Based on kernel version 2.6.25. Page generated on 2008-04-18 21:22 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 it's 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[AT]schaufler-ca[DOT]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 or the "/" (slash) character. 188 189 There are some predefined labels: 190 191 _ Pronounced "floor", a single underscore character. 192 ^ Pronounced "hat", a single circumflex character. 193 * Pronounced "star", a single asterisk character. 194 ? Pronounced "huh", a single question mark character. 195 196 Every task on a Smack system is assigned a label. System tasks, such as 197 init(8) and systems daemons, are run with the floor ("_") label. User tasks 198 are assigned labels according to the specification found in the 199 /etc/smack/user configuration file. 200 201 Access Rules 202 203 Smack uses the traditional access modes of Linux. These modes are read, 204 execute, write, and occasionally append. There are a few cases where the 205 access mode may not be obvious. These include: 206 207 Signals: A signal is a write operation from the subject task to 208 the object task. 209 Internet Domain IPC: Transmission of a packet is considered a 210 write operation from the source task to the destination task. 211 212 Smack restricts access based on the label attached to a subject and the label 213 attached to the object it is trying to access. The rules enforced are, in 214 order: 215 216 1. Any access requested by a task labeled "*" is denied. 217 2. A read or execute access requested by a task labeled "^" 218 is permitted. 219 3. A read or execute access requested on an object labeled "_" 220 is permitted. 221 4. Any access requested on an object labeled "*" is permitted. 222 5. Any access requested by a task on an object with the same 223 label is permitted. 224 6. Any access requested that is explicitly defined in the loaded 225 rule set is permitted. 226 7. Any other access is denied. 227 228 Smack Access Rules 229 230 With the isolation provided by Smack access separation is simple. There are 231 many interesting cases where limited access by subjects to objects with 232 different labels is desired. One example is the familiar spy model of 233 sensitivity, where a scientist working on a highly classified project would be 234 able to read documents of lower classifications and anything she writes will 235 be "born" highly classified. To accommodate such schemes Smack includes a 236 mechanism for specifying rules allowing access between labels. 237 238 Access Rule Format 239 240 The format of an access rule is: 241 242 subject-label object-label access 243 244 Where subject-label is the Smack label of the task, object-label is the Smack 245 label of the thing being accessed, and access is a string specifying the sort 246 of access allowed. The Smack labels are limited to 23 characters. The access 247 specification is searched for letters that describe access modes: 248 249 a: indicates that append access should be granted. 250 r: indicates that read access should be granted. 251 w: indicates that write access should be granted. 252 x: indicates that execute access should be granted. 253 254 Uppercase values for the specification letters are allowed as well. 255 Access mode specifications can be in any order. Examples of acceptable rules 256 are: 257 258 TopSecret Secret rx 259 Secret Unclass R 260 Manager Game x 261 User HR w 262 New Old rRrRr 263 Closed Off - 264 265 Examples of unacceptable rules are: 266 267 Top Secret Secret rx 268 Ace Ace r 269 Odd spells waxbeans 270 271 Spaces are not allowed in labels. Since a subject always has access to files 272 with the same label specifying a rule for that case is pointless. Only 273 valid letters (rwxaRWXA) and the dash ('-') character are allowed in 274 access specifications. The dash is a placeholder, so "a-r" is the same 275 as "ar". A lone dash is used to specify that no access should be allowed. 276 277 Applying Access Rules 278 279 The developers of Linux rarely define new sorts of things, usually importing 280 schemes and concepts from other systems. Most often, the other systems are 281 variants of Unix. Unix has many endearing properties, but consistency of 282 access control models is not one of them. Smack strives to treat accesses as 283 uniformly as is sensible while keeping with the spirit of the underlying 284 mechanism. 