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
22#define YAMA_SCOPE_DISABLED	0
23#define YAMA_SCOPE_RELATIONAL	1
24#define YAMA_SCOPE_CAPABILITY	2
25#define YAMA_SCOPE_NO_ATTACH	3
26
27static int ptrace_scope = YAMA_SCOPE_RELATIONAL;
28
29/* describe a ptrace relationship for potential exception */
30struct ptrace_relation {
31	struct task_struct *tracer;
32	struct task_struct *tracee;
33	bool invalid;
34	struct list_head node;
35	struct rcu_head rcu;
36};
37
38static LIST_HEAD(ptracer_relations);
39static DEFINE_SPINLOCK(ptracer_relations_lock);
40
41static void yama_relation_cleanup(struct work_struct *work);
42static DECLARE_WORK(yama_relation_work, yama_relation_cleanup);
43
44struct access_report_info {
45	struct callback_head work;
46	const char *access;
47	struct task_struct *target;
48	struct task_struct *agent;
49};
50
51static void __report_access(struct callback_head *work)
52{
53	struct access_report_info *info =
54		container_of(work, struct access_report_info, work);
55	char *target_cmd, *agent_cmd;
56
57	target_cmd = kstrdup_quotable_cmdline(info->target, GFP_KERNEL);
58	agent_cmd = kstrdup_quotable_cmdline(info->agent, GFP_KERNEL);
59
60	pr_notice_ratelimited(
61		"ptrace %s of \"%s\"[%d] was attempted by \"%s\"[%d]\n",
62		info->access, target_cmd, info->target->pid, agent_cmd,
63		info->agent->pid);
64
65	kfree(agent_cmd);
66	kfree(target_cmd);
67
68	put_task_struct(info->agent);
69	put_task_struct(info->target);
70	kfree(info);
71}
72
73/* defers execution because cmdline access can sleep */
74static void report_access(const char *access, struct task_struct *target,
75				struct task_struct *agent)
76{
77	struct access_report_info *info;
78	char agent_comm[sizeof(agent->comm)];
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,
89		    get_task_comm(agent_comm, agent), agent->pid);
90		return;
91	}
92
93	info = kmalloc(sizeof(*info), GFP_ATOMIC);
94	if (!info)
95		return;
96	init_task_work(&info->work, __report_access);
97	get_task_struct(target);
98	get_task_struct(agent);
99	info->access = access;
100	info->target = target;
101	info->agent = agent;
102	if (task_work_add(current, &info->work, TWA_RESUME) == 0)
103		return; /* success */
104
105	WARN(1, "report_access called from exiting task");
106	put_task_struct(target);
107	put_task_struct(agent);
108	kfree(info);
109}
110
111/**
112 * yama_relation_cleanup - remove invalid entries from the relation list
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 = current;
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		rcu_read_lock();
236		if (!thread_group_leader(myself))
237			myself = rcu_dereference(myself->group_leader);
238		get_task_struct(myself);
239		rcu_read_unlock();
240
241		if (arg2 == 0) {
242			yama_ptracer_del(NULL, myself);
243			rc = 0;
244		} else if (arg2 == PR_SET_PTRACER_ANY || (int)arg2 == -1) {
245			rc = yama_ptracer_add(NULL, myself);
246		} else {
247			struct task_struct *tracer;
248
249			tracer = find_get_task_by_vpid(arg2);
250			if (!tracer) {
251				rc = -EINVAL;
252			} else {
253				rc = yama_ptracer_add(tracer, myself);
254				put_task_struct(tracer);
255			}
256		}
257
258		put_task_struct(myself);
259		break;
260	}
261
262	return rc;
263}
264
265/**
266 * task_is_descendant - walk up a process family tree looking for a match
267 * @parent: the process to compare against while walking up from child
268 * @child: the process to start from while looking upwards for parent
269 *
270 * Returns 1 if child is a descendant of parent, 0 if not.
271 */
272static int task_is_descendant(struct task_struct *parent,
273			      struct task_struct *child)
274{
275	int rc = 0;
276	struct task_struct *walker = child;
277
278	if (!parent || !child)
279		return 0;
280
281	rcu_read_lock();
282	if (!thread_group_leader(parent))
283		parent = rcu_dereference(parent->group_leader);
284	while (walker->pid > 0) {
285		if (!thread_group_leader(walker))
286			walker = rcu_dereference(walker->group_leader);
287		if (walker == parent) {
288			rc = 1;
289			break;
290		}
291		walker = rcu_dereference(walker->real_parent);
292	}
293	rcu_read_unlock();
294
295	return rc;
296}
297
298/**
299 * ptracer_exception_found - tracer registered as exception for this tracee
300 * @tracer: the task_struct of the process attempting ptrace
301 * @tracee: the task_struct of the process to be ptraced
302 *
303 * Returns 1 if tracer has a ptracer exception ancestor for tracee.
