Based on kernel version 4.3. Page generated on 2015-11-02 12:51 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 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 - Basic utilities, which are helpful but not required 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 27 Smack kernels use the CIPSO IP option. Some network 28 configurations are intolerant of IP options and can impede 29 access to systems that use them as Smack does. 30 31 Smack is used in the Tizen operating system. Please 32 go to http://wiki.tizen.org for information about how 33 Smack is used in Tizen. 34 35 The current git repository for Smack user space is: 36 37 git://github.com/smack-team/smack.git 38 39 This should make and install on most modern distributions. 40 There are five commands included in smackutil: 41 42 chsmack - display or set Smack extended attribute values 43 smackctl - load the Smack access rules 44 smackaccess - report if a process with one label has access 45 to an object with another 46 47 These two commands are obsolete with the introduction of 48 the smackfs/load2 and smackfs/cipso2 interfaces. 49 50 smackload - properly formats data for writing to smackfs/load 51 smackcipso - properly formats data for writing to smackfs/cipso 52 53 In keeping with the intent of Smack, configuration data is 54 minimal and not strictly required. The most important 55 configuration step is mounting the smackfs pseudo filesystem. 56 If smackutil is installed the startup script will take care 57 of this, but it can be manually as well. 58 59 Add this line to /etc/fstab: 60 61 smackfs /sys/fs/smackfs smackfs defaults 0 0 62 63 The /sys/fs/smackfs directory is created by the kernel. 64 65 Smack uses extended attributes (xattrs) to store labels on filesystem 66 objects. The attributes are stored in the extended attribute security 67 name space. A process must have CAP_MAC_ADMIN to change any of these 68 attributes. 69 70 The extended attributes that Smack uses are: 71 72 SMACK64 73 Used to make access control decisions. In almost all cases 74 the label given to a new filesystem object will be the label 75 of the process that created it. 76 SMACK64EXEC 77 The Smack label of a process that execs a program file with 78 this attribute set will run with this attribute's value. 79 SMACK64MMAP 80 Don't allow the file to be mmapped by a process whose Smack 81 label does not allow all of the access permitted to a process 82 with the label contained in this attribute. This is a very 83 specific use case for shared libraries. 84 SMACK64TRANSMUTE 85 Can only have the value "TRUE". If this attribute is present 86 on a directory when an object is created in the directory and 87 the Smack rule (more below) that permitted the write access 88 to the directory includes the transmute ("t") mode the object 89 gets the label of the directory instead of the label of the 90 creating process. If the object being created is a directory 91 the SMACK64TRANSMUTE attribute is set as well. 92 SMACK64IPIN 93 This attribute is only available on file descriptors for sockets. 94 Use the Smack label in this attribute for access control 95 decisions on packets being delivered to this socket. 96 SMACK64IPOUT 97 This attribute is only available on file descriptors for sockets. 98 Use the Smack label in this attribute for access control 99 decisions on packets coming from this socket. 100 101 There are multiple ways to set a Smack label on a file: 102 103 # attr -S -s SMACK64 -V "value" path 104 # chsmack -a value path 105 106 A process can see the Smack label it is running with by 107 reading /proc/self/attr/current. A process with CAP_MAC_ADMIN 108 can set the process Smack by writing there. 109 110 Most Smack configuration is accomplished by writing to files 111 in the smackfs filesystem. This pseudo-filesystem is mounted 112 on /sys/fs/smackfs. 113 114 access 115 Provided for backward compatibility. The access2 interface 116 is preferred and should be used instead. 117 This interface reports whether a subject with the specified 118 Smack label has a particular access to an object with a 119 specified Smack label. Write a fixed format access rule to 120 this file. The next read will indicate whether the access 121 would be permitted. The text will be either "1" indicating 122 access, or "0" indicating denial. 123 access2 124 This interface reports whether a subject with the specified 125 Smack label has a particular access to an object with a 126 specified Smack label. Write a long format access rule to 127 this file. The next read will indicate whether the access 128 would be permitted. The text will be either "1" indicating 129 access, or "0" indicating denial. 