1 // SPDX-License-Identifier: GPL-2.0-or-later
2 /* Task credentials management - see Documentation/security/credentials.rst
3 *
4 * Copyright (C) 2008 Red Hat, Inc. All Rights Reserved.
5 * Written by David Howells (dhowells@redhat.com)
6 */
7
8 #define pr_fmt(fmt) "CRED: " fmt
9
10 #include <linux/export.h>
11 #include <linux/cred.h>
12 #include <linux/slab.h>
13 #include <linux/sched.h>
14 #include <linux/sched/coredump.h>
15 #include <linux/key.h>
16 #include <linux/keyctl.h>
17 #include <linux/init_task.h>
18 #include <linux/security.h>
19 #include <linux/binfmts.h>
20 #include <linux/cn_proc.h>
21 #include <linux/uidgid.h>
22
23 #if 0
24 #define kdebug(FMT, ...) \
25 printk("[%-5.5s%5u] " FMT "\n", \
26 current->comm, current->pid, ##__VA_ARGS__)
27 #else
28 #define kdebug(FMT, ...) \
29 do { \
30 if (0) \
31 no_printk("[%-5.5s%5u] " FMT "\n", \
32 current->comm, current->pid, ##__VA_ARGS__); \
33 } while (0)
34 #endif
35
36 static struct kmem_cache *cred_jar;
37
38 /* init to 2 - one for init_task, one to ensure it is never freed */
39 static struct group_info init_groups = { .usage = REFCOUNT_INIT(2) };
40
41 /*
42 * The initial credentials for the initial task
43 */
44 struct cred init_cred = {
45 .usage = ATOMIC_INIT(4),
46 .uid = GLOBAL_ROOT_UID,
47 .gid = GLOBAL_ROOT_GID,
48 .suid = GLOBAL_ROOT_UID,
49 .sgid = GLOBAL_ROOT_GID,
50 .euid = GLOBAL_ROOT_UID,
51 .egid = GLOBAL_ROOT_GID,
52 .fsuid = GLOBAL_ROOT_UID,
53 .fsgid = GLOBAL_ROOT_GID,
54 .securebits = SECUREBITS_DEFAULT,
55 .cap_inheritable = CAP_EMPTY_SET,
56 .cap_permitted = CAP_FULL_SET,
57 .cap_effective = CAP_FULL_SET,
58 .cap_bset = CAP_FULL_SET,
59 .user = INIT_USER,
60 .user_ns = &init_user_ns,
61 .group_info = &init_groups,
62 .ucounts = &init_ucounts,
63 };
64
65 /*
66 * The RCU callback to actually dispose of a set of credentials
67 */
68 static void put_cred_rcu(struct rcu_head *rcu)
69 {
70 struct cred *cred = container_of(rcu, struct cred, rcu);
71
72 kdebug("put_cred_rcu(%p)", cred);
73
74 if (atomic_long_read(&cred->usage) != 0)
75 panic("CRED: put_cred_rcu() sees %p with usage %ld\n",
76 cred, atomic_long_read(&cred->usage));
77
78 security_cred_free(cred);
79 key_put(cred->session_keyring);
80 key_put(cred->process_keyring);
81 key_put(cred->thread_keyring);
82 key_put(cred->request_key_auth);
83 if (cred->group_info)
84 put_group_info(cred->group_info);
85 free_uid(cred->user);
86 if (cred->ucounts)
87 put_ucounts(cred->ucounts);
88 put_user_ns(cred->user_ns);
89 kmem_cache_free(cred_jar, cred);
90 }
91
92 /**
93 * __put_cred - Destroy a set of credentials
94 * @cred: The record to release
95 *
96 * Destroy a set of credentials on which no references remain.
