Root/Documentation/credentials.txt

1                 ====================
2                 CREDENTIALS IN LINUX
3                 ====================
4
5By: David Howells <dhowells@redhat.com>
6
7Contents:
8
9 (*) Overview.
10
11 (*) Types of credentials.
12
13 (*) File markings.
14
15 (*) Task credentials.
16
17     - Immutable credentials.
18     - Accessing task credentials.
19     - Accessing another task's credentials.
20     - Altering credentials.
21     - Managing credentials.
22
23 (*) Open file credentials.
24
25 (*) Overriding the VFS's use of credentials.
26
27
28========
29OVERVIEW
30========
31
32There are several parts to the security check performed by Linux when one
33object acts upon another:
34
35 (1) Objects.
36
37     Objects are things in the system that may be acted upon directly by
38     userspace programs. Linux has a variety of actionable objects, including:
39
40    - Tasks
41    - Files/inodes
42    - Sockets
43    - Message queues
44    - Shared memory segments
45    - Semaphores
46    - Keys
47
48     As a part of the description of all these objects there is a set of
49     credentials. What's in the set depends on the type of object.
50
51 (2) Object ownership.
52
53     Amongst the credentials of most objects, there will be a subset that
54     indicates the ownership of that object. This is used for resource
55     accounting and limitation (disk quotas and task rlimits for example).
56
57     In a standard UNIX filesystem, for instance, this will be defined by the
58     UID marked on the inode.
59
60 (3) The objective context.
61
62     Also amongst the credentials of those objects, there will be a subset that
63     indicates the 'objective context' of that object. This may or may not be
64     the same set as in (2) - in standard UNIX files, for instance, this is the
65     defined by the UID and the GID marked on the inode.
66
67     The objective context is used as part of the security calculation that is
68     carried out when an object is acted upon.
69
70 (4) Subjects.
71
72     A subject is an object that is acting upon another object.
73
74     Most of the objects in the system are inactive: they don't act on other
75     objects within the system. Processes/tasks are the obvious exception:
76     they do stuff; they access and manipulate things.
77
78     Objects other than tasks may under some circumstances also be subjects.
79     For instance an open file may send SIGIO to a task using the UID and EUID
80     given to it by a task that called fcntl(F_SETOWN) upon it. In this case,
81     the file struct will have a subjective context too.
82
83 (5) The subjective context.
84
85     A subject has an additional interpretation of its credentials. A subset
86     of its credentials forms the 'subjective context'. The subjective context
87     is used as part of the security calculation that is carried out when a
88     subject acts.
89
90     A Linux task, for example, has the FSUID, FSGID and the supplementary
91     group list for when it is acting upon a file - which are quite separate
92     from the real UID and GID that normally form the objective context of the
93     task.
94
95 (6) Actions.
96
97     Linux has a number of actions available that a subject may perform upon an
98     object. The set of actions available depends on the nature of the subject
99     and the object.
100
101     Actions include reading, writing, creating and deleting files; forking or
102     signalling and tracing tasks.
103
104 (7) Rules, access control lists and security calculations.
105
106     When a subject acts upon an object, a security calculation is made. This
107     involves taking the subjective context, the objective context and the
108     action, and searching one or more sets of rules to see whether the subject
109     is granted or denied permission to act in the desired manner on the
110     object, given those contexts.
111
112     There are two main sources of rules:
113
114     (a) Discretionary access control (DAC):
115
116     Sometimes the object will include sets of rules as part of its
117     description. This is an 'Access Control List' or 'ACL'. A Linux
118     file may supply more than one ACL.
119
120     A traditional UNIX file, for example, includes a permissions mask that
121     is an abbreviated ACL with three fixed classes of subject ('user',
122     'group' and 'other'), each of which may be granted certain privileges
123     ('read', 'write' and 'execute' - whatever those map to for the object
124     in question). UNIX file permissions do not allow the arbitrary
125     specification of subjects, however, and so are of limited use.
126
127     A Linux file might also sport a POSIX ACL. This is a list of rules
128     that grants various permissions to arbitrary subjects.
129
130     (b) Mandatory access control (MAC):
131
132     The system as a whole may have one or more sets of rules that get
133     applied to all subjects and objects, regardless of their source.
134     SELinux and Smack are examples of this.
135
136     In the case of SELinux and Smack, each object is given a label as part
137     of its credentials. When an action is requested, they take the
138     subject label, the object label and the action and look for a rule
139     that says that this action is either granted or denied.
140
141
142====================
143TYPES OF CREDENTIALS
144====================
145
146The Linux kernel supports the following types of credentials:
147
148 (1) Traditional UNIX credentials.
149
150    Real User ID
151    Real Group ID
152
153     The UID and GID are carried by most, if not all, Linux objects, even if in
154     some cases it has to be invented (FAT or CIFS files for example, which are
155     derived from Windows). These (mostly) define the objective context of
156     that object, with tasks being slightly different in some cases.
