1 2 3 "Good for you, you've decided to clean the elevator!" 4 - The Elevator, from Dark Star 5 6Smack is the the Simplified Mandatory Access Control Kernel. 7Smack is a kernel based implementation of mandatory access 8control that includes simplicity in its primary design goals. 9 10Smack is not the only Mandatory Access Control scheme 11available for Linux. Those new to Mandatory Access Control 12are encouraged to compare Smack with the other mechanisms 13available to determine which is best suited to the problem 14at hand. 15 16Smack consists of three major components: 17 - The kernel 18 - A start-up script and a few modified applications 19 - Configuration data 20 21The kernel component of Smack is implemented as a Linux 22Security Modules (LSM) module. It requires netlabel and 23works best with file systems that support extended attributes, 24although xattr support is not strictly required. 25It is safe to run a Smack kernel under a "vanilla" distribution. 26Smack kernels use the CIPSO IP option. Some network 27configurations are intolerant of IP options and can impede 28access to systems that use them as Smack does. 29 30The startup script etc-init.d-smack should be installed 31in /etc/init.d/smack and should be invoked early in the 32start-up process. On Fedora rc5.d/S02smack is recommended. 33This script ensures that certain devices have the correct 34Smack attributes and loads the Smack configuration if 35any is defined. This script invokes two programs that 36ensure configuration data is properly formatted. These 37programs are /usr/sbin/smackload and /usr/sin/smackcipso. 38The system will run just fine without these programs, 39but it will be difficult to set access rules properly. 40 41A version of "ls" that provides a "-M" option to display 42Smack labels on long listing is available. 43 44A hacked version of sshd that allows network logins by users 45with specific Smack labels is available. This version does 46not work for scp. You must set the /etc/ssh/sshd_config 47line: 48 UsePrivilegeSeparation no 49 50The format of /etc/smack/usr is: 51 52 username smack 53 54In keeping with the intent of Smack, configuration data is 55minimal and not strictly required. The most important 56configuration step is mounting the smackfs pseudo filesystem. 57 58Add this line to /etc/fstab: 59 60 smackfs /smack smackfs smackfsdef=* 0 0 61 62and create the /smack directory for mounting. 63 64Smack uses extended attributes (xattrs) to store file labels. 65The command to set a Smack label on a file is: 66 67 # attr -S -s SMACK64 -V "value" path 68 69NOTE: 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 73If you don't do anything special all users will get the floor ("_") 74label when they log in. If you do want to log in via the hacked ssh 75at other labels use the attr command to set the smack value on the 76home directory and its contents. 77 78You can add access rules in /etc/smack/accesses. They take the form: 79 80 subjectlabel objectlabel access 81 82access is a combination of the letters rwxa which specify the 83kind of access permitted a subject with subjectlabel on an 84object with objectlabel. If there is no rule no access is allowed. 85 86A process can see the smack label it is running with by 87reading /proc/self/attr/current. A privileged process can 88set the process smack by writing there. 89 90Look for additional programs on http://schaufler-ca.com 91 92From the Smack Whitepaper: 93 94The Simplified Mandatory Access Control Kernel 95 96Casey Schaufler 97casey@schaufler-ca.com 98 99Mandatory Access Control 100 101Computer systems employ a variety of schemes to constrain how information is 102shared among the people and services using the machine. Some of these schemes 103allow the program or user to decide what other programs or users are allowed 104access to pieces of data. These schemes are called discretionary access 105control mechanisms because the access control is specified at the discretion 106of the user. Other schemes do not leave the decision regarding what a user or 107program can access up to users or programs. These schemes are called mandatory 108access control mechanisms because you don't have a choice regarding the users 109or programs that have access to pieces of data. 110 111Bell & LaPadula 112 113From the middle of the 1980's until the turn of the century Mandatory Access 114Control (MAC) was very closely associated with the Bell & LaPadula security 115model, a mathematical description of the United States Department of Defense 116policy for marking paper documents. MAC in this form enjoyed a following 117within the Capital Beltway and Scandinavian supercomputer centers but was 118often sited as failing to address general needs. 119 120Domain Type Enforcement 121 122Around the turn of the century Domain Type Enforcement (DTE) became popular. 