yama_lsm.c

  1// SPDX-License-Identifier: GPL-2.0-only
  2/*
  3 * Yama Linux Security Module
  4 *
  5 * Author: Kees Cook <keescook@chromium.org>
  6 *
  7 * Copyright (C) 2010 Canonical, Ltd.
  8 * Copyright (C) 2011 The Chromium OS Authors.
  9 */
 10
 11#include <linux/lsm_hooks.h>
 12#include <linux/sysctl.h>
 13#include <linux/ptrace.h>
 14#include <linux/prctl.h>
 15#include <linux/ratelimit.h>
 16#include <linux/workqueue.h>
 17#include <linux/string_helpers.h>
 18#include <linux/task_work.h>
 19#include <linux/sched.h>
 20#include <linux/spinlock.h>
 21#include <uapi/linux/lsm.h>
 22
 23#define YAMA_SCOPE_DISABLED	0
 24#define YAMA_SCOPE_RELATIONAL	1
 25#define YAMA_SCOPE_CAPABILITY	2
 26#define YAMA_SCOPE_NO_ATTACH	3
 27
 28static int ptrace_scope = YAMA_SCOPE_RELATIONAL;
 29
 30/* describe a ptrace relationship for potential exception */
 31struct ptrace_relation {
 32	struct task_struct *tracer;
 33	struct task_struct *tracee;
 34	bool invalid;
 35	struct list_head node;
 36	struct rcu_head rcu;
 37};
 38
 39static LIST_HEAD(ptracer_relations);
 40static DEFINE_SPINLOCK(ptracer_relations_lock);
 41
 42static void yama_relation_cleanup(struct work_struct *work);
 43static DECLARE_WORK(yama_relation_work, yama_relation_cleanup);
 44
 45struct access_report_info {
 46	struct callback_head work;
 47	const char *access;
 48	struct task_struct *target;
 49	struct task_struct *agent;
 50};
 51
 52static void __report_access(struct callback_head *work)
 53{
 54	struct access_report_info *info =
 55		container_of(work, struct access_report_info, work);
 56	char *target_cmd, *agent_cmd;
 57
 58	target_cmd = kstrdup_quotable_cmdline(info->target, GFP_KERNEL);
 59	agent_cmd = kstrdup_quotable_cmdline(info->agent, GFP_KERNEL);
 60
 61	pr_notice_ratelimited(
 62		"ptrace %s of \"%s\"[%d] was attempted by \"%s\"[%d]\n",
 63		info->access, target_cmd, info->target->pid, agent_cmd,
 64		info->agent->pid);
 65
 66	kfree(agent_cmd);
 67	kfree(target_cmd);
 68
 69	put_task_struct(info->agent);
 70	put_task_struct(info->target);
 71	kfree(info);
 72}
 73
 74/* defers execution because cmdline access can sleep */
 75static void report_access(const char *access, struct task_struct *target,
 76				struct task_struct *agent)
 77{
 78	struct access_report_info *info;
 79
 80	assert_spin_locked(&target->alloc_lock); /* for target->comm */
 81
 82	if (current->flags & PF_KTHREAD) {
 83		/* I don't think kthreads call task_work_run() before exiting.
 84		 * Imagine angry ranting about procfs here.