157
158    Effective, Saved and FS User ID
159    Effective, Saved and FS Group ID
160    Supplementary groups
161
162     These are additional credentials used by tasks only. Usually, an
163     EUID/EGID/GROUPS will be used as the subjective context, and real UID/GID
164     will be used as the objective. For tasks, it should be noted that this is
165     not always true.
166
167 (2) Capabilities.
168
169    Set of permitted capabilities
170    Set of inheritable capabilities
171    Set of effective capabilities
172    Capability bounding set
173
174     These are only carried by tasks. They indicate superior capabilities
175     granted piecemeal to a task that an ordinary task wouldn't otherwise have.
176     These are manipulated implicitly by changes to the traditional UNIX
177     credentials, but can also be manipulated directly by the capset() system
178     call.
179
180     The permitted capabilities are those caps that the process might grant
181     itself to its effective or permitted sets through capset(). This
182     inheritable set might also be so constrained.
183
184     The effective capabilities are the ones that a task is actually allowed to
185     make use of itself.
186
187     The inheritable capabilities are the ones that may get passed across
188     execve().
189
190     The bounding set limits the capabilities that may be inherited across
191     execve(), especially when a binary is executed that will execute as UID 0.
192
193 (3) Secure management flags (securebits).
194
195     These are only carried by tasks. These govern the way the above
196     credentials are manipulated and inherited over certain operations such as
197     execve(). They aren't used directly as objective or subjective
198     credentials.
199
200 (4) Keys and keyrings.
201
202     These are only carried by tasks. They carry and cache security tokens
203     that don't fit into the other standard UNIX credentials. They are for
204     making such things as network filesystem keys available to the file
205     accesses performed by processes, without the necessity of ordinary
206     programs having to know about security details involved.
207
208     Keyrings are a special type of key. They carry sets of other keys and can
209     be searched for the desired key. Each process may subscribe to a number
210     of keyrings:
211
212    Per-thread keying
213    Per-process keyring
214    Per-session keyring
215
216     When a process accesses a key, if not already present, it will normally be
217     cached on one of these keyrings for future accesses to find.
218
219     For more information on using keys, see Documentation/keys.txt.
220
221 (5) LSM
222
223     The Linux Security Module allows extra controls to be placed over the
224     operations that a task may do. Currently Linux supports two main
225     alternate LSM options: SELinux and Smack.
226
227     Both work by labelling the objects in a system and then applying sets of
228     rules (policies) that say what operations a task with one label may do to
229     an object with another label.
230
231 (6) AF_KEY
232
233     This is a socket-based approach to credential management for networking
234     stacks [RFC 2367]. It isn't discussed by this document as it doesn't
235     interact directly with task and file credentials; rather it keeps system
236     level credentials.
237
238
239When a file is opened, part of the opening task's subjective context is
240recorded in the file struct created. This allows operations using that file
241struct to use those credentials instead of the subjective context of the task
242that issued the operation. An example of this would be a file opened on a
243network filesystem where the credentials of the opened file should be presented
244to the server, regardless of who is actually doing a read or a write upon it.
245
246
247=============
248FILE MARKINGS
249=============
250
251Files on disk or obtained over the network may have annotations that form the
252objective security context of that file. Depending on the type of filesystem,
253this may include one or more of the following:
254
255 (*) UNIX UID, GID, mode;
256
257 (*) Windows user ID;
258
259 (*) Access control list;
260
261 (*) LSM security label;
262
263 (*) UNIX exec privilege escalation bits (SUID/SGID);
264
265 (*) File capabilities exec privilege escalation bits.
266
267These are compared to the task's subjective security context, and certain
268operations allowed or disallowed as a result. In the case of execve(), the
269privilege escalation bits come into play, and may allow the resulting process
270extra privileges, based on the annotations on the executable file.
271
272
273================
274TASK CREDENTIALS
275================
276
277In Linux, all of a task's credentials are held in (uid, gid) or through
278(groups, keys, LSM security) a refcounted structure of type 'struct cred'.
279Each task points to its credentials by a pointer called 'cred' in its
280task_struct.
281
282Once a set of credentials has been prepared and committed, it may not be
283changed, barring the following exceptions:
284
285 (1) its reference count may be changed;
286
287 (2) the reference count on the group_info struct it points to may be changed;
288
289 (3) the reference count on the security data it points to may be changed;
290
291 (4) the reference count on any keyrings it points to may be changed;
292
293 (5) any keyrings it points to may be revoked, expired or have their security
294     attributes changed; and
295
296 (6) the contents of any keyrings to which it points may be changed (the whole
297     point of keyrings being a shared set of credentials, modifiable by anyone
298     with appropriate access).