123This scheme organizes users, programs, and data into domains that are 124protected from each other. This scheme has been widely deployed as a component 125of popular Linux distributions. The administrative overhead required to 126maintain this scheme and the detailed understanding of the whole system 127necessary to provide a secure domain mapping leads to the scheme being 128disabled or used in limited ways in the majority of cases. 129 130Smack 131 132Smack is a Mandatory Access Control mechanism designed to provide useful MAC 133while avoiding the pitfalls of its predecessors. The limitations of Bell & 134LaPadula are addressed by providing a scheme whereby access can be controlled 135according to the requirements of the system and its purpose rather than those 136imposed by an arcane government policy. The complexity of Domain Type 137Enforcement and avoided by defining access controls in terms of the access 138modes already in use. 139 140Smack Terminology 141 142The jargon used to talk about Smack will be familiar to those who have dealt 143with other MAC systems and shouldn't be too difficult for the uninitiated to 144pick up. There are four terms that are used in a specific way and that are 145especially 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 160These definitions are consistent with the traditional use in the security 161community. 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 174Smack Basics 175 176Smack is an extension to a Linux system. It enforces additional restrictions 177on what subjects can access which objects, based on the labels attached to 178each of the subject and the object. 179 180Labels 181 182Smack labels are ASCII character strings, one to twenty-three characters in 183length. Single character labels using special characters, that being anything 184other than a letter or digit, are reserved for use by the Smack development 185team. Smack labels are unstructured, case sensitive, and the only operation 186ever performed on them is comparison for equality. Smack labels cannot 187contain unprintable characters, the "/" (slash), the "\" (backslash), the "'" 188(quote) and '"' (double-quote) characters. 189Smack labels cannot begin with a '-', which is reserved for special options. 190 191There 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 199Every task on a Smack system is assigned a label. System tasks, such as 200init(8) and systems daemons, are run with the floor ("_") label. User tasks 201are assigned labels according to the specification found in the 202/etc/smack/user configuration file. 203 204Access Rules 205 206Smack uses the traditional access modes of Linux. These modes are read, 207execute, write, and occasionally append. There are a few cases where the 208access 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 215Smack restricts access based on the label attached to a subject and the label 216attached to the object it is trying to access. The rules enforced are, in 217order: 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 231Smack Access Rules 232 233With the isolation provided by Smack access separation is simple. There are 234many interesting cases where limited access by subjects to objects with 235different labels is desired. One example is the familiar spy model of 236sensitivity, where a scientist working on a highly classified project would be 237able to read documents of lower classifications and anything she writes will 238be "born" highly classified. To accommodate such schemes Smack includes a 239mechanism for specifying rules allowing access between labels. 240 241Access Rule Format 242 243The format of an access rule is: 244 245 subject-label object-label access 246 247Where subject-label is the Smack label of the task, object-label is the Smack 248label of the thing being accessed, and access is a string specifying the sort 249of access allowed. The Smack labels are limited to 23 characters. The access 250specification 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 257Uppercase values for the specification letters are allowed as well. 258Access mode specifications can be in any order. Examples of acceptable rules 259are: 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 268Examples of unacceptable rules are: 269 270 Top Secret Secret rx 271 Ace Ace r 272 Odd spells waxbeans 273 274Spaces are not allowed in labels. Since a subject always has access to files 275with the same label specifying a rule for that case is pointless. Only 276valid letters (rwxaRWXA) and the dash ('-') character are allowed in 277access specifications. The dash is a placeholder, so "a-r" is the same 278as "ar". A lone dash is used to specify that no access should be allowed. 279 280Applying Access Rules 281 282The developers of Linux rarely define new sorts of things, usually importing 283schemes and concepts from other systems. Most often, the other systems are 284variants of Unix. Unix has many endearing properties, but consistency of 285access control models is not one of them. Smack strives to treat accesses as 286uniformly as is sensible while keeping with the spirit of the underlying 287mechanism. 