97 */
98 void __put_cred(struct cred *cred)
99 {
100 kdebug("__put_cred(%p{%ld})", cred,
101 atomic_long_read(&cred->usage));
102
103 BUG_ON(atomic_long_read(&cred->usage) != 0);
104 BUG_ON(cred == current->cred);
105 BUG_ON(cred == current->real_cred);
106
107 if (cred->non_rcu)
108 put_cred_rcu(&cred->rcu);
109 else
110 call_rcu(&cred->rcu, put_cred_rcu);
111 }
112 EXPORT_SYMBOL(__put_cred);
113
114 /*
115 * Clean up a task's credentials when it exits
116 */
117 void exit_creds(struct task_struct *tsk)
118 {
119 struct cred *real_cred, *cred;
120
121 kdebug("exit_creds(%u,%p,%p,{%ld})", tsk->pid, tsk->real_cred, tsk->cred,
122 atomic_long_read(&tsk->cred->usage));
123
124 real_cred = (struct cred *) tsk->real_cred;
125 tsk->real_cred = NULL;
126
127 cred = (struct cred *) tsk->cred;
128 tsk->cred = NULL;
129
130 if (real_cred == cred) {
131 put_cred_many(cred, 2);
132 } else {
133 put_cred(real_cred);
134 put_cred(cred);
135 }
136
137 #ifdef CONFIG_KEYS_REQUEST_CACHE
138 key_put(tsk->cached_requested_key);
139 tsk->cached_requested_key = NULL;
140 #endif
141 }
142
143 /**
144 * get_task_cred - Get another task's objective credentials
145 * @task: The task to query
146 *
147 * Get the objective credentials of a task, pinning them so that they can't go
148 * away. Accessing a task's credentials directly is not permitted.
149 *
150 * The caller must also make sure task doesn't get deleted, either by holding a
151 * ref on task or by holding tasklist_lock to prevent it from being unlinked.
152 */
153 const struct cred *get_task_cred(struct task_struct *task)
154 {
155 const struct cred *cred;
156
157 rcu_read_lock();
158
159 do {
160 cred = __task_cred((task));
161 BUG_ON(!cred);
162 } while (!get_cred_rcu(cred));
163
164 rcu_read_unlock();
165 return cred;
166 }
167 EXPORT_SYMBOL(get_task_cred);
168
169 /*
170 * Allocate blank credentials, such that the credentials can be filled in at a
171 * later date without risk of ENOMEM.
172 */
173 struct cred *cred_alloc_blank(void)
174 {
175 struct cred *new;
176
177 new = kmem_cache_zalloc(cred_jar, GFP_KERNEL);
178 if (!new)
179 return NULL;
180
181 atomic_long_set(&new->usage, 1);
182 if (security_cred_alloc_blank(new, GFP_KERNEL_ACCOUNT) < 0)
183 goto error;
184
185 return new;
186
187 error:
188 abort_creds(new);
189 return NULL;
190 }
191
192 /**
193 * prepare_creds - Prepare a new set of credentials for modification
194 *
195 * Prepare a new set of task credentials for modification. A task's creds
196 * shouldn't generally be modified directly, therefore this function is used to
197 * prepare a new copy, which the caller then modifies and then commits by
198 * calling commit_creds().
199 *
200 * Preparation involves making a copy of the objective creds for modification.
201 *
202 * Returns a pointer to the new creds-to-be if successful, NULL otherwise.
203 *
204 * Call commit_creds() or abort_creds() to clean up.
205 */
206 struct cred *prepare_creds(void)
207 {
208 struct task_struct *task = current;
209 const struct cred *old;
210 struct cred *new;
211
212 new = kmem_cache_alloc(cred_jar, GFP_KERNEL);
213 if (!new)
214 return NULL;
215
216 kdebug("prepare_creds() alloc %p", new);
217
218 old = task->cred;
219 memcpy(new, old, sizeof(struct cred));
220
221 new->non_rcu = 0;
222 atomic_long_set(&new->usage, 1);
223 get_group_info(new->group_info);
224 get_uid(new->user);
225 get_user_ns(new->user_ns);
226
227 #ifdef CONFIG_KEYS
228 key_get(new->session_keyring);
229 key_get(new->process_keyring);
230 key_get(new->thread_keyring);
231 key_get(new->request_key_auth);
232 #endif
233
234 #ifdef CONFIG_SECURITY
235 new->security = NULL;
236 #endif
237
238 new->ucounts = get_ucounts(new->ucounts);
239 if (!new->ucounts)
240 goto error;
241
242 if (security_prepare_creds(new, old, GFP_KERNEL_ACCOUNT) < 0)
243 goto error;
244
245 return new;
246
247 error:
248 abort_creds(new);
249 return NULL;
250 }
251 EXPORT_SYMBOL(prepare_creds);
252
253 /*
254 * Prepare credentials for current to perform an execve()
255 * - The caller must hold ->cred_guard_mutex
256 */
257 struct cred *prepare_exec_creds(void)
258 {
259 struct cred *new;
260
261 new = prepare_creds();
262 if (!new)
263 return new;
264
265 #ifdef CONFIG_KEYS
266 /* newly exec'd tasks don't get a thread keyring */
267 key_put(new->thread_keyring);
268 new->thread_keyring = NULL;
269
270 /* inherit the session keyring; new process keyring */
271 key_put(new->process_keyring);
272 new->process_keyring = NULL;
273 #endif
274
275 new->suid = new->fsuid = new->euid;
276 new->sgid = new->fsgid = new->egid;
277
278 return new;
279 }
280
281 /*
282 * Copy credentials for the new process created by fork()
283 *
284 * We share if we can, but under some circumstances we have to generate a new
285 * set.
