Based on kernel version 2.6.39.1. Page generated on 2011-06-03 13:47 EST.
1 2 3 "Good for you, you've decided to clean the elevator!" 4 - The Elevator, from Dark Star 5 6 Smack is the the Simplified Mandatory Access Control Kernel. 7 Smack is a kernel based implementation of mandatory access 8 control that includes simplicity in its primary design goals. 9 10 Smack is not the only Mandatory Access Control scheme 11 available for Linux. Those new to Mandatory Access Control 12 are encouraged to compare Smack with the other mechanisms 13 available to determine which is best suited to the problem 14 at hand. 15 16 Smack consists of three major components: 17 - The kernel 18 - A start-up script and a few modified applications 19 - Configuration data 20 21 The kernel component of Smack is implemented as a Linux 22 Security Modules (LSM) module. It requires netlabel and 23 works best with file systems that support extended attributes, 24 although xattr support is not strictly required. 25 It is safe to run a Smack kernel under a "vanilla" distribution. 26 Smack kernels use the CIPSO IP option. Some network 27 configurations are intolerant of IP options and can impede 28 access to systems that use them as Smack does. 29 30 The startup script etc-init.d-smack should be installed 31 in /etc/init.d/smack and should be invoked early in the 32 start-up process. On Fedora rc5.d/S02smack is recommended. 33 This script ensures that certain devices have the correct 34 Smack attributes and loads the Smack configuration if 35 any is defined. This script invokes two programs that 36 ensure configuration data is properly formatted. These 37 programs are /usr/sbin/smackload and /usr/sin/smackcipso. 38 The system will run just fine without these programs, 39 but it will be difficult to set access rules properly. 40 41 A version of "ls" that provides a "-M" option to display 42 Smack labels on long listing is available. 43 44 A hacked version of sshd that allows network logins by users 45 with specific Smack labels is available. This version does 46 not work for scp. You must set the /etc/ssh/sshd_config 47 line: 48 UsePrivilegeSeparation no 49 50 The format of /etc/smack/usr is: 51 52 username smack 53 54 In keeping with the intent of Smack, configuration data is 55 minimal and not strictly required. The most important 56 configuration step is mounting the smackfs pseudo filesystem. 57 58 Add this line to /etc/fstab: 59 60 smackfs /smack smackfs smackfsdef=* 0 0 61 62 and create the /smack directory for mounting. 63 64 Smack uses extended attributes (xattrs) to store file labels. 65 The command to set a Smack label on a file is: 66 67 # attr -S -s SMACK64 -V "value" path 68 69 NOTE: Smack labels are limited to 23 characters. The attr command 70 does not enforce this restriction and can be used to set 71 invalid Smack labels on files. 72 73 If you don't do anything special all users will get the floor ("_") 74 label when they log in. If you do want to log in via the hacked ssh 75 at other labels use the attr command to set the smack value on the 76 home directory and its contents. 77 78 You can add access rules in /etc/smack/accesses. They take the form: 79 80 subjectlabel objectlabel access 81 82 access is a combination of the letters rwxa which specify the 83 kind of access permitted a subject with subjectlabel on an 84 object with objectlabel. If there is no rule no access is allowed. 85 86 A process can see the smack label it is running with by 87 reading /proc/self/attr/current. A privileged process can 88 set the process smack by writing there. 89 90 Look for additional programs on http://schaufler-ca.com 91 92 From the Smack Whitepaper: 93 94 The Simplified Mandatory Access Control Kernel 95 96 Casey Schaufler 97 casey@schaufler-ca.com 98 99 Mandatory Access Control 100 101 Computer systems employ a variety of schemes to constrain how information is 102 shared among the people and services using the machine. Some of these schemes 103 allow the program or user to decide what other programs or users are allowed 104 access to pieces of data. These schemes are called discretionary access 105 control mechanisms because the access control is specified at the discretion 106 of the user. Other schemes do not leave the decision regarding what a user or 107 program can access up to users or programs. These schemes are called mandatory 108 access control mechanisms because you don't have a choice regarding the users 109 or programs that have access to pieces of data. 110 111 Bell & LaPadula 112 113 From the middle of the 1980's until the turn of the century Mandatory Access 114 Control (MAC) was very closely associated with the Bell & LaPadula security 115 model, a mathematical description of the United States Department of Defense 116 policy for marking paper documents. MAC in this form enjoyed a following 117 within the Capital Beltway and Scandinavian supercomputer centers but was 118 often sited as failing to address general needs. 