130 ambient 131 This contains the Smack label applied to unlabeled network 132 packets. 133 change-rule 134 This interface allows modification of existing access control rules. 135 The format accepted on write is: 136 "%s %s %s %s" 137 where the first string is the subject label, the second the 138 object label, the third the access to allow and the fourth the 139 access to deny. The access strings may contain only the characters 140 "rwxat-". If a rule for a given subject and object exists it will be 141 modified by enabling the permissions in the third string and disabling 142 those in the fourth string. If there is no such rule it will be 143 created using the access specified in the third and the fourth strings. 144 cipso 145 Provided for backward compatibility. The cipso2 interface 146 is preferred and should be used instead. 147 This interface allows a specific CIPSO header to be assigned 148 to a Smack label. The format accepted on write is: 149 "%24s%4d%4d"["%4d"]... 150 The first string is a fixed Smack label. The first number is 151 the level to use. The second number is the number of categories. 152 The following numbers are the categories. 153 "level-3-cats-5-19 3 2 5 19" 154 cipso2 155 This interface allows a specific CIPSO header to be assigned 156 to a Smack label. The format accepted on write is: 157 "%s%4d%4d"["%4d"]... 158 The first string is a long Smack label. The first number is 159 the level to use. The second number is the number of categories. 160 The following numbers are the categories. 161 "level-3-cats-5-19 3 2 5 19" 162 direct 163 This contains the CIPSO level used for Smack direct label 164 representation in network packets. 165 doi 166 This contains the CIPSO domain of interpretation used in 167 network packets. 168 ipv6host 169 This interface allows specific IPv6 internet addresses to be 170 treated as single label hosts. Packets are sent to single 171 label hosts only from processes that have Smack write access 172 to the host label. All packets received from single label hosts 173 are given the specified label. The format accepted on write is: 174 "%h:%h:%h:%h:%h:%h:%h:%h label" or 175 "%h:%h:%h:%h:%h:%h:%h:%h/%d label". 176 The "::" address shortcut is not supported. 177 If label is "-DELETE" a matched entry will be deleted. 178 load 179 Provided for backward compatibility. The load2 interface 180 is preferred and should be used instead. 181 This interface allows access control rules in addition to 182 the system defined rules to be specified. The format accepted 183 on write is: 184 "%24s%24s%5s" 185 where the first string is the subject label, the second the 186 object label, and the third the requested access. The access 187 string may contain only the characters "rwxat-", and specifies 188 which sort of access is allowed. The "-" is a placeholder for 189 permissions that are not allowed. The string "r-x--" would 190 specify read and execute access. Labels are limited to 23 191 characters in length. 192 load2 193 This interface allows access control rules in addition to 194 the system defined rules to be specified. The format accepted 195 on write is: 196 "%s %s %s" 197 where the first string is the subject label, the second the 198 object label, and the third the requested access. The access 199 string may contain only the characters "rwxat-", and specifies 200 which sort of access is allowed. The "-" is a placeholder for 201 permissions that are not allowed. The string "r-x--" would 202 specify read and execute access. 203 load-self 204 Provided for backward compatibility. The load-self2 interface 205 is preferred and should be used instead. 206 This interface allows process specific access rules to be 207 defined. These rules are only consulted if access would 208 otherwise be permitted, and are intended to provide additional 209 restrictions on the process. The format is the same as for 210 the load interface. 211 load-self2 212 This interface allows process specific access rules to be 213 defined. These rules are only consulted if access would 214 otherwise be permitted, and are intended to provide additional 215 restrictions on the process. The format is the same as for 216 the load2 interface. 217 logging 218 This contains the Smack logging state. 219 mapped 220 This contains the CIPSO level used for Smack mapped label 221 representation in network packets. 222 netlabel 223 This interface allows specific internet addresses to be 224 treated as single label hosts. Packets are sent to single 225 label hosts without CIPSO headers, but only from processes 226 that have Smack write access to the host label. All packets 227 received from single label hosts are given the specified 228 label. The format accepted on write is: 229 "%d.