285 286 File system objects including files, directories, named pipes, symbolic links, 287 and devices require access permissions that closely match those used by mode 288 bit access. To open a file for reading read access is required on the file. To 289 search a directory requires execute access. Creating a file with write access 290 requires both read and write access on the containing directory. Deleting a 291 file requires read and write access to the file and to the containing 292 directory. It is possible that a user may be able to see that a file exists 293 but not any of its attributes by the circumstance of having read access to the 294 containing directory but not to the differently labeled file. This is an 295 artifact of the file name being data in the directory, not a part of the file. 296 297 IPC objects, message queues, semaphore sets, and memory segments exist in flat 298 namespaces and access requests are only required to match the object in 299 question. 300 301 Process objects reflect tasks on the system and the Smack label used to access 302 them is the same Smack label that the task would use for its own access 303 attempts. Sending a signal via the kill() system call is a write operation 304 from the signaler to the recipient. Debugging a process requires both reading 305 and writing. Creating a new task is an internal operation that results in two 306 tasks with identical Smack labels and requires no access checks. 307 308 Sockets are data structures attached to processes and sending a packet from 309 one process to another requires that the sender have write access to the 310 receiver. The receiver is not required to have read access to the sender. 311 312 Setting Access Rules 313 314 The configuration file /etc/smack/accesses contains the rules to be set at 315 system startup. The contents are written to the special file /smack/load. 316 Rules can be written to /smack/load at any time and take effect immediately. 317 For any pair of subject and object labels there can be only one rule, with the 318 most recently specified overriding any earlier specification. 319 320 The program smackload is provided to ensure data is formatted 321 properly when written to /smack/load. This program reads lines 322 of the form 323 324 subjectlabel objectlabel mode. 325 326 Task Attribute 327 328 The Smack label of a process can be read from /proc/<pid>/attr/current. A 329 process can read its own Smack label from /proc/self/attr/current. A 330 privileged process can change its own Smack label by writing to 331 /proc/self/attr/current but not the label of another process. 332 333 File Attribute 334 335 The Smack label of a filesystem object is stored as an extended attribute 336 named SMACK64 on the file. This attribute is in the security namespace. It can 337 only be changed by a process with privilege. 338 339 Privilege 340 341 A process with CAP_MAC_OVERRIDE is privileged. 342 343 Smack Networking 344 345 As mentioned before, Smack enforces access control on network protocol 346 transmissions. Every packet sent by a Smack process is tagged with its Smack 347 label. This is done by adding a CIPSO tag to the header of the IP packet. Each 348 packet received is expected to have a CIPSO tag that identifies the label and 349 if it lacks such a tag the network ambient label is assumed. Before the packet 350 is delivered a check is made to determine that a subject with the label on the 351 packet has write access to the receiving process and if that is not the case 352 the packet is dropped. 353 354 CIPSO Configuration 355 356 It is normally unnecessary to specify the CIPSO configuration. The default 357 values used by the system handle all internal cases. Smack will compose CIPSO 358 label values to match the Smack labels being used without administrative 359 intervention. Unlabeled packets that come into the system will be given the 360 ambient label. 361 362 Smack requires configuration in the case where packets from a system that is 363 not smack that speaks CIPSO may be encountered. Usually this will be a Trusted 364 Solaris system, but there are other, less widely deployed systems out there. 365 CIPSO provides 3 important values, a Domain Of Interpretation (DOI), a level, 366 and a category set with each packet. The DOI is intended to identify a group 367 of systems that use compatible labeling schemes, and the DOI specified on the 368 smack system must match that of the remote system or packets will be 369 discarded. The DOI is 3 by default. The value can be read from /smack/doi and 370 can be changed by writing to /smack/doi. 371 372 The label and category set are mapped to a Smack label as defined in 373 /etc/smack/cipso. 