286 *
287 * The new process gets the current process's subjective credentials as its
288 * objective and subjective credentials
289 */
290 int copy_creds(struct task_struct *p, unsigned long clone_flags)
291 {
292 struct cred *new;
293 int ret;
294
295 #ifdef CONFIG_KEYS_REQUEST_CACHE
296 p->cached_requested_key = NULL;
297 #endif
298
299 if (
300 #ifdef CONFIG_KEYS
301 !p->cred->thread_keyring &&
302 #endif
303 clone_flags & CLONE_THREAD
304 ) {
305 p->real_cred = get_cred_many(p->cred, 2);
306 kdebug("share_creds(%p{%ld})",
307 p->cred, atomic_long_read(&p->cred->usage));
308 inc_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1);
309 return 0;
310 }
311
312 new = prepare_creds();
313 if (!new)
314 return -ENOMEM;
315
316 if (clone_flags & CLONE_NEWUSER) {
317 ret = create_user_ns(new);
318 if (ret < 0)
319 goto error_put;
320 ret = set_cred_ucounts(new);
321 if (ret < 0)
322 goto error_put;
323 }
324
325 #ifdef CONFIG_KEYS
326 /* new threads get their own thread keyrings if their parent already
327 * had one */
328 if (new->thread_keyring) {
329 key_put(new->thread_keyring);
330 new->thread_keyring = NULL;
331 if (clone_flags & CLONE_THREAD)
332 install_thread_keyring_to_cred(new);
333 }
334
335 /* The process keyring is only shared between the threads in a process;
336 * anything outside of those threads doesn't inherit.
337 */
338 if (!(clone_flags & CLONE_THREAD)) {
339 key_put(new->process_keyring);
340 new->process_keyring = NULL;
341 }
342 #endif
343
344 p->cred = p->real_cred = get_cred(new);
345 inc_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1);
346 return 0;
347
348 error_put:
349 put_cred(new);
350 return ret;
351 }
352
353 static bool cred_cap_issubset(const struct cred *set, const struct cred *subset)
354 {
355 const struct user_namespace *set_ns = set->user_ns;
356 const struct user_namespace *subset_ns = subset->user_ns;
357
358 /* If the two credentials are in the same user namespace see if
359 * the capabilities of subset are a subset of set.
360 */
361 if (set_ns == subset_ns)
362 return cap_issubset(subset->cap_permitted, set->cap_permitted);
363
364 /* The credentials are in a different user namespaces
365 * therefore one is a subset of the other only if a set is an
366 * ancestor of subset and set->euid is owner of subset or one
367 * of subsets ancestors.
368 */
369 for (;subset_ns != &init_user_ns; subset_ns = subset_ns->parent) {
370 if ((set_ns == subset_ns->parent) &&
371 uid_eq(subset_ns->owner, set->euid))
372 return true;
373 }
374
375 return false;
376 }
377
378 /**
379 * commit_creds - Install new credentials upon the current task
380 * @new: The credentials to be assigned
381 *
382 * Install a new set of credentials to the current task, using RCU to replace
383 * the old set. Both the objective and the subjective credentials pointers are
384 * updated. This function may not be called if the subjective credentials are
385 * in an overridden state.