119 120 Domain Type Enforcement 121 122 Around the turn of the century Domain Type Enforcement (DTE) became popular. 123 This scheme organizes users, programs, and data into domains that are 124 protected from each other. This scheme has been widely deployed as a component 125 of popular Linux distributions. The administrative overhead required to 126 maintain this scheme and the detailed understanding of the whole system 127 necessary to provide a secure domain mapping leads to the scheme being 128 disabled or used in limited ways in the majority of cases. 129 130 Smack 131 132 Smack is a Mandatory Access Control mechanism designed to provide useful MAC 133 while avoiding the pitfalls of its predecessors. The limitations of Bell & 134 LaPadula are addressed by providing a scheme whereby access can be controlled 135 according to the requirements of the system and its purpose rather than those 136 imposed by an arcane government policy. The complexity of Domain Type 137 Enforcement and avoided by defining access controls in terms of the access 138 modes already in use. 139 140 Smack Terminology 141 142 The jargon used to talk about Smack will be familiar to those who have dealt 143 with other MAC systems and shouldn't be too difficult for the uninitiated to 144 pick up. There are four terms that are used in a specific way and that are 145 especially important: 146 147 Subject: A subject is an active entity on the computer system. 148 On Smack a subject is a task, which is in turn the basic unit 149 of execution. 150 151 Object: An object is a passive entity on the computer system. 152 On Smack files of all types, IPC, and tasks can be objects. 153 154 Access: Any attempt by a subject to put information into or get 155 information from an object is an access. 156 157 Label: Data that identifies the Mandatory Access Control 158 characteristics of a subject or an object. 159 160 These definitions are consistent with the traditional use in the security 161 community. There are also some terms from Linux that are likely to crop up: 162 163 Capability: A task that possesses a capability has permission to 164 violate an aspect of the system security policy, as identified by 165 the specific capability. A task that possesses one or more 166 capabilities is a privileged task, whereas a task with no 167 capabilities is an unprivileged task. 168 169 Privilege: A task that is allowed to violate the system security 170 policy is said to have privilege. As of this writing a task can 171 have privilege either by possessing capabilities or by having an 172 effective user of root. 173 174 Smack Basics 175 176 Smack is an extension to a Linux system. It enforces additional restrictions 177 on what subjects can access which objects, based on the labels attached to 178 each of the subject and the object. 179 180 Labels 181 182 Smack labels are ASCII character strings, one to twenty-three characters in 183 length. Single character labels using special characters, that being anything 184 other than a letter or digit, are reserved for use by the Smack development 185 team. Smack labels are unstructured, case sensitive, and the only operation 186 ever performed on them is comparison for equality. Smack labels cannot 187 contain unprintable characters, the "/" (slash), the "\" (backslash), the "'" 188 (quote) and '"' (double-quote) characters. 189 Smack labels cannot begin with a '-', which is reserved for special options. 190 191 There are some predefined labels: 192 193 _ Pronounced "floor", a single underscore character. 194 ^ Pronounced "hat", a single circumflex character. 195 * Pronounced "star", a single asterisk character. 196 ? Pronounced "huh", a single question mark character. 197 @ Pronounced "Internet", a single at sign character. 198 199 Every task on a Smack system is assigned a label. System tasks, such as 200 init(8) and systems daemons, are run with the floor ("_") label. User tasks 201 are assigned labels according to the specification found in the 202 /etc/smack/user configuration file. 203 204 Access Rules 205 206 Smack uses the traditional access modes of Linux. These modes are read, 207 execute, write, and occasionally append. There are a few cases where the 208 access mode may not be obvious. These include: 209 210 Signals: A signal is a write operation from the subject task to 211 the object task. 212 Internet Domain IPC: Transmission of a packet is considered a 213 write operation from the source task to the destination task. 214 215 Smack restricts access based on the label attached to a subject and the label 216 attached to the object it is trying to access. The rules enforced are, in 217 order: 218 219 1. Any access requested by a task labeled "*" is denied. 