Based on kernel version 3.15.4. Page generated on 2014-07-07 09:04 EST.
1 IMA Template Management Mechanism 2 3 4 ==== INTRODUCTION ==== 5 6 The original 'ima' template is fixed length, containing the filedata hash 7 and pathname. The filedata hash is limited to 20 bytes (md5/sha1). 8 The pathname is a null terminated string, limited to 255 characters. 9 To overcome these limitations and to add additional file metadata, it is 10 necessary to extend the current version of IMA by defining additional 11 templates. For example, information that could be possibly reported are 12 the inode UID/GID or the LSM labels either of the inode and of the process 13 that is accessing it. 14 15 However, the main problem to introduce this feature is that, each time 16 a new template is defined, the functions that generate and display 17 the measurements list would include the code for handling a new format 18 and, thus, would significantly grow over the time. 19 20 The proposed solution solves this problem by separating the template 21 management from the remaining IMA code. The core of this solution is the 22 definition of two new data structures: a template descriptor, to determine 23 which information should be included in the measurement list; a template 24 field, to generate and display data of a given type. 25 26 Managing templates with these structures is very simple. To support 27 a new data type, developers define the field identifier and implement 28 two functions, init() and show(), respectively to generate and display 29 measurement entries. Defining a new template descriptor requires 30 specifying the template format, a string of field identifiers separated 31 by the '|' character. While in the current implementation it is possible 32 to define new template descriptors only by adding their definition in the 33 template specific code (ima_template.c), in a future version it will be 34 possible to register a new template on a running kernel by supplying to IMA 35 the desired format string. In this version, IMA initializes at boot time 36 all defined template descriptors by translating the format into an array 37 of template fields structures taken from the set of the supported ones. 38 39 After the initialization step, IMA will call ima_alloc_init_template() 40 (new function defined within the patches for the new template management 41 mechanism) to generate a new measurement entry by using the template 42 descriptor chosen through the kernel configuration or through the newly 43 introduced 'ima_template=' kernel command line parameter. It is during this 44 phase that the advantages of the new architecture are clearly shown: 45 the latter function will not contain specific code to handle a given template 46 but, instead, it simply calls the init() method of the template fields 47 associated to the chosen template descriptor and store the result (pointer 48 to allocated data and data length) in the measurement entry structure. 49 50 The same mechanism is employed to display measurements entries. 51 The functions ima[_ascii]_measurements_show() retrieve, for each entry, 52 the template descriptor used to produce that entry and call the show() 53 method for each item of the array of template fields structures. 54 55 56 57 ==== SUPPORTED TEMPLATE FIELDS AND DESCRIPTORS ==== 58 59 In the following, there is the list of supported template fields 60 ('<identifier>': description), that can be used to define new template 61 descriptors by adding their identifier to the format string 62 (support for more data types will be added later): 63 64 - 'd': the digest of the event (i.e. the digest of a measured file), 65 calculated with the SHA1 or MD5 hash algorithm; 66 - 'n': the name of the event (i.e. the file name), with size up to 255 bytes; 67 - 'd-ng': the digest of the event, calculated with an arbitrary hash 68 algorithm (field format: [<hash algo>:]digest, where the digest 69 prefix is shown only if the hash algorithm is not SHA1 or MD5); 70 - 'n-ng': the name of the event, without size limitations; 71 - 'sig': the file signature. 72 73 74 Below, there is the list of defined template descriptors: 75 - "ima": its format is 'd|n'; 76 - "ima-ng" (default): its format is 'd-ng|n-ng'; 77 - "ima-sig": its format is 'd-ng|n-ng|sig'. 78 79 80 81 ==== USE ==== 82 83 To specify the template descriptor to be used to generate measurement entries, 84 currently the following methods are supported: 85 86 - select a template descriptor among those supported in the kernel 87 configuration ('ima-ng' is the default choice); 88 - specify a template descriptor name from the kernel command line through 89 the 'ima_template=' parameter.