288 289File system objects including files, directories, named pipes, symbolic links, 290and devices require access permissions that closely match those used by mode 291bit access. To open a file for reading read access is required on the file. To 292search a directory requires execute access. Creating a file with write access 293requires both read and write access on the containing directory. Deleting a 294file requires read and write access to the file and to the containing 295directory. It is possible that a user may be able to see that a file exists 296but not any of its attributes by the circumstance of having read access to the 297containing directory but not to the differently labeled file. This is an 298artifact of the file name being data in the directory, not a part of the file. 299 300IPC objects, message queues, semaphore sets, and memory segments exist in flat 301namespaces and access requests are only required to match the object in 302question. 303 304Process objects reflect tasks on the system and the Smack label used to access 305them is the same Smack label that the task would use for its own access 306attempts. Sending a signal via the kill() system call is a write operation 307from the signaler to the recipient. Debugging a process requires both reading 308and writing. Creating a new task is an internal operation that results in two 309tasks with identical Smack labels and requires no access checks. 310 311Sockets are data structures attached to processes and sending a packet from 312one process to another requires that the sender have write access to the 313receiver. The receiver is not required to have read access to the sender. 314 315Setting Access Rules 316 317The configuration file /etc/smack/accesses contains the rules to be set at 318system startup. The contents are written to the special file /smack/load. 319Rules can be written to /smack/load at any time and take effect immediately. 320For any pair of subject and object labels there can be only one rule, with the 321most recently specified overriding any earlier specification. 322 323The program smackload is provided to ensure data is formatted 324properly when written to /smack/load. This program reads lines 325of the form 326 327 subjectlabel objectlabel mode. 328 329Task Attribute 330 331The Smack label of a process can be read from /proc/<pid>/attr/current. A 332process can read its own Smack label from /proc/self/attr/current. A 333privileged process can change its own Smack label by writing to 334/proc/self/attr/current but not the label of another process. 335 336File Attribute 337 338The Smack label of a filesystem object is stored as an extended attribute 339named SMACK64 on the file. This attribute is in the security namespace. It can 340only be changed by a process with privilege. 341 342Privilege 343 344A process with CAP_MAC_OVERRIDE is privileged. 345 346Smack Networking 347 348As mentioned before, Smack enforces access control on network protocol 349transmissions. Every packet sent by a Smack process is tagged with its Smack 350label. This is done by adding a CIPSO tag to the header of the IP packet. Each 351packet received is expected to have a CIPSO tag that identifies the label and 352if it lacks such a tag the network ambient label is assumed. Before the packet 353is delivered a check is made to determine that a subject with the label on the 354packet has write access to the receiving process and if that is not the case 355the packet is dropped. 356 357CIPSO Configuration 358 359It is normally unnecessary to specify the CIPSO configuration. The default 360values used by the system handle all internal cases. Smack will compose CIPSO 361label values to match the Smack labels being used without administrative 362intervention. Unlabeled packets that come into the system will be given the 363ambient label. 364 365Smack requires configuration in the case where packets from a system that is 366not smack that speaks CIPSO may be encountered. Usually this will be a Trusted 367Solaris system, but there are other, less widely deployed systems out there. 368CIPSO provides 3 important values, a Domain Of Interpretation (DOI), a level, 369and a category set with each packet. The DOI is intended to identify a group 370of systems that use compatible labeling schemes, and the DOI specified on the 371smack system must match that of the remote system or packets will be 372discarded. The DOI is 3 by default. The value can be read from /smack/doi and 373can be changed by writing to /smack/doi. 374 375The label and category set are mapped to a Smack label as defined in 376/etc/smack/cipso. 377 378A Smack/CIPSO mapping has the form: 379 380 smack level [category [category]*] 381 382Smack does not expect the level or category sets to be related in any 383particular way and does not assume or assign accesses based on them. Some 384examples 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 391The ":" and "," characters are permitted in a Smack label but have no special 392meaning. 