220 2. A read or execute access requested by a task labeled "^" 221 is permitted. 222 3. A read or execute access requested on an object labeled "_" 223 is permitted. 224 4. Any access requested on an object labeled "*" is permitted. 225 5. Any access requested by a task on an object with the same 226 label is permitted. 227 6. Any access requested that is explicitly defined in the loaded 228 rule set is permitted. 229 7. Any other access is denied. 230 231 Smack Access Rules 232 233 With the isolation provided by Smack access separation is simple. There are 234 many interesting cases where limited access by subjects to objects with 235 different labels is desired. One example is the familiar spy model of 236 sensitivity, where a scientist working on a highly classified project would be 237 able to read documents of lower classifications and anything she writes will 238 be "born" highly classified. To accommodate such schemes Smack includes a 239 mechanism for specifying rules allowing access between labels. 240 241 Access Rule Format 242 243 The format of an access rule is: 244 245 subject-label object-label access 246 247 Where subject-label is the Smack label of the task, object-label is the Smack 248 label of the thing being accessed, and access is a string specifying the sort 249 of access allowed. The Smack labels are limited to 23 characters. The access 250 specification is searched for letters that describe access modes: 251 252 a: indicates that append access should be granted. 253 r: indicates that read access should be granted. 254 w: indicates that write access should be granted. 255 x: indicates that execute access should be granted. 256 257 Uppercase values for the specification letters are allowed as well. 258 Access mode specifications can be in any order. Examples of acceptable rules 259 are: 260 261 TopSecret Secret rx 262 Secret Unclass R 263 Manager Game x 264 User HR w 265 New Old rRrRr 266 Closed Off - 267 268 Examples of unacceptable rules are: 269 270 Top Secret Secret rx 271 Ace Ace r 272 Odd spells waxbeans 273 274 Spaces are not allowed in labels. Since a subject always has access to files 275 with the same label specifying a rule for that case is pointless. Only 276 valid letters (rwxaRWXA) and the dash ('-') character are allowed in 277 access specifications. The dash is a placeholder, so "a-r" is the same 278 as "ar". A lone dash is used to specify that no access should be allowed. 279 280 Applying Access Rules 281 282 The developers of Linux rarely define new sorts of things, usually importing 283 schemes and concepts from other systems. Most often, the other systems are 284 variants of Unix. Unix has many endearing properties, but consistency of 285 access control models is not one of them. Smack strives to treat accesses as 286 uniformly as is sensible while keeping with the spirit of the underlying 287 mechanism. 288 289 File system objects including files, directories, named pipes, symbolic links, 290 and devices require access permissions that closely match those used by mode 291 bit access. To open a file for reading read access is required on the file. To 292 search a directory requires execute access. Creating a file with write access 293 requires both read and write access on the containing directory. Deleting a 294 file requires read and write access to the file and to the containing 295 directory. It is possible that a user may be able to see that a file exists 296 but not any of its attributes by the circumstance of having read access to the 297 containing directory but not to the differently labeled file. This is an 298 artifact of the file name being data in the directory, not a part of the file. 299 300 IPC objects, message queues, semaphore sets, and memory segments exist in flat 301 namespaces and access requests are only required to match the object in 302 question. 303 304 Process objects reflect tasks on the system and the Smack label used to access 305 them is the same Smack label that the task would use for its own access 306 attempts. Sending a signal via the kill() system call is a write operation 307 from the signaler to the recipient. Debugging a process requires both reading 308 and writing. Creating a new task is an internal operation that results in two 309 tasks with identical Smack labels and requires no access checks. 310 311 Sockets are data structures attached to processes and sending a packet from 312 one process to another requires that the sender have write access to the 313 receiver. The receiver is not required to have read access to the sender. 