%d.%d.%d label" or "%d.%d.%d.%d/%d label". 230 If the label specified is "-CIPSO" the address is treated 231 as a host that supports CIPSO headers. 232 onlycap 233 This contains labels processes must have for CAP_MAC_ADMIN 234 and CAP_MAC_OVERRIDE to be effective. If this file is empty 235 these capabilities are effective at for processes with any 236 label. The values are set by writing the desired labels, separated 237 by spaces, to the file or cleared by writing "-" to the file. 238 ptrace 239 This is used to define the current ptrace policy 240 0 - default: this is the policy that relies on Smack access rules. 241 For the PTRACE_READ a subject needs to have a read access on 242 object. For the PTRACE_ATTACH a read-write access is required. 243 1 - exact: this is the policy that limits PTRACE_ATTACH. Attach is 244 only allowed when subject's and object's labels are equal. 245 PTRACE_READ is not affected. Can be overridden with CAP_SYS_PTRACE. 246 2 - draconian: this policy behaves like the 'exact' above with an 247 exception that it can't be overridden with CAP_SYS_PTRACE. 248 revoke-subject 249 Writing a Smack label here sets the access to '-' for all access 250 rules with that subject label. 251 unconfined 252 If the kernel is configured with CONFIG_SECURITY_SMACK_BRINGUP 253 a process with CAP_MAC_ADMIN can write a label into this interface. 254 Thereafter, accesses that involve that label will be logged and 255 the access permitted if it wouldn't be otherwise. Note that this 256 is dangerous and can ruin the proper labeling of your system. 257 It should never be used in production. 258 259 If you are using the smackload utility 260 you can add access rules in /etc/smack/accesses. They take the form: 261 262 subjectlabel objectlabel access 263 264 access is a combination of the letters rwxatb which specify the 265 kind of access permitted a subject with subjectlabel on an 266 object with objectlabel. If there is no rule no access is allowed. 267 268 Look for additional programs on http://schaufler-ca.com 269 270 From the Smack Whitepaper: 271 272 The Simplified Mandatory Access Control Kernel 273 274 Casey Schaufler 275 email@example.com 276 277 Mandatory Access Control 278 279 Computer systems employ a variety of schemes to constrain how information is 280 shared among the people and services using the machine. Some of these schemes 281 allow the program or user to decide what other programs or users are allowed 282 access to pieces of data. These schemes are called discretionary access 283 control mechanisms because the access control is specified at the discretion 284 of the user. Other schemes do not leave the decision regarding what a user or 285 program can access up to users or programs. These schemes are called mandatory 286 access control mechanisms because you don't have a choice regarding the users 287 or programs that have access to pieces of data. 288 289 Bell & LaPadula 290 291 From the middle of the 1980's until the turn of the century Mandatory Access 292 Control (MAC) was very closely associated with the Bell & LaPadula security 293 model, a mathematical description of the United States Department of Defense 294 policy for marking paper documents. MAC in this form enjoyed a following 295 within the Capital Beltway and Scandinavian supercomputer centers but was 296 often sited as failing to address general needs. 297 298 Domain Type Enforcement 299 300 Around the turn of the century Domain Type Enforcement (DTE) became popular. 301 This scheme organizes users, programs, and data into domains that are 302 protected from each other. This scheme has been widely deployed as a component 303 of popular Linux distributions. The administrative overhead required to 304 maintain this scheme and the detailed understanding of the whole system 305 necessary to provide a secure domain mapping leads to the scheme being 306 disabled or used in limited ways in the majority of cases. 307 308 Smack 309 310 Smack is a Mandatory Access Control mechanism designed to provide useful MAC 311 while avoiding the pitfalls of its predecessors. The limitations of Bell & 312 LaPadula are addressed by providing a scheme whereby access can be controlled 313 according to the requirements of the system and its purpose rather than those 314 imposed by an arcane government policy. The complexity of Domain Type 315 Enforcement and avoided by defining access controls in terms of the access 316 modes already in use. 317 318 Smack Terminology 319 320 The jargon used to talk about Smack will be familiar to those who have dealt 321 with other MAC systems and shouldn't be too difficult for the uninitiated to 322 pick up. There are four terms that are used in a specific way and that are 323 especially important: 324 325 Subject: A subject is an active entity on the computer system. 