299
300To alter anything in the cred struct, the copy-and-replace principle must be
301adhered to. First take a copy, then alter the copy and then use RCU to change
302the task pointer to make it point to the new copy. There are wrappers to aid
303with this (see below).
304
305A task may only alter its _own_ credentials; it is no longer permitted for a
306task to alter another's credentials. This means the capset() system call is no
307longer permitted to take any PID other than the one of the current process.
308Also keyctl_instantiate() and keyctl_negate() functions no longer permit
309attachment to process-specific keyrings in the requesting process as the
310instantiating process may need to create them.
311
312
313IMMUTABLE CREDENTIALS
314---------------------
315
316Once a set of credentials has been made public (by calling commit_creds() for
317example), it must be considered immutable, barring two exceptions:
318
319 (1) The reference count may be altered.
320
321 (2) Whilst the keyring subscriptions of a set of credentials may not be
322     changed, the keyrings subscribed to may have their contents altered.
323
324To catch accidental credential alteration at compile time, struct task_struct
325has _const_ pointers to its credential sets, as does struct file. Furthermore,
326certain functions such as get_cred() and put_cred() operate on const pointers,
327thus rendering casts unnecessary, but require to temporarily ditch the const
328qualification to be able to alter the reference count.
329
330
331ACCESSING TASK CREDENTIALS
332--------------------------
333
334A task being able to alter only its own credentials permits the current process
335to read or replace its own credentials without the need for any form of locking
336- which simplifies things greatly. It can just call:
337
338    const struct cred *current_cred()
339
340to get a pointer to its credentials structure, and it doesn't have to release
341it afterwards.
342
343There are convenience wrappers for retrieving specific aspects of a task's
344credentials (the value is simply returned in each case):
345
346    uid_t current_uid(void) Current's real UID
347    gid_t current_gid(void) Current's real GID
348    uid_t current_euid(void) Current's effective UID
349    gid_t current_egid(void) Current's effective GID
350    uid_t current_fsuid(void) Current's file access UID
351    gid_t current_fsgid(void) Current's file access GID
352    kernel_cap_t current_cap(void) Current's effective capabilities
353    void *current_security(void) Current's LSM security pointer
354    struct user_struct *current_user(void) Current's user account
355
356There are also convenience wrappers for retrieving specific associated pairs of
357a task's credentials:
358
359    void current_uid_gid(uid_t *, gid_t *);
360    void current_euid_egid(uid_t *, gid_t *);
361    void current_fsuid_fsgid(uid_t *, gid_t *);
362
363which return these pairs of values through their arguments after retrieving
364them from the current task's credentials.
365
366
367In addition, there is a function for obtaining a reference on the current
368process's current set of credentials:
369
370    const struct cred *get_current_cred(void);
371
372and functions for getting references to one of the credentials that don't
373actually live in struct cred:
374
375    struct user_struct *get_current_user(void);
376    struct group_info *get_current_groups(void);
377
378which get references to the current process's user accounting structure and
379supplementary groups list respectively.
380
381Once a reference has been obtained, it must be released with put_cred(),
382free_uid() or put_group_info() as appropriate.
383
384
385ACCESSING ANOTHER TASK'S CREDENTIALS
386------------------------------------
387
388Whilst a task may access its own credentials without the need for locking, the
389same is not true of a task wanting to access another task's credentials. It
390must use the RCU read lock and rcu_dereference().
391
392The rcu_dereference() is wrapped by:
393
394    const struct cred *__task_cred(struct task_struct *task);
395
396This should be used inside the RCU read lock, as in the following example:
397
398    void foo(struct task_struct *t, struct foo_data *f)
399    {
400        const struct cred *tcred;
401        ...
402        rcu_read_lock();
403        tcred = __task_cred(t);
404        f->uid = tcred->uid;
405        f->gid = tcred->gid;
406        f->groups = get_group_info(tcred->groups);
407        rcu_read_unlock();
408        ...
409    }
410
411A function need not get RCU read lock to use __task_cred() if it is holding a
412spinlock at the time as this implicitly holds the RCU read lock.
413
414Should it be necessary to hold another task's credentials for a long period of
415time, and possibly to sleep whilst doing so, then the caller should get a
416reference on them using:
417
418    const struct cred *get_task_cred(struct task_struct *task);
419
420This does all the RCU magic inside of it. The caller must call put_cred() on
421the credentials so obtained when they're finished with.
422
423There are a couple of convenience functions to access bits of another task's
424credentials, hiding the RCU magic from the caller:
425
426    uid_t task_uid(task) Task's real UID
427    uid_t task_euid(task) Task's effective UID
428
429If the caller is holding a spinlock or the RCU read lock at the time anyway,
430then:
431
432    __task_cred(task)->uid
433    __task_cred(task)->euid
434
435should be used instead. Similarly, if multiple aspects of a task's credentials
436need to be accessed, RCU read lock or a spinlock should be used, __task_cred()
437called, the result stored in a temporary pointer and then the credential
438aspects called from that before dropping the lock. This prevents the
439potentially expensive RCU magic from being invoked multiple times.