374 375 A Smack/CIPSO mapping has the form: 376 377 smack level [category [category]*] 378 379 Smack does not expect the level or category sets to be related in any 380 particular way and does not assume or assign accesses based on them. Some 381 examples of mappings: 382 383 TopSecret 7 384 TS:A,B 7 1 2 385 SecBDE 5 2 4 6 386 RAFTERS 7 12 26 387 388 The ":" and "," characters are permitted in a Smack label but have no special 389 meaning. 390 391 The mapping of Smack labels to CIPSO values is defined by writing to 392 /smack/cipso. Again, the format of data written to this special file 393 is highly restrictive, so the program smackcipso is provided to 394 ensure the writes are done properly. This program takes mappings 395 on the standard input and sends them to /smack/cipso properly. 396 397 In addition to explicit mappings Smack supports direct CIPSO mappings. One 398 CIPSO level is used to indicate that the category set passed in the packet is 399 in fact an encoding of the Smack label. The level used is 250 by default. The 400 value can be read from /smack/direct and changed by writing to /smack/direct. 401 402 Socket Attributes 403 404 There are two attributes that are associated with sockets. These attributes 405 can only be set by privileged tasks, but any task can read them for their own 406 sockets. 407 408 SMACK64IPIN: The Smack label of the task object. A privileged 409 program that will enforce policy may set this to the star label. 410 411 SMACK64IPOUT: The Smack label transmitted with outgoing packets. 412 A privileged program may set this to match the label of another 413 task with which it hopes to communicate. 414 415 Writing Applications for Smack 416 417 There are three sorts of applications that will run on a Smack system. How an 418 application interacts with Smack will determine what it will have to do to 419 work properly under Smack. 420 421 Smack Ignorant Applications 422 423 By far the majority of applications have no reason whatever to care about the 424 unique properties of Smack. Since invoking a program has no impact on the 425 Smack label associated with the process the only concern likely to arise is 426 whether the process has execute access to the program. 427 428 Smack Relevant Applications 429 430 Some programs can be improved by teaching them about Smack, but do not make 431 any security decisions themselves. The utility ls(1) is one example of such a 432 program. 433 434 Smack Enforcing Applications 435 436 These are special programs that not only know about Smack, but participate in 437 the enforcement of system policy. In most cases these are the programs that 438 set up user sessions. There are also network services that provide information 439 to processes running with various labels. 440 441 File System Interfaces 442 443 Smack maintains labels on file system objects using extended attributes. The 444 Smack label of a file, directory, or other file system object can be obtained 445 using getxattr(2). 446 447 len = getxattr("/", "security.SMACK64", value, sizeof (value)); 448 449 will put the Smack label of the root directory into value. A privileged 450 process can set the Smack label of a file system object with setxattr(2). 451 452 len = strlen("Rubble"); 453 rc = setxattr("/foo", "security.SMACK64", "Rubble", len, 0); 454 455 will set the Smack label of /foo to "Rubble" if the program has appropriate 456 privilege. 457 458 Socket Interfaces 459 460 The socket attributes can be read using fgetxattr(2). 461 462 A privileged process can set the Smack label of outgoing packets with 463 fsetxattr(2). 464 465 len = strlen("Rubble"); 466 rc = fsetxattr(fd, "security.SMACK64IPOUT", "Rubble", len, 0); 467 468 will set the Smack label "Rubble" on packets going out from the socket if the 469 program has appropriate privilege. 470 471 rc = fsetxattr(fd, "security.SMACK64IPIN, "*", strlen("*"), 0); 472 473 will set the Smack label "*" as the object label against which incoming 474 packets will be checked if the program has appropriate privilege. 475 476 Administration 477 478 Smack supports some mount options: 479 480 smackfsdef=label: specifies the label to give files that lack 481 the Smack label extended attribute. 482 483 smackfsroot=label: specifies the label to assign the root of the 484 file system if it lacks the Smack extended attribute. 485 486 smackfshat=label: specifies a label that must have read access to 487 all labels set on the filesystem. Not yet enforced. 488 489 smackfsfloor=label: specifies a label to which all labels set on the 490 filesystem must have read access. Not yet enforced. 491 492 These mount options apply to all file system types.