326 On Smack a subject is a task, which is in turn the basic unit 327 of execution. 328 329 Object: An object is a passive entity on the computer system. 330 On Smack files of all types, IPC, and tasks can be objects. 331 332 Access: Any attempt by a subject to put information into or get 333 information from an object is an access. 334 335 Label: Data that identifies the Mandatory Access Control 336 characteristics of a subject or an object. 337 338 These definitions are consistent with the traditional use in the security 339 community. There are also some terms from Linux that are likely to crop up: 340 341 Capability: A task that possesses a capability has permission to 342 violate an aspect of the system security policy, as identified by 343 the specific capability. A task that possesses one or more 344 capabilities is a privileged task, whereas a task with no 345 capabilities is an unprivileged task. 346 347 Privilege: A task that is allowed to violate the system security 348 policy is said to have privilege. As of this writing a task can 349 have privilege either by possessing capabilities or by having an 350 effective user of root. 351 352 Smack Basics 353 354 Smack is an extension to a Linux system. It enforces additional restrictions 355 on what subjects can access which objects, based on the labels attached to 356 each of the subject and the object. 357 358 Labels 359 360 Smack labels are ASCII character strings. They can be up to 255 characters 361 long, but keeping them to twenty-three characters is recommended. 362 Single character labels using special characters, that being anything 363 other than a letter or digit, are reserved for use by the Smack development 364 team. Smack labels are unstructured, case sensitive, and the only operation 365 ever performed on them is comparison for equality. Smack labels cannot 366 contain unprintable characters, the "/" (slash), the "\" (backslash), the "'" 367 (quote) and '"' (double-quote) characters. 368 Smack labels cannot begin with a '-'. This is reserved for special options. 369 370 There are some predefined labels: 371 372 _ Pronounced "floor", a single underscore character. 373 ^ Pronounced "hat", a single circumflex character. 374 * Pronounced "star", a single asterisk character. 375 ? Pronounced "huh", a single question mark character. 376 @ Pronounced "web", a single at sign character. 377 378 Every task on a Smack system is assigned a label. The Smack label 379 of a process will usually be assigned by the system initialization 380 mechanism. 381 382 Access Rules 383 384 Smack uses the traditional access modes of Linux. These modes are read, 385 execute, write, and occasionally append. There are a few cases where the 386 access mode may not be obvious. These include: 387 388 Signals: A signal is a write operation from the subject task to 389 the object task. 390 Internet Domain IPC: Transmission of a packet is considered a 391 write operation from the source task to the destination task. 392 393 Smack restricts access based on the label attached to a subject and the label 394 attached to the object it is trying to access. The rules enforced are, in 395 order: 396 397 1. Any access requested by a task labeled "*" is denied. 398 2. A read or execute access requested by a task labeled "^" 399 is permitted. 400 3. A read or execute access requested on an object labeled "_" 401 is permitted. 402 4. Any access requested on an object labeled "*" is permitted. 403 5. Any access requested by a task on an object with the same 404 label is permitted. 405 6. Any access requested that is explicitly defined in the loaded 406 rule set is permitted. 407 7. Any other access is denied. 408 409 Smack Access Rules 410 411 With the isolation provided by Smack access separation is simple. There are 412 many interesting cases where limited access by subjects to objects with 413 different labels is desired. One example is the familiar spy model of 414 sensitivity, where a scientist working on a highly classified project would be 415 able to read documents of lower classifications and anything she writes will 416 be "born" highly classified. To accommodate such schemes Smack includes a 417 mechanism for specifying rules allowing access between labels. 418 419 Access Rule Format 420 421 The format of an access rule is: 422 423 subject-label object-label access 424 425 Where subject-label is the Smack label of the task, object-label is the Smack 426 label of the thing being accessed, and access is a string specifying the sort 427 of access allowed. The access specification is searched for letters that 428 describe access modes: 429 430 a: indicates that append access should be granted. 