440
441Should some other single aspect of another task's credentials need to be
442accessed, then this can be used:
443
444    task_cred_xxx(task, member)
445
446where 'member' is a non-pointer member of the cred struct. For instance:
447
448    uid_t task_cred_xxx(task, suid);
449
450will retrieve 'struct cred::suid' from the task, doing the appropriate RCU
451magic. This may not be used for pointer members as what they point to may
452disappear the moment the RCU read lock is dropped.
453
454
455ALTERING CREDENTIALS
456--------------------
457
458As previously mentioned, a task may only alter its own credentials, and may not
459alter those of another task. This means that it doesn't need to use any
460locking to alter its own credentials.
461
462To alter the current process's credentials, a function should first prepare a
463new set of credentials by calling:
464
465    struct cred *prepare_creds(void);
466
467this locks current->cred_replace_mutex and then allocates and constructs a
468duplicate of the current process's credentials, returning with the mutex still
469held if successful. It returns NULL if not successful (out of memory).
470
471The mutex prevents ptrace() from altering the ptrace state of a process whilst
472security checks on credentials construction and changing is taking place as
473the ptrace state may alter the outcome, particularly in the case of execve().
474
475The new credentials set should be altered appropriately, and any security
476checks and hooks done. Both the current and the proposed sets of credentials
477are available for this purpose as current_cred() will return the current set
478still at this point.
479
480
481When the credential set is ready, it should be committed to the current process
482by calling:
483
484    int commit_creds(struct cred *new);
485
486This will alter various aspects of the credentials and the process, giving the
487LSM a chance to do likewise, then it will use rcu_assign_pointer() to actually
488commit the new credentials to current->cred, it will release
489current->cred_replace_mutex to allow ptrace() to take place, and it will notify
490the scheduler and others of the changes.
491
492This function is guaranteed to return 0, so that it can be tail-called at the
493end of such functions as sys_setresuid().
494
495Note that this function consumes the caller's reference to the new credentials.
496The caller should _not_ call put_cred() on the new credentials afterwards.
497
498Furthermore, once this function has been called on a new set of credentials,
499those credentials may _not_ be changed further.
500
501
502Should the security checks fail or some other error occur after prepare_creds()
503has been called, then the following function should be invoked:
504
505    void abort_creds(struct cred *new);
506
507This releases the lock on current->cred_replace_mutex that prepare_creds() got
508and then releases the new credentials.
509
510
511A typical credentials alteration function would look something like this:
512
513    int alter_suid(uid_t suid)
514    {
515        struct cred *new;
516        int ret;
517
518        new = prepare_creds();
519        if (!new)
520            return -ENOMEM;
521
522        new->suid = suid;
523        ret = security_alter_suid(new);
524        if (ret < 0) {
525            abort_creds(new);
526            return ret;
527        }
528
529        return commit_creds(new);
530    }
531
532
533MANAGING CREDENTIALS
534--------------------
535
536There are some functions to help manage credentials:
537
538 (*) void put_cred(const struct cred *cred);
539
540     This releases a reference to the given set of credentials. If the
541     reference count reaches zero, the credentials will be scheduled for
542     destruction by the RCU system.
543
544 (*) const struct cred *get_cred(const struct cred *cred);
545
546     This gets a reference on a live set of credentials, returning a pointer to
547     that set of credentials.
548
549 (*) struct cred *get_new_cred(struct cred *cred);
550
551     This gets a reference on a set of credentials that is under construction
552     and is thus still mutable, returning a pointer to that set of credentials.
553
554
555=====================
556OPEN FILE CREDENTIALS
557=====================
558
559When a new file is opened, a reference is obtained on the opening task's
560credentials and this is attached to the file struct as 'f_cred' in place of
561'f_uid' and 'f_gid'. Code that used to access file->f_uid and file->f_gid
562should now access file->f_cred->fsuid and file->f_cred->fsgid.
563
564It is safe to access f_cred without the use of RCU or locking because the
565pointer will not change over the lifetime of the file struct, and nor will the
566contents of the cred struct pointed to, barring the exceptions listed above
567(see the Task Credentials section).
568
569
570=======================================
571OVERRIDING THE VFS'S USE OF CREDENTIALS
572=======================================
573
574Under some circumstances it is desirable to override the credentials used by
575the VFS, and that can be done by calling into such as vfs_mkdir() with a
576different set of credentials. This is done in the following places:
577
578 (*) sys_faccessat().
579
580 (*) do_coredump().
581
582 (*) nfs4recover.c.
583

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