 85		 */
 86		pr_notice_ratelimited(
 87		    "ptrace %s of \"%s\"[%d] was attempted by \"%s\"[%d]\n",
 88		    access, target->comm, target->pid, agent->comm, agent->pid);
 89		return;
 90	}
 91
 92	info = kmalloc(sizeof(*info), GFP_ATOMIC);
 93	if (!info)
 94		return;
 95	init_task_work(&info->work, __report_access);
 96	get_task_struct(target);
 97	get_task_struct(agent);
 98	info->access = access;
 99	info->target = target;
100	info->agent = agent;
101	if (task_work_add(current, &info->work, TWA_RESUME) == 0)
102		return; /* success */
103
104	WARN(1, "report_access called from exiting task");
105	put_task_struct(target);
106	put_task_struct(agent);
107	kfree(info);
108}
109
110/**
111 * yama_relation_cleanup - remove invalid entries from the relation list
112 * @work: unused
113 *
114 */
115static void yama_relation_cleanup(struct work_struct *work)
116{
117	struct ptrace_relation *relation;
118
119	spin_lock(&ptracer_relations_lock);
120	rcu_read_lock();
121	list_for_each_entry_rcu(relation, &ptracer_relations, node) {
122		if (relation->invalid) {
123			list_del_rcu(&relation->node);
124			kfree_rcu(relation, rcu);
125		}
126	}
127	rcu_read_unlock();
128	spin_unlock(&ptracer_relations_lock);
129}
130
131/**
132 * yama_ptracer_add - add/replace an exception for this tracer/tracee pair
133 * @tracer: the task_struct of the process doing the ptrace
134 * @tracee: the task_struct of the process to be ptraced
135 *
136 * Each tracee can have, at most, one tracer registered. Each time this
137 * is called, the prior registered tracer will be replaced for the tracee.
138 *
139 * Returns 0 if relationship was added, -ve on error.
140 */
141static int yama_ptracer_add(struct task_struct *tracer,
142			    struct task_struct *tracee)
143{
144	struct ptrace_relation *relation, *added;
145
146	added = kmalloc(sizeof(*added), GFP_KERNEL);
147	if (!added)
148		return -ENOMEM;
149
150	added->tracee = tracee;
151	added->tracer = tracer;
152	added->invalid = false;
153
154	spin_lock(&ptracer_relations_lock);
155	rcu_read_lock();
156	list_for_each_entry_rcu(relation, &ptracer_relations, node) {
157		if (relation->invalid)
158			continue;
159		if (relation->tracee == tracee) {
160			list_replace_rcu(&relation->node, &added->node);
161			kfree_rcu(relation, rcu);
162			goto out;
163		}
164	}
165
166	list_add_rcu(&added->node, &ptracer_relations);
167
168out:
169	rcu_read_unlock();
170	spin_unlock(&ptracer_relations_lock);
171	return 0;
172}
173
174/**
175 * yama_ptracer_del - remove exceptions related to the given tasks
176 * @tracer: remove any relation where tracer task matches
177 * @tracee: remove any relation where tracee task matches
178 */
179static void yama_ptracer_del(struct task_struct *tracer,
180			     struct task_struct *tracee)
181{
182	struct ptrace_relation *relation;
183	bool marked = false;
184
185	rcu_read_lock();
186	list_for_each_entry_rcu(relation, &ptracer_relations, node) {
187		if (relation->invalid)
188			continue;
189		if (relation->tracee == tracee ||
190		    (tracer && relation->tracer == tracer)) {
191			relation->invalid = true;
192			marked = true;
193		}
194	}
195	rcu_read_unlock();
196
197	if (marked)
198		schedule_work(&yama_relation_work);
199}
200
201/**
202 * yama_task_free - check for task_pid to remove from exception list
203 * @task: task being removed
204 */
205static void yama_task_free(struct task_struct *task)
206{
207	yama_ptracer_del(task, task);
208}
209
210/**
211 * yama_task_prctl - check for Yama-specific prctl operations
212 * @option: operation
213 * @arg2: argument
214 * @arg3: argument
215 * @arg4: argument
216 * @arg5: argument
217 *
218 * Return 0 on success, -ve on error.  -ENOSYS is returned when Yama
219 * does not handle the given option.
220 */
221static int yama_task_prctl(int option, unsigned long arg2, unsigned long arg3,
222			   unsigned long arg4, unsigned long arg5)
223{
224	int rc = -ENOSYS;
225	struct task_struct *myself;
226
227	switch (option) {
228	case PR_SET_PTRACER:
229		/* Since a thread can call prctl(), find the group leader
230		 * before calling _add() or _del() on it, since we want
231		 * process-level granularity of control. The tracer group
232		 * leader checking is handled later when walking the ancestry
233		 * at the time of PTRACE_ATTACH check.