431 r: indicates that read access should be granted. 432 w: indicates that write access should be granted. 433 x: indicates that execute access should be granted. 434 t: indicates that the rule requests transmutation. 435 b: indicates that the rule should be reported for bring-up. 436 437 Uppercase values for the specification letters are allowed as well. 438 Access mode specifications can be in any order. Examples of acceptable rules 439 are: 440 441 TopSecret Secret rx 442 Secret Unclass R 443 Manager Game x 444 User HR w 445 Snap Crackle rwxatb 446 New Old rRrRr 447 Closed Off - 448 449 Examples of unacceptable rules are: 450 451 Top Secret Secret rx 452 Ace Ace r 453 Odd spells waxbeans 454 455 Spaces are not allowed in labels. Since a subject always has access to files 456 with the same label specifying a rule for that case is pointless. Only 457 valid letters (rwxatbRWXATB) and the dash ('-') character are allowed in 458 access specifications. The dash is a placeholder, so "a-r" is the same 459 as "ar". A lone dash is used to specify that no access should be allowed. 460 461 Applying Access Rules 462 463 The developers of Linux rarely define new sorts of things, usually importing 464 schemes and concepts from other systems. Most often, the other systems are 465 variants of Unix. Unix has many endearing properties, but consistency of 466 access control models is not one of them. Smack strives to treat accesses as 467 uniformly as is sensible while keeping with the spirit of the underlying 468 mechanism. 469 470 File system objects including files, directories, named pipes, symbolic links, 471 and devices require access permissions that closely match those used by mode 472 bit access. To open a file for reading read access is required on the file. To 473 search a directory requires execute access. Creating a file with write access 474 requires both read and write access on the containing directory. Deleting a 475 file requires read and write access to the file and to the containing 476 directory. It is possible that a user may be able to see that a file exists 477 but not any of its attributes by the circumstance of having read access to the 478 containing directory but not to the differently labeled file. This is an 479 artifact of the file name being data in the directory, not a part of the file. 480 481 If a directory is marked as transmuting (SMACK64TRANSMUTE=TRUE) and the 482 access rule that allows a process to create an object in that directory 483 includes 't' access the label assigned to the new object will be that 484 of the directory, not the creating process. This makes it much easier 485 for two processes with different labels to share data without granting 486 access to all of their files. 487 488 IPC objects, message queues, semaphore sets, and memory segments exist in flat 489 namespaces and access requests are only required to match the object in 490 question. 491 492 Process objects reflect tasks on the system and the Smack label used to access 493 them is the same Smack label that the task would use for its own access 494 attempts. Sending a signal via the kill() system call is a write operation 495 from the signaler to the recipient. Debugging a process requires both reading 496 and writing. Creating a new task is an internal operation that results in two 497 tasks with identical Smack labels and requires no access checks. 498 499 Sockets are data structures attached to processes and sending a packet from 500 one process to another requires that the sender have write access to the 501 receiver. The receiver is not required to have read access to the sender. 502 503 Setting Access Rules 504 505 The configuration file /etc/smack/accesses contains the rules to be set at 506 system startup. The contents are written to the special file 507 /sys/fs/smackfs/load2. Rules can be added at any time and take effect 508 immediately. For any pair of subject and object labels there can be only 509 one rule, with the most recently specified overriding any earlier 510 specification. 511 512 Task Attribute 513 514 The Smack label of a process can be read from /proc/<pid>/attr/current. A 515 process can read its own Smack label from /proc/self/attr/current. A 516 privileged process can change its own Smack label by writing to 517 /proc/self/attr/current but not the label of another process. 518 519 File Attribute 520 521 The Smack label of a filesystem object is stored as an extended attribute 522 named SMACK64 on the file. This attribute is in the security namespace. It can 523 only be changed by a process with privilege. 