314 315 Setting Access Rules 316 317 The configuration file /etc/smack/accesses contains the rules to be set at 318 system startup. The contents are written to the special file /smack/load. 319 Rules can be written to /smack/load at any time and take effect immediately. 320 For any pair of subject and object labels there can be only one rule, with the 321 most recently specified overriding any earlier specification. 322 323 The program smackload is provided to ensure data is formatted 324 properly when written to /smack/load. This program reads lines 325 of the form 326 327 subjectlabel objectlabel mode. 328 329 Task Attribute 330 331 The Smack label of a process can be read from /proc/<pid>/attr/current. A 332 process can read its own Smack label from /proc/self/attr/current. A 333 privileged process can change its own Smack label by writing to 334 /proc/self/attr/current but not the label of another process. 335 336 File Attribute 337 338 The Smack label of a filesystem object is stored as an extended attribute 339 named SMACK64 on the file. This attribute is in the security namespace. It can 340 only be changed by a process with privilege. 341 342 Privilege 343 344 A process with CAP_MAC_OVERRIDE is privileged. 345 346 Smack Networking 347 348 As mentioned before, Smack enforces access control on network protocol 349 transmissions. Every packet sent by a Smack process is tagged with its Smack 350 label. This is done by adding a CIPSO tag to the header of the IP packet. Each 351 packet received is expected to have a CIPSO tag that identifies the label and 352 if it lacks such a tag the network ambient label is assumed. Before the packet 353 is delivered a check is made to determine that a subject with the label on the 354 packet has write access to the receiving process and if that is not the case 355 the packet is dropped. 356 357 CIPSO Configuration 358 359 It is normally unnecessary to specify the CIPSO configuration. The default 360 values used by the system handle all internal cases. Smack will compose CIPSO 361 label values to match the Smack labels being used without administrative 362 intervention. Unlabeled packets that come into the system will be given the 363 ambient label. 364 365 Smack requires configuration in the case where packets from a system that is 366 not smack that speaks CIPSO may be encountered. Usually this will be a Trusted 367 Solaris system, but there are other, less widely deployed systems out there. 368 CIPSO provides 3 important values, a Domain Of Interpretation (DOI), a level, 369 and a category set with each packet. The DOI is intended to identify a group 370 of systems that use compatible labeling schemes, and the DOI specified on the 371 smack system must match that of the remote system or packets will be 372 discarded. The DOI is 3 by default. The value can be read from /smack/doi and 373 can be changed by writing to /smack/doi. 374 375 The label and category set are mapped to a Smack label as defined in 376 /etc/smack/cipso. 377 378 A Smack/CIPSO mapping has the form: 379 380 smack level [category [category]*] 381 382 Smack does not expect the level or category sets to be related in any 383 particular way and does not assume or assign accesses based on them. Some 384 examples of mappings: 385 386 TopSecret 7 387 TS:A,B 7 1 2 388 SecBDE 5 2 4 6 389 RAFTERS 7 12 26 390 391 The ":" and "," characters are permitted in a Smack label but have no special 392 meaning. 393 394 The mapping of Smack labels to CIPSO values is defined by writing to 395 /smack/cipso. Again, the format of data written to this special file 396 is highly restrictive, so the program smackcipso is provided to 397 ensure the writes are done properly. This program takes mappings 398 on the standard input and sends them to /smack/cipso properly. 399 400 In addition to explicit mappings Smack supports direct CIPSO mappings. One 401 CIPSO level is used to indicate that the category set passed in the packet is 402 in fact an encoding of the Smack label. The level used is 250 by default. The 403 value can be read from /smack/direct and changed by writing to /smack/direct. 404 405 Socket Attributes 406 407 There are two attributes that are associated with sockets. These attributes 408 can only be set by privileged tasks, but any task can read them for their own 409 sockets. 410 411 SMACK64IPIN: The Smack label of the task object. A privileged 412 program that will enforce policy may set this to the star label. 413 414 SMACK64IPOUT: The Smack label transmitted with outgoing packets. 415 A privileged program may set this to match the label of another 416 task with which it hopes to communicate. 