cred.c

  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, ...)						\
 29do {									\
 30	if (0)								\
 31		no_printk("[%-5.5s%5u] " FMT "\n",			\
 32			  current->comm, current->pid, ##__VA_ARGS__);	\
 33} while (0)
 34#endif
 35
 36static struct kmem_cache *cred_jar;
 37
 38/*
 39 * The RCU callback to actually dispose of a set of credentials
 40 */
 41static void put_cred_rcu(struct rcu_head *rcu)
 42{
 43	struct cred *cred = container_of(rcu, struct cred, rcu);
 44
 45	kdebug("put_cred_rcu(%p)", cred);
 46
 47	if (atomic_long_read(&cred->usage) != 0)
 48		panic("CRED: put_cred_rcu() sees %p with usage %ld\n",
 49		      cred, atomic_long_read(&cred->usage));
 50
 51	security_cred_free(cred);
 52	key_put(cred->session_keyring);
 53	key_put(cred->process_keyring);
 54	key_put(cred->thread_keyring);
 55	key_put(cred->request_key_auth);
 56	if (cred->group_info)
 57		put_group_info(cred->group_info);
 58	free_uid(cred->user);
 59	if (cred->ucounts)
 60		put_ucounts(cred->ucounts);
 61	put_user_ns(cred->user_ns);
 62	kmem_cache_free(cred_jar, cred);
 63}
 64
 65/**
 66 * __put_cred - Destroy a set of credentials
 67 * @cred: The record to release
 68 *
 69 * Destroy a set of credentials on which no references remain.
 70 */
 71void __put_cred(struct cred *cred)
 72{
 73	kdebug("__put_cred(%p{%ld})", cred,
 74	       atomic_long_read(&cred->usage));
 75
 76	BUG_ON(atomic_long_read(&cred->usage) != 0);
 77	BUG_ON(cred == current->cred);
 78	BUG_ON(cred == current->real_cred);
 79
 80	if (cred->non_rcu)
 81		put_cred_rcu(&cred->rcu);
 82	else
 83		call_rcu(&cred->rcu, put_cred_rcu);
 84}
 85EXPORT_SYMBOL(__put_cred);
 86
 87/*
 88 * Clean up a task's credentials when it exits
 89 */
 90void exit_creds(struct task_struct *tsk)
 91{
 92	struct cred *real_cred, *cred;
 93
 94	kdebug("exit_creds(%u,%p,%p,{%ld})", tsk->pid, tsk->real_cred, tsk->cred,
 95	       atomic_long_read(&tsk->cred->usage));
 96
 97	real_cred = (struct cred *) tsk->real_cred;
 98	tsk->real_cred = NULL;
 99
100	cred = (struct cred *) tsk->cred;
101	tsk->cred = NULL;
102
103	if (real_cred == cred) {
104		put_cred_many(cred, 2);
105	} else {
106		put_cred(real_cred);
107		put_cred(cred);
108	}
109
110#ifdef CONFIG_KEYS_REQUEST_CACHE
111	key_put(tsk->cached_requested_key);
112	tsk->cached_requested_key = NULL;
113#endif
114}
115
116/**
117 * get_task_cred - Get another task's objective credentials
118 * @task: The task to query
119 *
120 * Get the objective credentials of a task, pinning them so that they can't go
121 * away.  Accessing a task's credentials directly is not permitted.
122 *
123 * The caller must also make sure task doesn't get deleted, either by holding a
124 * ref on task or by holding tasklist_lock to prevent it from being unlinked.
125 */
126const struct cred *get_task_cred(struct task_struct *task)
127{
128	const struct cred *cred;
129
130	rcu_read_lock();
131
132	do {
133		cred = __task_cred((task));
134		BUG_ON(!cred);
135	} while (!get_cred_rcu(cred));
136
137	rcu_read_unlock();
138	return cred;
139}
140EXPORT_SYMBOL(get_task_cred);
141
142/*
143 * Allocate blank credentials, such that the credentials can be filled in at a
144 * later date without risk of ENOMEM.
