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diff --git a/Documentation/security/IMA-templates.rst b/Documentation/security/IMA-templates.rst new file mode 100644 index 0000000000..15b4add314 --- /dev/null +++ b/Documentation/security/IMA-templates.rst @@ -0,0 +1,111 @@ +================================= +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); + - 'd-ngv2': same as d-ng, but prefixed with the "ima" or "verity" digest type + (field format: <digest type>:<hash algo>:digest); + - '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, based on either the file's/fsverity's digest[1], + or the EVM portable signature, if 'security.ima' contains a file hash. + - 'modsig' the appended file signature; + - 'buf': the buffer data that was used to generate the hash without size limitations; + - 'evmsig': the EVM portable signature; + - 'iuid': the inode UID; + - 'igid': the inode GID; + - 'imode': the inode mode; + - 'xattrnames': a list of xattr names (separated by ``|``), only if the xattr is + present; + - 'xattrlengths': a list of xattr lengths (u32), only if the xattr is present; + - 'xattrvalues': a list of xattr values; + + +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-ngv2": its format is ``d-ngv2|n-ng``; + - "ima-sig": its format is ``d-ng|n-ng|sig``; + - "ima-sigv2": its format is ``d-ngv2|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``; + - "evm-sig": its format is ``d-ng|n-ng|evmsig|xattrnames|xattrlengths|xattrvalues|iuid|igid|imode``; + + +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=``. |