of this solution is the
  25definition of two new data structures: a template descriptor, to determine
  26which information should be included in the measurement list; a template
  27field, to generate and display data of a given type.
  29Managing templates with these structures is very simple. To support
  30a new data type, developers define the field identifier and implement
  31two functions, init() and show(), respectively to generate and display
  32measurement entries. Defining a new template descriptor requires
  33specifying the template format (a string of field identifiers separated
  34by the ``|`` character) through the ``ima_template_fmt`` kernel command line
  35parameter. At boot time, IMA initializes the chosen template descriptor
  36by translating the format into an array of template fields structures taken
  37from the set of the supported ones.
  39After the initialization step, IMA will call ``ima_alloc_init_template()``
  40(new function defined within the patches for the new template management
  41mechanism) to generate a new measurement entry by using the template
  42descriptor chosen through the kernel configuration or through the newly
  43introduced ``ima_template`` and ``ima_template_fmt`` kernel command line parameters.
  44It is during this phase that the advantages of the new architecture are
  45clearly shown: the latter function will not contain specific code to handle
  46a given template but, instead, it simply calls the ``init()`` method of the template
  47fields associated to the chosen template descriptor and store the result
  48(pointer to allocated data and data length) in the measurement entry structure.
  50The same mechanism is employed to display measurements entries.
  51The functions ``ima[_ascii]_measurements_show()`` retrieve, for each entry,
  52the template descriptor used to produce that entry and call the show()
  53method for each item of the array of template fields structures.
  57Supported Template Fields and Descriptors
  60In the following, there is the list of supported template fields
  61``('<identifier>': description)``, that can be used to define new template
  62descriptors by adding their identifier to the format string
  63(support for more data types will be added later):
  65 - 'd': the digest of the event (i.e. the digest of a measured file),
  66   calculated with the SHA1 or MD5 hash algorithm;
  67 - 'n': the name of the event (i.e. the file name), with size up to 255 bytes;
  68 - 'd-ng': the digest of the event, calculated with an arbitrary hash
  69   algorithm (field format: [<hash algo>:]digest, where the digest
  70   prefix is shown only if the hash algorithm is not SHA1 or MD5);
  71 - 'd-modsig': the digest of the event without the appended modsig;
  72 - 'n-ng': the name of the event, without size limitations;
  73 - 'sig': the file signature, or the EVM portable signature if the file
  74   signature is not found;
  75 - 'modsig' the appended file signature;
  76 - 'buf': the buffer data that was used to generate the hash without size limitations;
  77 - 'evmsig': the EVM portable signature;
  78 - 'iuid': the inode UID;
  79 - 'igid': the inode GID;
  80 - 'imode': the inode mode;
  81 - 'xattrnames': a list of xattr names (separated by ``|``), only if the xattr is
  82    present;
  83 - 'xattrlengths': a list of xattr lengths (u32), only if the xattr is present;
  84 - 'xattrvalues': a list of xattr values;
  87Below, there is the list of defined template descriptors:
  89 - "ima": its format is ``d|n``;
  90 - "ima-ng" (default): its format is ``d-ng|n-ng``;
  91 - "ima-sig": its format is ``d-ng|n-ng|sig``;
  92 - "ima-buf": its format is ``d-ng|n-ng|buf``;
  93 - "ima-modsig": its format is ``d-ng|n-ng|sig|d-modsig|modsig``;
  94 - "evm-sig": its format is ``d-ng|n-ng|evmsig|xattrnames|xattrlengths|xattrvalues|iuid|igid|imode``;
 100To specify the template descriptor to be used to generate measurement entries,
 101currently the following methods are supported:
 103 - select a template descriptor among those supported in the kernel
 104   configuration (``ima-ng`` is the default choice);
 105 - specify a template descriptor name from the kernel command line through
 106   the ``ima_template=`` parameter;
 107 - register a new template descriptor with custom format through the kernel
 108   command line parameter ``ima_template_fmt=``.