234		 */
235		myself = current->group_leader;
236
237		if (arg2 == 0) {
238			yama_ptracer_del(NULL, myself);
239			rc = 0;
240		} else if (arg2 == PR_SET_PTRACER_ANY || (int)arg2 == -1) {
241			rc = yama_ptracer_add(NULL, myself);
242		} else {
243			struct task_struct *tracer;
244
245			tracer = find_get_task_by_vpid(arg2);
246			if (!tracer) {
247				rc = -EINVAL;
248			} else {
249				rc = yama_ptracer_add(tracer, myself);
250				put_task_struct(tracer);
251			}
252		}
253
254		break;
255	}
256
257	return rc;
258}
259
260/**
261 * task_is_descendant - walk up a process family tree looking for a match
262 * @parent: the process to compare against while walking up from child
263 * @child: the process to start from while looking upwards for parent
264 *
265 * Returns 1 if child is a descendant of parent, 0 if not.
266 */
267static int task_is_descendant(struct task_struct *parent,
268			      struct task_struct *child)
269{
270	int rc = 0;
271	struct task_struct *walker = child;
272
273	if (!parent || !child)
274		return 0;
275
276	rcu_read_lock();
277	if (!thread_group_leader(parent))
278		parent = rcu_dereference(parent->group_leader);
279	while (walker->pid > 0) {
280		if (!thread_group_leader(walker))
281			walker = rcu_dereference(walker->group_leader);
282		if (walker == parent) {
283			rc = 1;
284			break;
285		}
286		walker = rcu_dereference(walker->real_parent);
287	}
288	rcu_read_unlock();
289
290	return rc;
291}
292
293/**
294 * ptracer_exception_found - tracer registered as exception for this tracee
295 * @tracer: the task_struct of the process attempting ptrace
296 * @tracee: the task_struct of the process to be ptraced
297 *
298 * Returns 1 if tracer has a ptracer exception ancestor for tracee.
299 */
300static int ptracer_exception_found(struct task_struct *tracer,
301				   struct task_struct *tracee)
302{
303	int rc = 0;
304	struct ptrace_relation *relation;
305	struct task_struct *parent = NULL;
306	bool found = false;
307
308	rcu_read_lock();
309
310	/*
311	 * If there's already an active tracing relationship, then make an
312	 * exception for the sake of other accesses, like process_vm_rw().
313	 */
314	parent = ptrace_parent(tracee);
315	if (parent != NULL && same_thread_group(parent, tracer)) {
316		rc = 1;
317		goto unlock;
318	}
319
320	/* Look for a PR_SET_PTRACER relationship. */
321	if (!thread_group_leader(tracee))
322		tracee = rcu_dereference(tracee->group_leader);
323	list_for_each_entry_rcu(relation, &ptracer_relations, node) {
324		if (relation->invalid)
325			continue;
326		if (relation->tracee == tracee) {
327			parent = relation->tracer;
328			found = true;
329			break;
330		}
331	}
332
333	if (found && (parent == NULL || task_is_descendant(parent, tracer)))
334		rc = 1;
335
336unlock:
337	rcu_read_unlock();
338
339	return rc;
340}
341
342/**
343 * yama_ptrace_access_check - validate PTRACE_ATTACH calls
344 * @child: task that current task is attempting to ptrace
345 * @mode: ptrace attach mode
346 *
347 * Returns 0 if following the ptrace is allowed, -ve on error.