145 */
146struct cred *cred_alloc_blank(void)
147{
148	struct cred *new;
149
150	new = kmem_cache_zalloc(cred_jar, GFP_KERNEL);
151	if (!new)
152		return NULL;
153
154	atomic_long_set(&new->usage, 1);
155	if (security_cred_alloc_blank(new, GFP_KERNEL_ACCOUNT) < 0)
156		goto error;
157
158	return new;
159
160error:
161	abort_creds(new);
162	return NULL;
163}
164
165/**
166 * prepare_creds - Prepare a new set of credentials for modification
167 *
168 * Prepare a new set of task credentials for modification.  A task's creds
169 * shouldn't generally be modified directly, therefore this function is used to
170 * prepare a new copy, which the caller then modifies and then commits by
171 * calling commit_creds().
172 *
173 * Preparation involves making a copy of the objective creds for modification.
174 *
175 * Returns a pointer to the new creds-to-be if successful, NULL otherwise.
176 *
177 * Call commit_creds() or abort_creds() to clean up.
178 */
179struct cred *prepare_creds(void)
180{
181	struct task_struct *task = current;
182	const struct cred *old;
183	struct cred *new;
184
185	new = kmem_cache_alloc(cred_jar, GFP_KERNEL);
186	if (!new)
187		return NULL;
188
189	kdebug("prepare_creds() alloc %p", new);
190
191	old = task->cred;
192	memcpy(new, old, sizeof(struct cred));
193
194	new->non_rcu = 0;
195	atomic_long_set(&new->usage, 1);
196	get_group_info(new->group_info);
197	get_uid(new->user);
198	get_user_ns(new->user_ns);
199
200#ifdef CONFIG_KEYS
201	key_get(new->session_keyring);
202	key_get(new->process_keyring);
203	key_get(new->thread_keyring);
204	key_get(new->request_key_auth);
205#endif
206
207#ifdef CONFIG_SECURITY
208	new->security = NULL;
209#endif
210
211	new->ucounts = get_ucounts(new->ucounts);
212	if (!new->ucounts)
213		goto error;
214
215	if (security_prepare_creds(new, old, GFP_KERNEL_ACCOUNT) < 0)
216		goto error;
217
218	return new;
219
220error:
221	abort_creds(new);
222	return NULL;
223}
224EXPORT_SYMBOL(prepare_creds);
225
226/*
227 * Prepare credentials for current to perform an execve()
228 * - The caller must hold ->cred_guard_mutex
229 */
230struct cred *prepare_exec_creds(void)
231{
232	struct cred *new;
233
234	new = prepare_creds();
235	if (!new)
236		return new;
237
238#ifdef CONFIG_KEYS
239	/* newly exec'd tasks don't get a thread keyring */
240	key_put(new->thread_keyring);
241	new->thread_keyring = NULL;
242
243	/* inherit the session keyring; new process keyring */
244	key_put(new->process_keyring);
245	new->process_keyring = NULL;
246#endif
247
248	new->suid = new->fsuid = new->euid;
249	new->sgid = new->fsgid = new->egid;
250
251	return new;
252}
253
254/*
255 * Copy credentials for the new process created by fork()
256 *
257 * We share if we can, but under some circumstances we have to generate a new
258 * set.
259 *
260 * The new process gets the current process's subjective credentials as its
261 * objective and subjective credentials
262 */
263int copy_creds(struct task_struct *p, u64 clone_flags)
264{
265	struct cred *new;
266	int ret;
267
268#ifdef CONFIG_KEYS_REQUEST_CACHE
269	p->cached_requested_key = NULL;
270#endif
271
272	if (
273#ifdef CONFIG_KEYS
274		!p->cred->thread_keyring &&
275#endif
276		clone_flags & CLONE_THREAD
277	    ) {
278		p->real_cred = get_cred_many(p->cred, 2);
279		kdebug("share_creds(%p{%ld})",
280		       p->cred, atomic_long_read(&p->cred->usage));
281		inc_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1);
282		get_cred_namespaces(p);
283		return 0;
284	}
285
286	new = prepare_creds();
287	if (!new)
288		return -ENOMEM;
289
290	if (clone_flags & CLONE_NEWUSER) {
291		ret = create_user_ns(new);
292		if (ret < 0)
293			goto error_put;
294		ret = set_cred_ucounts(new);
295		if (ret < 0)
296			goto error_put;
297	}
298
299#ifdef CONFIG_KEYS
300	/* new threads get their own thread keyrings if their parent already
301	 * had one */
302	if (new->thread_keyring) {
303		key_put(new->thread_keyring);
304		new->thread_keyring = NULL;
305		if (clone_flags & CLONE_THREAD)
306			install_thread_keyring_to_cred(new);
307	}
308
309	/* The process keyring is only shared between the threads in a process;
310	 * anything outside of those threads doesn't inherit.