417 418 Smack Netlabel Exceptions 419 420 You will often find that your labeled application has to talk to the outside, 421 unlabeled world. To do this there's a special file /smack/netlabel where you can 422 add some exceptions in the form of : 423 @IP1 LABEL1 or 424 @IP2/MASK LABEL2 425 426 It means that your application will have unlabeled access to @IP1 if it has 427 write access on LABEL1, and access to the subnet @IP2/MASK if it has write 428 access on LABEL2. 429 430 Entries in the /smack/netlabel file are matched by longest mask first, like in 431 classless IPv4 routing. 432 433 A special label '@' and an option '-CIPSO' can be used there : 434 @ means Internet, any application with any label has access to it 435 -CIPSO means standard CIPSO networking 436 437 If you don't know what CIPSO is and don't plan to use it, you can just do : 438 echo 127.0.0.1 -CIPSO > /smack/netlabel 439 echo 0.0.0.0/0 @ > /smack/netlabel 440 441 If you use CIPSO on your 192.168.0.0/16 local network and need also unlabeled 442 Internet access, you can have : 443 echo 127.0.0.1 -CIPSO > /smack/netlabel 444 echo 192.168.0.0/16 -CIPSO > /smack/netlabel 445 echo 0.0.0.0/0 @ > /smack/netlabel 446 447 448 Writing Applications for Smack 449 450 There are three sorts of applications that will run on a Smack system. How an 451 application interacts with Smack will determine what it will have to do to 452 work properly under Smack. 453 454 Smack Ignorant Applications 455 456 By far the majority of applications have no reason whatever to care about the 457 unique properties of Smack. Since invoking a program has no impact on the 458 Smack label associated with the process the only concern likely to arise is 459 whether the process has execute access to the program. 460 461 Smack Relevant Applications 462 463 Some programs can be improved by teaching them about Smack, but do not make 464 any security decisions themselves. The utility ls(1) is one example of such a 465 program. 466 467 Smack Enforcing Applications 468 469 These are special programs that not only know about Smack, but participate in 470 the enforcement of system policy. In most cases these are the programs that 471 set up user sessions. There are also network services that provide information 472 to processes running with various labels. 473 474 File System Interfaces 475 476 Smack maintains labels on file system objects using extended attributes. The 477 Smack label of a file, directory, or other file system object can be obtained 478 using getxattr(2). 479 480 len = getxattr("/", "security.SMACK64", value, sizeof (value)); 481 482 will put the Smack label of the root directory into value. A privileged 483 process can set the Smack label of a file system object with setxattr(2). 484 485 len = strlen("Rubble"); 486 rc = setxattr("/foo", "security.SMACK64", "Rubble", len, 0); 487 488 will set the Smack label of /foo to "Rubble" if the program has appropriate 489 privilege. 490 491 Socket Interfaces 492 493 The socket attributes can be read using fgetxattr(2). 494 495 A privileged process can set the Smack label of outgoing packets with 496 fsetxattr(2). 497 498 len = strlen("Rubble"); 499 rc = fsetxattr(fd, "security.SMACK64IPOUT", "Rubble", len, 0); 500 501 will set the Smack label "Rubble" on packets going out from the socket if the 502 program has appropriate privilege. 503 504 rc = fsetxattr(fd, "security.SMACK64IPIN, "*", strlen("*"), 0); 505 506 will set the Smack label "*" as the object label against which incoming 507 packets will be checked if the program has appropriate privilege. 508 509 Administration 510 511 Smack supports some mount options: 512 513 smackfsdef=label: specifies the label to give files that lack 514 the Smack label extended attribute. 515 516 smackfsroot=label: specifies the label to assign the root of the 517 file system if it lacks the Smack extended attribute. 518 519 smackfshat=label: specifies a label that must have read access to 520 all labels set on the filesystem. Not yet enforced. 521 522 smackfsfloor=label: specifies a label to which all labels set on the 523 filesystem must have read access. Not yet enforced. 524 525 These mount options apply to all file system types. 526 527 Smack auditing 528 529 If you want Smack auditing of security events, you need to set CONFIG_AUDIT 530 in your kernel configuration. 531 By default, all denied events will be audited. You can change this behavior by 532 writing a single character to the /smack/logging file : 533 0 : no logging 534 1 : log denied (default) 535 2 : log accepted 536 3 : log denied & accepted 537 538 Events are logged as 'key=value' pairs, for each event you at least will get 539 the subjet, the object, the rights requested, the action, the kernel function 540 that triggered the event, plus other pairs depending on the type of event 541 audited.