348 */
349static int yama_ptrace_access_check(struct task_struct *child,
350				    unsigned int mode)
351{
352	int rc = 0;
353
354	/* require ptrace target be a child of ptracer on attach */
355	if (mode & PTRACE_MODE_ATTACH) {
356		switch (ptrace_scope) {
357		case YAMA_SCOPE_DISABLED:
358			/* No additional restrictions. */
359			break;
360		case YAMA_SCOPE_RELATIONAL:
361			rcu_read_lock();
362			if (!pid_alive(child))
363				rc = -EPERM;
364			if (!rc && !task_is_descendant(current, child) &&
365			    !ptracer_exception_found(current, child) &&
366			    !ns_capable(__task_cred(child)->user_ns, CAP_SYS_PTRACE))
367				rc = -EPERM;
368			rcu_read_unlock();
369			break;
370		case YAMA_SCOPE_CAPABILITY:
371			rcu_read_lock();
372			if (!ns_capable(__task_cred(child)->user_ns, CAP_SYS_PTRACE))
373				rc = -EPERM;
374			rcu_read_unlock();
375			break;
376		case YAMA_SCOPE_NO_ATTACH:
377		default:
378			rc = -EPERM;
379			break;
380		}
381	}
382
383	if (rc && (mode & PTRACE_MODE_NOAUDIT) == 0)
384		report_access("attach", child, current);
385
386	return rc;
387}
388
389/**
390 * yama_ptrace_traceme - validate PTRACE_TRACEME calls
391 * @parent: task that will become the ptracer of the current task
392 *
393 * Returns 0 if following the ptrace is allowed, -ve on error.
394 */
395static int yama_ptrace_traceme(struct task_struct *parent)
396{
397	int rc = 0;
398
399	/* Only disallow PTRACE_TRACEME on more aggressive settings. */
400	switch (ptrace_scope) {
401	case YAMA_SCOPE_CAPABILITY:
402		if (!has_ns_capability(parent, current_user_ns(), CAP_SYS_PTRACE))
403			rc = -EPERM;
404		break;
405	case YAMA_SCOPE_NO_ATTACH:
406		rc = -EPERM;
407		break;
408	}
409
410	if (rc) {
411		task_lock(current);
412		report_access("traceme", current, parent);
413		task_unlock(current);
414	}
415
416	return rc;
417}
418
419static const struct lsm_id yama_lsmid = {
420	.name = "yama",
421	.id = LSM_ID_YAMA,
422};
423
424static struct security_hook_list yama_hooks[] __ro_after_init = {
425	LSM_HOOK_INIT(ptrace_access_check, yama_ptrace_access_check),
426	LSM_HOOK_INIT(ptrace_traceme, yama_ptrace_traceme),
427	LSM_HOOK_INIT(task_prctl, yama_task_prctl),
428	LSM_HOOK_INIT(task_free, yama_task_free),
429};
430
431#ifdef CONFIG_SYSCTL
432static int yama_dointvec_minmax(const struct ctl_table *table, int write,
433				void *buffer, size_t *lenp, loff_t *ppos)
434{
435	struct ctl_table table_copy;
436
437	if (write && !capable(CAP_SYS_PTRACE))
438		return -EPERM;
439
440	/* Lock the max value if it ever gets set. */
441	table_copy = *table;
442	if (*(int *)table_copy.data == *(int *)table_copy.extra2)
443		table_copy.extra1 = table_copy.extra2;
444
445	return proc_dointvec_minmax(&table_copy, write, buffer, lenp, ppos);
446}
447
448static int max_scope = YAMA_SCOPE_NO_ATTACH;
449
450static const struct ctl_table yama_sysctl_table[] = {
451	{
452		.procname       = "ptrace_scope",
453		.data           = &ptrace_scope,
454		.maxlen         = sizeof(int),
455		.mode           = 0644,
456		.proc_handler   = yama_dointvec_minmax,
457		.extra1         = SYSCTL_ZERO,
458		.extra2         = &max_scope,
459	},
460};
461static void __init yama_init_sysctl(void)
462{
463	if (!register_sysctl("kernel/yama", yama_sysctl_table))
464		panic("Yama: sysctl registration failed.\n");
465}
466#else
467static inline void yama_init_sysctl(void) { }
468#endif /* CONFIG_SYSCTL */
469
470static int __init yama_init(void)
471{
472	pr_info("Yama: becoming mindful.\n");
473	security_add_hooks(yama_hooks, ARRAY_SIZE(yama_hooks), &yama_lsmid);
474	yama_init_sysctl();
475	return 0;
476}
477
478DEFINE_LSM(yama) = {
479	.id = &yama_lsmid,
480	.init = yama_init,
481};