524 525 Privilege 526 527 A process with CAP_MAC_OVERRIDE or CAP_MAC_ADMIN is privileged. 528 CAP_MAC_OVERRIDE allows the process access to objects it would 529 be denied otherwise. CAP_MAC_ADMIN allows a process to change 530 Smack data, including rules and attributes. 531 532 Smack Networking 533 534 As mentioned before, Smack enforces access control on network protocol 535 transmissions. Every packet sent by a Smack process is tagged with its Smack 536 label. This is done by adding a CIPSO tag to the header of the IP packet. Each 537 packet received is expected to have a CIPSO tag that identifies the label and 538 if it lacks such a tag the network ambient label is assumed. Before the packet 539 is delivered a check is made to determine that a subject with the label on the 540 packet has write access to the receiving process and if that is not the case 541 the packet is dropped. 542 543 CIPSO Configuration 544 545 It is normally unnecessary to specify the CIPSO configuration. The default 546 values used by the system handle all internal cases. Smack will compose CIPSO 547 label values to match the Smack labels being used without administrative 548 intervention. Unlabeled packets that come into the system will be given the 549 ambient label. 550 551 Smack requires configuration in the case where packets from a system that is 552 not Smack that speaks CIPSO may be encountered. Usually this will be a Trusted 553 Solaris system, but there are other, less widely deployed systems out there. 554 CIPSO provides 3 important values, a Domain Of Interpretation (DOI), a level, 555 and a category set with each packet. The DOI is intended to identify a group 556 of systems that use compatible labeling schemes, and the DOI specified on the 557 Smack system must match that of the remote system or packets will be 558 discarded. The DOI is 3 by default. The value can be read from 559 /sys/fs/smackfs/doi and can be changed by writing to /sys/fs/smackfs/doi. 560 561 The label and category set are mapped to a Smack label as defined in 562 /etc/smack/cipso. 563 564 A Smack/CIPSO mapping has the form: 565 566 smack level [category [category]*] 567 568 Smack does not expect the level or category sets to be related in any 569 particular way and does not assume or assign accesses based on them. Some 570 examples of mappings: 571 572 TopSecret 7 573 TS:A,B 7 1 2 574 SecBDE 5 2 4 6 575 RAFTERS 7 12 26 576 577 The ":" and "," characters are permitted in a Smack label but have no special 578 meaning. 579 580 The mapping of Smack labels to CIPSO values is defined by writing to 581 /sys/fs/smackfs/cipso2. 582 583 In addition to explicit mappings Smack supports direct CIPSO mappings. One 584 CIPSO level is used to indicate that the category set passed in the packet is 585 in fact an encoding of the Smack label. The level used is 250 by default. The 586 value can be read from /sys/fs/smackfs/direct and changed by writing to 587 /sys/fs/smackfs/direct. 588 589 Socket Attributes 590 591 There are two attributes that are associated with sockets. These attributes 592 can only be set by privileged tasks, but any task can read them for their own 593 sockets. 594 595 SMACK64IPIN: The Smack label of the task object. A privileged 596 program that will enforce policy may set this to the star label. 597 598 SMACK64IPOUT: The Smack label transmitted with outgoing packets. 599 A privileged program may set this to match the label of another 600 task with which it hopes to communicate. 601 602 Smack Netlabel Exceptions 603 604 You will often find that your labeled application has to talk to the outside, 605 unlabeled world. To do this there's a special file /sys/fs/smackfs/netlabel 606 where you can add some exceptions in the form of : 607 @IP1 LABEL1 or 608 @IP2/MASK LABEL2 609 610 It means that your application will have unlabeled access to @IP1 if it has 611 write access on LABEL1, and access to the subnet @IP2/MASK if it has write 612 access on LABEL2. 613 614 Entries in the /sys/fs/smackfs/netlabel file are matched by longest mask 615 first, like in classless IPv4 routing. 616 617 A special label '@' and an option '-CIPSO' can be used there : 618 @ means Internet, any application with any label has access to it 619 -CIPSO means standard CIPSO networking 620 621 If you don't know what CIPSO is and don't plan to use it, you can just do : 622 echo 127.0.0.1 -CIPSO > /sys/fs/smackfs/netlabel 623 echo 0.0.0.0/0 @ > /sys/fs/smackfs/netlabel 624 625 If you use CIPSO on your 192.168.0.0/16 local network and need also unlabeled 626 Internet access, you can have : 627 echo 127.0.0.1 -CIPSO > /sys/fs/smackfs/netlabel 628 echo 192.168.0.0/16 -CIPSO > /sys/fs/smackfs/netlabel 629 echo 0.0.0.0/0 @ > /sys/fs/smackfs/netlabel 630 631 632 Writing Applications for Smack 633 634 There are three sorts of applications that will run on a Smack system. How an 635 application interacts with Smack will determine what it will have to do to 636 work properly under Smack. 