311	 */
312	if (!(clone_flags & CLONE_THREAD)) {
313		key_put(new->process_keyring);
314		new->process_keyring = NULL;
315	}
316#endif
317
318	p->cred = p->real_cred = get_cred(new);
319	inc_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1);
320	get_cred_namespaces(p);
321
322	return 0;
323
324error_put:
325	put_cred(new);
326	return ret;
327}
328
329static bool cred_cap_issubset(const struct cred *set, const struct cred *subset)
330{
331	const struct user_namespace *set_ns = set->user_ns;
332	const struct user_namespace *subset_ns = subset->user_ns;
333
334	/* If the two credentials are in the same user namespace see if
335	 * the capabilities of subset are a subset of set.
336	 */
337	if (set_ns == subset_ns)
338		return cap_issubset(subset->cap_permitted, set->cap_permitted);
339
340	/* The credentials are in a different user namespaces
341	 * therefore one is a subset of the other only if a set is an
342	 * ancestor of subset and set->euid is owner of subset or one
343	 * of subsets ancestors.
344	 */
345	for (;subset_ns != &init_user_ns; subset_ns = subset_ns->parent) {
346		if ((set_ns == subset_ns->parent)  &&
347		    uid_eq(subset_ns->owner, set->euid))
348			return true;
349	}
350
351	return false;
352}
353
354/**
355 * commit_creds - Install new credentials upon the current task
356 * @new: The credentials to be assigned
357 *
358 * Install a new set of credentials to the current task, using RCU to replace
359 * the old set.  Both the objective and the subjective credentials pointers are
360 * updated.  This function may not be called if the subjective credentials are
361 * in an overridden state.
362 *
363 * This function eats the caller's reference to the new credentials.
364 *
365 * Always returns 0 thus allowing this function to be tail-called at the end
366 * of, say, sys_setgid().
367 */
368int commit_creds(struct cred *new)
369{
370	struct task_struct *task = current;
371	const struct cred *old = task->real_cred;
372
373	kdebug("commit_creds(%p{%ld})", new,
374	       atomic_long_read(&new->usage));
375
376	BUG_ON(task->cred != old);
377	BUG_ON(atomic_long_read(&new->usage) < 1);
378
379	get_cred(new); /* we will require a ref for the subj creds too */
380
381	/* dumpability changes */
382	if (!uid_eq(old->euid, new->euid) ||
383	    !gid_eq(old->egid, new->egid) ||
384	    !uid_eq(old->fsuid, new->fsuid) ||
385	    !gid_eq(old->fsgid, new->fsgid) ||
386	    !cred_cap_issubset(old, new)) {
387		if (task->mm)
388			set_dumpable(task->mm, suid_dumpable);
389		task->pdeath_signal = 0;
390		/*
391		 * If a task drops privileges and becomes nondumpable,
392		 * the dumpability change must become visible before
393		 * the credential change; otherwise, a __ptrace_may_access()
394		 * racing with this change may be able to attach to a task it
395		 * shouldn't be able to attach to (as if the task had dropped
396		 * privileges without becoming nondumpable).
397		 * Pairs with a read barrier in __ptrace_may_access().
398		 */
399		smp_wmb();
400	}
401
402	/* alter the thread keyring */
403	if (!uid_eq(new->fsuid, old->fsuid))
404		key_fsuid_changed(new);
405	if (!gid_eq(new->fsgid, old->fsgid))
406		key_fsgid_changed(new);
407
408	/* do it
409	 * RLIMIT_NPROC limits on user->processes have already been checked
410	 * in set_user().