386 *
387 * This function eats the caller's reference to the new credentials.
388 *
389 * Always returns 0 thus allowing this function to be tail-called at the end
390 * of, say, sys_setgid().
391 */
392 int commit_creds(struct cred *new)
393 {
394 struct task_struct *task = current;
395 const struct cred *old = task->real_cred;
396
397 kdebug("commit_creds(%p{%ld})", new,
398 atomic_long_read(&new->usage));
399
400 BUG_ON(task->cred != old);
401 BUG_ON(atomic_long_read(&new->usage) < 1);
402
403 get_cred(new); /* we will require a ref for the subj creds too */
404
405 /* dumpability changes */
406 if (!uid_eq(old->euid, new->euid) ||
407 !gid_eq(old->egid, new->egid) ||
408 !uid_eq(old->fsuid, new->fsuid) ||
409 !gid_eq(old->fsgid, new->fsgid) ||
410 !cred_cap_issubset(old, new)) {
411 if (task->mm)
412 set_dumpable(task->mm, suid_dumpable);
413 task->pdeath_signal = 0;
414 /*
415 * If a task drops privileges and becomes nondumpable,
416 * the dumpability change must become visible before
417 * the credential change; otherwise, a __ptrace_may_access()
418 * racing with this change may be able to attach to a task it
419 * shouldn't be able to attach to (as if the task had dropped
420 * privileges without becoming nondumpable).
421 * Pairs with a read barrier in __ptrace_may_access().
422 */
423 smp_wmb();
424 }
425
426 /* alter the thread keyring */
427 if (!uid_eq(new->fsuid, old->fsuid))
428 key_fsuid_changed(new);
429 if (!gid_eq(new->fsgid, old->fsgid))
430 key_fsgid_changed(new);
431
432 /* do it
433 * RLIMIT_NPROC limits on user->processes have already been checked
434 * in set_user().
435 */
436 if (new->user != old->user || new->user_ns != old->user_ns)
437 inc_rlimit_ucounts(new->ucounts, UCOUNT_RLIMIT_NPROC, 1);
438 rcu_assign_pointer(task->real_cred, new);
439 rcu_assign_pointer(task->cred, new);
440 if (new->user != old->user || new->user_ns != old->user_ns)
441 dec_rlimit_ucounts(old->ucounts, UCOUNT_RLIMIT_NPROC, 1);
442
443 /* send notifications */
444 if (!uid_eq(new->uid, old->uid) ||
445 !uid_eq(new->euid, old->euid) ||
446 !uid_eq(new->suid, old->suid) ||
447 !uid_eq(new->fsuid, old->fsuid))
448 proc_id_connector(task, PROC_EVENT_UID);
449
450 if (!gid_eq(new->gid, old->gid) ||
451 !gid_eq(new->egid, old->egid) ||
452 !gid_eq(new->sgid, old->sgid) ||
453 !gid_eq(new->fsgid, old->fsgid))
454 proc_id_connector(task, PROC_EVENT_GID);
455
456 /* release the old obj and subj refs both */
457 put_cred_many(old, 2);
458 return 0;
459 }
460 EXPORT_SYMBOL(commit_creds);
461
462 /**
463 * abort_creds - Discard a set of credentials and unlock the current task
464 * @new: The credentials that were going to be applied
465 *
466 * Discard a set of credentials that were under construction and unlock the
467 * current task.
468 */
469 void abort_creds(struct cred *new)
470 {
471 kdebug("abort_creds(%p{%ld})", new,
472 atomic_long_read(&new->usage));
473
474 BUG_ON(atomic_long_read(&new->usage) < 1);
475 put_cred(new);
476 }
477 EXPORT_SYMBOL(abort_creds);
478
479 /**
480 * override_creds - Override the current process's subjective credentials
481 * @new: The credentials to be assigned
482 *
483 * Install a set of temporary override subjective credentials on the current
484 * process, returning the old set for later reversion.