304 */
305static int ptracer_exception_found(struct task_struct *tracer,
306				   struct task_struct *tracee)
307{
308	int rc = 0;
309	struct ptrace_relation *relation;
310	struct task_struct *parent = NULL;
311	bool found = false;
312
313	rcu_read_lock();
314
315	/*
316	 * If there's already an active tracing relationship, then make an
317	 * exception for the sake of other accesses, like process_vm_rw().
318	 */
319	parent = ptrace_parent(tracee);
320	if (parent != NULL && same_thread_group(parent, tracer)) {
321		rc = 1;
322		goto unlock;
323	}
324
325	/* Look for a PR_SET_PTRACER relationship. */
326	if (!thread_group_leader(tracee))
327		tracee = rcu_dereference(tracee->group_leader);
328	list_for_each_entry_rcu(relation, &ptracer_relations, node) {
329		if (relation->invalid)
330			continue;
331		if (relation->tracee == tracee) {
332			parent = relation->tracer;
333			found = true;
334			break;
335		}
336	}
337
338	if (found && (parent == NULL || task_is_descendant(parent, tracer)))
339		rc = 1;
340
341unlock:
342	rcu_read_unlock();
343
344	return rc;
345}
346
347/**
348 * yama_ptrace_access_check - validate PTRACE_ATTACH calls
349 * @child: task that current task is attempting to ptrace
350 * @mode: ptrace attach mode
351 *
352 * Returns 0 if following the ptrace is allowed, -ve on error.
353 */
354static int yama_ptrace_access_check(struct task_struct *child,
355				    unsigned int mode)
356{
357	int rc = 0;
358
359	/* require ptrace target be a child of ptracer on attach */
360	if (mode & PTRACE_MODE_ATTACH) {
361		switch (ptrace_scope) {
362		case YAMA_SCOPE_DISABLED:
363			/* No additional restrictions. */
364			break;
365		case YAMA_SCOPE_RELATIONAL:
366			rcu_read_lock();
367			if (!pid_alive(child))
368				rc = -EPERM;
369			if (!rc && !task_is_descendant(current, child) &&
370			    !ptracer_exception_found(current, child) &&
371			    !ns_capable(__task_cred(child)->user_ns, CAP_SYS_PTRACE))
372				rc = -EPERM;
373			rcu_read_unlock();
374			break;
375		case YAMA_SCOPE_CAPABILITY:
376			rcu_read_lock();
377			if (!ns_capable(__task_cred(child)->user_ns, CAP_SYS_PTRACE))
378				rc = -EPERM;
379			rcu_read_unlock();
380			break;
381		case YAMA_SCOPE_NO_ATTACH:
382		default:
383			rc = -EPERM;
384			break;
385		}
386	}
387
388	if (rc && (mode & PTRACE_MODE_NOAUDIT) == 0)
389		report_access("attach", child, current);
390
391	return rc;
392}
393
394/**
395 * yama_ptrace_traceme - validate PTRACE_TRACEME calls
396 * @parent: task that will become the ptracer of the current task
397 *
398 * Returns 0 if following the ptrace is allowed, -ve on error.
399 */
400static int yama_ptrace_traceme(struct task_struct *parent)
401{
402	int rc = 0;
403
404	/* Only disallow PTRACE_TRACEME on more aggressive settings. */
405	switch (ptrace_scope) {
406	case YAMA_SCOPE_CAPABILITY:
407		if (!has_ns_capability(parent, current_user_ns(), CAP_SYS_PTRACE))
408			rc = -EPERM;
409		break;
410	case YAMA_SCOPE_NO_ATTACH:
411		rc = -EPERM;
412		break;
413	}
414
415	if (rc) {
416		task_lock(current);
417		report_access("traceme", current, parent);
418		task_unlock(current);
419	}
420
421	return rc;
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(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 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	{ }
461};
462static void __init yama_init_sysctl(void)
463{
464	if (!register_sysctl("kernel/yama", yama_sysctl_table))
465		panic("Yama: sysctl registration failed.\n");
466}
467#else
468static inline void yama_init_sysctl(void) { }
469#endif /* CONFIG_SYSCTL */
470
471static int __init yama_init(void)
472{
473	pr_info("Yama: becoming mindful.\n");
474	security_add_hooks(yama_hooks, ARRAY_SIZE(yama_hooks), "yama");
475	yama_init_sysctl();
476	return 0;
477}
478
479DEFINE_LSM(yama) = {
480	.name = "yama",
481	.init = yama_init,
482};
483