411	 */
412	if (new->user != old->user || new->user_ns != old->user_ns)
413		inc_rlimit_ucounts(new->ucounts, UCOUNT_RLIMIT_NPROC, 1);
414
415	rcu_assign_pointer(task->real_cred, new);
416	rcu_assign_pointer(task->cred, new);
417	if (new->user != old->user || new->user_ns != old->user_ns)
418		dec_rlimit_ucounts(old->ucounts, UCOUNT_RLIMIT_NPROC, 1);
419	if (new->user_ns != old->user_ns)
420		switch_cred_namespaces(old, new);
421
422	/* send notifications */
423	if (!uid_eq(new->uid,   old->uid)  ||
424	    !uid_eq(new->euid,  old->euid) ||
425	    !uid_eq(new->suid,  old->suid) ||
426	    !uid_eq(new->fsuid, old->fsuid))
427		proc_id_connector(task, PROC_EVENT_UID);
428
429	if (!gid_eq(new->gid,   old->gid)  ||
430	    !gid_eq(new->egid,  old->egid) ||
431	    !gid_eq(new->sgid,  old->sgid) ||
432	    !gid_eq(new->fsgid, old->fsgid))
433		proc_id_connector(task, PROC_EVENT_GID);
434
435	/* release the old obj and subj refs both */
436	put_cred_many(old, 2);
437	return 0;
438}
439EXPORT_SYMBOL(commit_creds);
440
441/**
442 * abort_creds - Discard a set of credentials and unlock the current task
443 * @new: The credentials that were going to be applied
444 *
445 * Discard a set of credentials that were under construction and unlock the
446 * current task.
447 */
448void abort_creds(struct cred *new)
449{
450	kdebug("abort_creds(%p{%ld})", new,
451	       atomic_long_read(&new->usage));
452
453	BUG_ON(atomic_long_read(&new->usage) < 1);
454	put_cred(new);
455}
456EXPORT_SYMBOL(abort_creds);
457
458/**
459 * cred_fscmp - Compare two credentials with respect to filesystem access.
460 * @a: The first credential
461 * @b: The second credential
462 *
463 * cred_cmp() will return zero if both credentials have the same
464 * fsuid, fsgid, and supplementary groups.  That is, if they will both
465 * provide the same access to files based on mode/uid/gid.
466 * If the credentials are different, then either -1 or 1 will
467 * be returned depending on whether @a comes before or after @b
468 * respectively in an arbitrary, but stable, ordering of credentials.
469 *
470 * Return: -1, 0, or 1 depending on comparison
471 */
472int cred_fscmp(const struct cred *a, const struct cred *b)
473{
474	struct group_info *ga, *gb;
475	int g;
476
477	if (a == b)
478		return 0;
479	if (uid_lt(a->fsuid, b->fsuid))
480		return -1;
481	if (uid_gt(a->fsuid, b->fsuid))
482		return 1;
483
484	if (gid_lt(a->fsgid, b->fsgid))
485		return -1;
486	if (gid_gt(a->fsgid, b->fsgid))
487		return 1;
488
489	ga = a->group_info;
490	gb = b->group_info;
491	if (ga == gb)
492		return 0;
493	if (ga == NULL)
494		return -1;
495	if (gb == NULL)
496		return 1;
497	if (ga->ngroups < gb->ngroups)
498		return -1;
499	if (ga->ngroups > gb->ngroups)
500		return 1;
501
502	for (g = 0; g < ga->ngroups; g++) {
503		if (gid_lt(ga->gid[g], gb->gid[g]))
504			return -1;
505		if (gid_gt(ga->gid[g], gb->gid[g]))
506			return 1;
507	}
508	return 0;
509}
510EXPORT_SYMBOL(cred_fscmp);
511
512int set_cred_ucounts(struct cred *new)
513{
514	struct ucounts *new_ucounts, *old_ucounts = new->ucounts;
515
516	/*
517	 * This optimization is needed because alloc_ucounts() uses locks
518	 * for table lookups.
519	 */
520	if (old_ucounts->ns == new->user_ns && uid_eq(old_ucounts->uid, new->uid))
521		return 0;
522
523	if (!(new_ucounts = alloc_ucounts(new->user_ns, new->uid)))
524		return -EAGAIN;
525
526	new->ucounts = new_ucounts;
527	put_ucounts(old_ucounts);
528
529	return 0;
530}
531
532/*
533 * initialise the credentials stuff
534 */
535void __init cred_init(void)
536{
537	/* allocate a slab in which we can store credentials */
538	cred_jar = KMEM_CACHE(cred,
539			      SLAB_HWCACHE_ALIGN | SLAB_PANIC | SLAB_ACCOUNT);
540}
541
542/**
543 * prepare_kernel_cred - Prepare a set of credentials for a kernel service
544 * @daemon: A userspace daemon to be used as a reference
545 *
546 * Prepare a set of credentials for a kernel service.  This can then be used to
547 * override a task's own credentials so that work can be done on behalf of that
548 * task that requires a different subjective context.