485 */
486 const struct cred *override_creds(const struct cred *new)
487 {
488 const struct cred *old = current->cred;
489
490 kdebug("override_creds(%p{%ld})", new,
491 atomic_long_read(&new->usage));
492
493 /*
494 * NOTE! This uses 'get_new_cred()' rather than 'get_cred()'.
495 *
496 * That means that we do not clear the 'non_rcu' flag, since
497 * we are only installing the cred into the thread-synchronous
498 * '->cred' pointer, not the '->real_cred' pointer that is
499 * visible to other threads under RCU.
500 */
501 get_new_cred((struct cred *)new);
502 rcu_assign_pointer(current->cred, new);
503
504 kdebug("override_creds() = %p{%ld}", old,
505 atomic_long_read(&old->usage));
506 return old;
507 }
508 EXPORT_SYMBOL(override_creds);
509
510 /**
511 * revert_creds - Revert a temporary subjective credentials override
512 * @old: The credentials to be restored
513 *
514 * Revert a temporary set of override subjective credentials to an old set,
515 * discarding the override set.
516 */
517 void revert_creds(const struct cred *old)
518 {
519 const struct cred *override = current->cred;
520
521 kdebug("revert_creds(%p{%ld})", old,
522 atomic_long_read(&old->usage));
523
524 rcu_assign_pointer(current->cred, old);
525 put_cred(override);
526 }
527 EXPORT_SYMBOL(revert_creds);
528
529 /**
530 * cred_fscmp - Compare two credentials with respect to filesystem access.
531 * @a: The first credential
532 * @b: The second credential
533 *
534 * cred_cmp() will return zero if both credentials have the same
535 * fsuid, fsgid, and supplementary groups. That is, if they will both
536 * provide the same access to files based on mode/uid/gid.
537 * If the credentials are different, then either -1 or 1 will
538 * be returned depending on whether @a comes before or after @b
539 * respectively in an arbitrary, but stable, ordering of credentials.
540 *
541 * Return: -1, 0, or 1 depending on comparison
542 */
543 int cred_fscmp(const struct cred *a, const struct cred *b)
544 {
545 struct group_info *ga, *gb;
546 int g;
547
548 if (a == b)
549 return 0;
550 if (uid_lt(a->fsuid, b->fsuid))
551 return -1;
552 if (uid_gt(a->fsuid, b->fsuid))
553 return 1;
554
555 if (gid_lt(a->fsgid, b->fsgid))
556 return -1;
557 if (gid_gt(a->fsgid, b->fsgid))
558 return 1;
559
560 ga = a->group_info;
561 gb = b->group_info;
562 if (ga == gb)
563 return 0;
564 if (ga == NULL)
565 return -1;
566 if (gb == NULL)
567 return 1;
568 if (ga->ngroups < gb->ngroups)
569 return -1;
570 if (ga->ngroups > gb->ngroups)
571 return 1;
572
573 for (g = 0; g < ga->ngroups; g++) {
574 if (gid_lt(ga->gid[g], gb->gid[g]))
575 return -1;
576 if (gid_gt(ga->gid[g], gb->gid[g]))
577 return 1;
578 }
579 return 0;
580 }
581 EXPORT_SYMBOL(cred_fscmp);
582
583 int set_cred_ucounts(struct cred *new)
584 {
585 struct ucounts *new_ucounts, *old_ucounts = new->ucounts;
586
587 /*
588 * This optimization is needed because alloc_ucounts() uses locks
589 * for table lookups.
590 */
591 if (old_ucounts->ns == new->user_ns && uid_eq(old_ucounts->uid, new->uid))
592 return 0;
593
594 if (!(new_ucounts = alloc_ucounts(new->user_ns, new->uid)))
595 return -EAGAIN;
596
597 new->ucounts = new_ucounts;
598 put_ucounts(old_ucounts);
599
600 return 0;
601 }
602
603 /*
604 * initialise the credentials stuff
605 */
606 void __init cred_init(void)
607 {
608 /* allocate a slab in which we can store credentials */
609 cred_jar = KMEM_CACHE(cred,
610 SLAB_HWCACHE_ALIGN | SLAB_PANIC | SLAB_ACCOUNT);
611 }
612
613 /**
614 * prepare_kernel_cred - Prepare a set of credentials for a kernel service
615 * @daemon: A userspace daemon to be used as a reference
616 *
617 * Prepare a set of credentials for a kernel service. This can then be used to
618 * override a task's own credentials so that work can be done on behalf of that
619 * task that requires a different subjective context.