393 394The 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 396is highly restrictive, so the program smackcipso is provided to 397ensure the writes are done properly. This program takes mappings 398on the standard input and sends them to /smack/cipso properly. 399 400In addition to explicit mappings Smack supports direct CIPSO mappings. One 401CIPSO level is used to indicate that the category set passed in the packet is 402in fact an encoding of the Smack label. The level used is 250 by default. The 403value can be read from /smack/direct and changed by writing to /smack/direct. 404 405Socket Attributes 406 407There are two attributes that are associated with sockets. These attributes 408can only be set by privileged tasks, but any task can read them for their own 409sockets. 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 418Smack Netlabel Exceptions 419 420You will often find that your labeled application has to talk to the outside, 421unlabeled world. To do this there's a special file /smack/netlabel where you can 422add some exceptions in the form of : 423@IP1 LABEL1 or 424@IP2/MASK LABEL2 425 426It means that your application will have unlabeled access to @IP1 if it has 427write access on LABEL1, and access to the subnet @IP2/MASK if it has write 428access on LABEL2. 429 430Entries in the /smack/netlabel file are matched by longest mask first, like in 431classless IPv4 routing. 432 433A 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 437If you don't know what CIPSO is and don't plan to use it, you can just do : 438echo 127.0.0.1 -CIPSO > /smack/netlabel 439echo 0.0.0.0/0 @ > /smack/netlabel 440 441If you use CIPSO on your 192.168.0.0/16 local network and need also unlabeled 442Internet access, you can have : 443echo 127.0.0.1 -CIPSO > /smack/netlabel 444echo 192.168.0.0/16 -CIPSO > /smack/netlabel 445echo 0.0.0.0/0 @ > /smack/netlabel 446 447 448Writing Applications for Smack 449 450There are three sorts of applications that will run on a Smack system. How an 451application interacts with Smack will determine what it will have to do to 452work properly under Smack. 453 454Smack Ignorant Applications 455 456By far the majority of applications have no reason whatever to care about the 457unique properties of Smack. Since invoking a program has no impact on the 458Smack label associated with the process the only concern likely to arise is 459whether the process has execute access to the program. 460 461Smack Relevant Applications 462 463Some programs can be improved by teaching them about Smack, but do not make 464any security decisions themselves. The utility ls(1) is one example of such a 465program. 466 467Smack Enforcing Applications 468 469These are special programs that not only know about Smack, but participate in 470the enforcement of system policy. In most cases these are the programs that 471set up user sessions. There are also network services that provide information 472to processes running with various labels. 473 474File System Interfaces 475 476Smack maintains labels on file system objects using extended attributes. The 477Smack label of a file, directory, or other file system object can be obtained 478using getxattr(2). 479 480 len = getxattr("/", "security.SMACK64", value, sizeof (value)); 481 482will put the Smack label of the root directory into value. A privileged 483process 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 488will set the Smack label of /foo to "Rubble" if the program has appropriate 489privilege. 490 491Socket Interfaces 492 493The socket attributes can be read using fgetxattr(2). 494 495A privileged process can set the Smack label of outgoing packets with 496fsetxattr(2). 497 498 len = strlen("Rubble"); 499 rc = fsetxattr(fd, "security.SMACK64IPOUT", "Rubble", len, 0); 500 501will set the Smack label "Rubble" on packets going out from the socket if the 502program has appropriate privilege. 503 504 rc = fsetxattr(fd, "security.SMACK64IPIN, "*", strlen("*"), 0); 505 506will set the Smack label "*" as the object label against which incoming 507packets will be checked if the program has appropriate privilege. 508 509Administration 510 511Smack 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 525These mount options apply to all file system types. 526 527Smack auditing 528 529If you want Smack auditing of security events, you need to set CONFIG_AUDIT 530in your kernel configuration. 531By default, all denied events will be audited. You can change this behavior by 532writing a single character to the /smack/logging file : 5330 : no logging 5341 : log denied (default) 5352 : log accepted 5363 : log denied & accepted 537 538Events are logged as 'key=value' pairs, for each event you at least will get 539the subjet, the object, the rights requested, the action, the kernel function 540that triggered the event, plus other pairs depending on the type of event 541audited. 542