549 *
550 * @daemon is used to provide a base cred, with the security data derived from
551 * that; if this is "&init_task", they'll be set to 0, no groups, full
552 * capabilities, and no keys.
553 *
554 * The caller may change these controls afterwards if desired.
555 *
556 * Returns the new credentials or NULL if out of memory.
557 */
558struct cred *prepare_kernel_cred(struct task_struct *daemon)
559{
560	const struct cred *old;
561	struct cred *new;
562
563	if (WARN_ON_ONCE(!daemon))
564		return NULL;
565
566	new = kmem_cache_alloc(cred_jar, GFP_KERNEL);
567	if (!new)
568		return NULL;
569
570	kdebug("prepare_kernel_cred() alloc %p", new);
571
572	old = get_task_cred(daemon);
573
574	*new = *old;
575	new->non_rcu = 0;
576	atomic_long_set(&new->usage, 1);
577	get_uid(new->user);
578	get_user_ns(new->user_ns);
579	get_group_info(new->group_info);
580
581#ifdef CONFIG_KEYS
582	new->session_keyring = NULL;
583	new->process_keyring = NULL;
584	new->thread_keyring = NULL;
585	new->request_key_auth = NULL;
586	new->jit_keyring = KEY_REQKEY_DEFL_THREAD_KEYRING;
587#endif
588
589#ifdef CONFIG_SECURITY
590	new->security = NULL;
591#endif
592	new->ucounts = get_ucounts(new->ucounts);
593	if (!new->ucounts)
594		goto error;
595
596	if (security_prepare_creds(new, old, GFP_KERNEL_ACCOUNT) < 0)
597		goto error;
598
599	put_cred(old);
600	return new;
601
602error:
603	put_cred(new);
604	put_cred(old);
605	return NULL;
606}
607EXPORT_SYMBOL(prepare_kernel_cred);
608
609/**
610 * set_security_override - Set the security ID in a set of credentials
611 * @new: The credentials to alter
612 * @secid: The LSM security ID to set
613 *
614 * Set the LSM security ID in a set of credentials so that the subjective
615 * security is overridden when an alternative set of credentials is used.
616 */
617int set_security_override(struct cred *new, u32 secid)
618{
619	return security_kernel_act_as(new, secid);
620}
621EXPORT_SYMBOL(set_security_override);
622
623/**
624 * set_security_override_from_ctx - Set the security ID in a set of credentials
625 * @new: The credentials to alter
626 * @secctx: The LSM security context to generate the security ID from.
627 *
628 * Set the LSM security ID in a set of credentials so that the subjective
629 * security is overridden when an alternative set of credentials is used.  The
630 * security ID is specified in string form as a security context to be
631 * interpreted by the LSM.
632 */
633int set_security_override_from_ctx(struct cred *new, const char *secctx)
634{
635	u32 secid;
636	int ret;
637
638	ret = security_secctx_to_secid(secctx, strlen(secctx), &secid);
639	if (ret < 0)
640		return ret;
641
642	return set_security_override(new, secid);
643}
644EXPORT_SYMBOL(set_security_override_from_ctx);
645
646/**
647 * set_create_files_as - Set the LSM file create context in a set of credentials
648 * @new: The credentials to alter
649 * @inode: The inode to take the context from
650 *
651 * Change the LSM file creation context in a set of credentials to be the same
652 * as the object context of the specified inode, so that the new inodes have
653 * the same MAC context as that inode.
654 */
655int set_create_files_as(struct cred *new, struct inode *inode)
656{
657	if (!uid_valid(inode->i_uid) || !gid_valid(inode->i_gid))
658		return -EINVAL;
659	new->fsuid = inode->i_uid;
660	new->fsgid = inode->i_gid;
661	return security_kernel_create_files_as(new, inode);
662}
663EXPORT_SYMBOL(set_create_files_as);