620 *
621 * @daemon is used to provide a base cred, with the security data derived from
622 * that; if this is "&init_task", they'll be set to 0, no groups, full
623 * capabilities, and no keys.
624 *
625 * The caller may change these controls afterwards if desired.
626 *
627 * Returns the new credentials or NULL if out of memory.
628 */
629 struct cred *prepare_kernel_cred(struct task_struct *daemon)
630 {
631 const struct cred *old;
632 struct cred *new;
633
634 if (WARN_ON_ONCE(!daemon))
635 return NULL;
636
637 new = kmem_cache_alloc(cred_jar, GFP_KERNEL);
638 if (!new)
639 return NULL;
640
641 kdebug("prepare_kernel_cred() alloc %p", new);
642
643 old = get_task_cred(daemon);
644
645 *new = *old;
646 new->non_rcu = 0;
647 atomic_long_set(&new->usage, 1);
648 get_uid(new->user);
649 get_user_ns(new->user_ns);
650 get_group_info(new->group_info);
651
652 #ifdef CONFIG_KEYS
653 new->session_keyring = NULL;
654 new->process_keyring = NULL;
655 new->thread_keyring = NULL;
656 new->request_key_auth = NULL;
657 new->jit_keyring = KEY_REQKEY_DEFL_THREAD_KEYRING;
658 #endif
659
660 #ifdef CONFIG_SECURITY
661 new->security = NULL;
662 #endif
663 new->ucounts = get_ucounts(new->ucounts);
664 if (!new->ucounts)
665 goto error;
666
667 if (security_prepare_creds(new, old, GFP_KERNEL_ACCOUNT) < 0)
668 goto error;
669
670 put_cred(old);
671 return new;
672
673 error:
674 put_cred(new);
675 put_cred(old);
676 return NULL;
677 }
678 EXPORT_SYMBOL(prepare_kernel_cred);
679
680 /**
681 * set_security_override - Set the security ID in a set of credentials
682 * @new: The credentials to alter
683 * @secid: The LSM security ID to set
684 *
685 * Set the LSM security ID in a set of credentials so that the subjective
686 * security is overridden when an alternative set of credentials is used.
687 */
688 int set_security_override(struct cred *new, u32 secid)
689 {
690 return security_kernel_act_as(new, secid);
691 }
692 EXPORT_SYMBOL(set_security_override);
693
694 /**
695 * set_security_override_from_ctx - Set the security ID in a set of credentials
696 * @new: The credentials to alter
697 * @secctx: The LSM security context to generate the security ID from.
698 *
699 * Set the LSM security ID in a set of credentials so that the subjective
700 * security is overridden when an alternative set of credentials is used. The
701 * security ID is specified in string form as a security context to be
702 * interpreted by the LSM.
703 */
704 int set_security_override_from_ctx(struct cred *new, const char *secctx)
705 {
706 u32 secid;
707 int ret;
708
709 ret = security_secctx_to_secid(secctx, strlen(secctx), &secid);
710 if (ret < 0)
711 return ret;
712
713 return set_security_override(new, secid);
714 }
715 EXPORT_SYMBOL(set_security_override_from_ctx);
716
717 /**
718 * set_create_files_as - Set the LSM file create context in a set of credentials
719 * @new: The credentials to alter
720 * @inode: The inode to take the context from
721 *
722 * Change the LSM file creation context in a set of credentials to be the same
723 * as the object context of the specified inode, so that the new inodes have
724 * the same MAC context as that inode.
725 */
726 int set_create_files_as(struct cred *new, struct inode *inode)
727 {
728 if (!uid_valid(inode->i_uid) || !gid_valid(inode->i_gid))
729 return -EINVAL;
730 new->fsuid = inode->i_uid;
731 new->fsgid = inode->i_gid;
732 return security_kernel_create_files_as(new, inode);
733 }
734 EXPORT_SYMBOL(set_create_files_as);
735
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