Based on kernel version 4.9. Page generated on 2016-12-21 14:37 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) through the 'ima_template_fmt' kernel command line 32 parameter. At boot time, IMA initializes the chosen template descriptor 33 by translating the format into an array of template fields structures taken 34 from the set of the supported ones. 35 36 After the initialization step, IMA will call ima_alloc_init_template() 37 (new function defined within the patches for the new template management 38 mechanism) to generate a new measurement entry by using the template 39 descriptor chosen through the kernel configuration or through the newly 40 introduced 'ima_template' and 'ima_template_fmt' kernel command line parameters. 41 It is during this phase that the advantages of the new architecture are 42 clearly shown: the latter function will not contain specific code to handle 43 a given template but, instead, it simply calls the init() method of the template 44 fields associated to the chosen template descriptor and store the result 45 (pointer to allocated data and data length) in the measurement entry structure. 46 47 The same mechanism is employed to display measurements entries. 48 The functions ima[_ascii]_measurements_show() retrieve, for each entry, 49 the template descriptor used to produce that entry and call the show() 50 method for each item of the array of template fields structures. 51 52 53 54 ==== SUPPORTED TEMPLATE FIELDS AND DESCRIPTORS ==== 55 56 In the following, there is the list of supported template fields 57 ('<identifier>': description), that can be used to define new template 58 descriptors by adding their identifier to the format string 59 (support for more data types will be added later): 60 61 - 'd': the digest of the event (i.e. the digest of a measured file), 62 calculated with the SHA1 or MD5 hash algorithm; 63 - 'n': the name of the event (i.e. the file name), with size up to 255 bytes; 64 - 'd-ng': the digest of the event, calculated with an arbitrary hash 65 algorithm (field format: [<hash algo>:]digest, where the digest 66 prefix is shown only if the hash algorithm is not SHA1 or MD5); 67 - 'n-ng': the name of the event, without size limitations; 68 - 'sig': the file signature. 69 70 71 Below, there is the list of defined template descriptors: 72 - "ima": its format is 'd|n'; 73 - "ima-ng" (default): its format is 'd-ng|n-ng'; 74 - "ima-sig": its format is 'd-ng|n-ng|sig'. 75 76 77 78 ==== USE ==== 79 80 To specify the template descriptor to be used to generate measurement entries, 81 currently the following methods are supported: 82 83 - select a template descriptor among those supported in the kernel 84 configuration ('ima-ng' is the default choice); 85 - specify a template descriptor name from the kernel command line through 86 the 'ima_template=' parameter; 87 - register a new template descriptor with custom format through the kernel 88 command line parameter 'ima_template_fmt='.