637 638 Smack Ignorant Applications 639 640 By far the majority of applications have no reason whatever to care about the 641 unique properties of Smack. Since invoking a program has no impact on the 642 Smack label associated with the process the only concern likely to arise is 643 whether the process has execute access to the program. 644 645 Smack Relevant Applications 646 647 Some programs can be improved by teaching them about Smack, but do not make 648 any security decisions themselves. The utility ls(1) is one example of such a 649 program. 650 651 Smack Enforcing Applications 652 653 These are special programs that not only know about Smack, but participate in 654 the enforcement of system policy. In most cases these are the programs that 655 set up user sessions. There are also network services that provide information 656 to processes running with various labels. 657 658 File System Interfaces 659 660 Smack maintains labels on file system objects using extended attributes. The 661 Smack label of a file, directory, or other file system object can be obtained 662 using getxattr(2). 663 664 len = getxattr("/", "security.SMACK64", value, sizeof (value)); 665 666 will put the Smack label of the root directory into value. A privileged 667 process can set the Smack label of a file system object with setxattr(2). 668 669 len = strlen("Rubble"); 670 rc = setxattr("/foo", "security.SMACK64", "Rubble", len, 0); 671 672 will set the Smack label of /foo to "Rubble" if the program has appropriate 673 privilege. 674 675 Socket Interfaces 676 677 The socket attributes can be read using fgetxattr(2). 678 679 A privileged process can set the Smack label of outgoing packets with 680 fsetxattr(2). 681 682 len = strlen("Rubble"); 683 rc = fsetxattr(fd, "security.SMACK64IPOUT", "Rubble", len, 0); 684 685 will set the Smack label "Rubble" on packets going out from the socket if the 686 program has appropriate privilege. 687 688 rc = fsetxattr(fd, "security.SMACK64IPIN, "*", strlen("*"), 0); 689 690 will set the Smack label "*" as the object label against which incoming 691 packets will be checked if the program has appropriate privilege. 692 693 Administration 694 695 Smack supports some mount options: 696 697 smackfsdef=label: specifies the label to give files that lack 698 the Smack label extended attribute. 699 700 smackfsroot=label: specifies the label to assign the root of the 701 file system if it lacks the Smack extended attribute. 702 703 smackfshat=label: specifies a label that must have read access to 704 all labels set on the filesystem. Not yet enforced. 705 706 smackfsfloor=label: specifies a label to which all labels set on the 707 filesystem must have read access. Not yet enforced. 708 709 These mount options apply to all file system types. 710 711 Smack auditing 712 713 If you want Smack auditing of security events, you need to set CONFIG_AUDIT 714 in your kernel configuration. 715 By default, all denied events will be audited. You can change this behavior by 716 writing a single character to the /sys/fs/smackfs/logging file : 717 0 : no logging 718 1 : log denied (default) 719 2 : log accepted 720 3 : log denied & accepted 721 722 Events are logged as 'key=value' pairs, for each event you at least will get 723 the subject, the object, the rights requested, the action, the kernel function 724 that triggered the event, plus other pairs depending on the type of event 725 audited. 726 727 Bringup Mode 728 729 Bringup mode provides logging features that can make application 730 configuration and system bringup easier. Configure the kernel with 731 CONFIG_SECURITY_SMACK_BRINGUP to enable these features. When bringup 732 mode is enabled accesses that succeed due to rules marked with the "b" 733 access mode will logged. When a new label is introduced for processes 734 rules can be added aggressively, marked with the "b". The logging allows 735 tracking of which rules actual get used for that label. 736 737 Another feature of bringup mode is the "unconfined" option. Writing 738 a label to /sys/fs/smackfs/unconfined makes subjects with that label 739 able to access any object, and objects with that label accessible to 740 all subjects. Any access that is granted because a label is unconfined 741 is logged. This feature is dangerous, as files and directories may 742 be created in places they couldn't if the policy were being enforced.