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	char agent_comm[sizeof(agent->comm)];
80
81	assert_spin_locked(&target->alloc_lock); /* for target->comm */
82
83	if (current->flags & PF_KTHREAD) {
84		/* I don't think kthreads call task_work_run() before exiting.
85		 * Imagine angry ranting about procfs here.
86		 */
87		pr_notice_ratelimited(
88		    "ptrace %s of \"%s\"[%d] was attempted by \"%s\"[%d]\n",
89		    access, target->comm, target->pid,
90		    get_task_comm(agent_comm, agent), agent->pid);
91		return;
92	}
93
94	info = kmalloc(sizeof(*info), GFP_ATOMIC);
95	if (!info)
96		return;
97	init_task_work(&info->work, __report_access);
98	get_task_struct(target);
99	get_task_struct(agent);
100	info->access = access;
101	info->target = target;
102	info->agent = agent;
103	if (task_work_add(current, &info->work, TWA_RESUME) == 0)
104		return; /* success */
105
106	WARN(1, "report_access called from exiting task");
107	put_task_struct(target);
108	put_task_struct(agent);
109	kfree(info);
110}
111
112/**
113 * yama_relation_cleanup - remove invalid entries from the relation list
114 *
115 */
116static void yama_relation_cleanup(struct work_struct *work)
117{
118	struct ptrace_relation *relation;
119
120	spin_lock(&ptracer_relations_lock);
121	rcu_read_lock();
122	list_for_each_entry_rcu(relation, &ptracer_relations, node) {
123		if (relation->invalid) {
124			list_del_rcu(&relation->node);
125			kfree_rcu(relation, rcu);
126		}
127	}
128	rcu_read_unlock();
129	spin_unlock(&ptracer_relations_lock);
130}
131
132/**
133 * yama_ptracer_add - add/replace an exception for this tracer/tracee pair
134 * @tracer: the task_struct of the process doing the ptrace
135 * @tracee: the task_struct of the process to be ptraced
136 *
137 * Each tracee can have, at most, one tracer registered. Each time this
138 * is called, the prior registered tracer will be replaced for the tracee.
139 *
140 * Returns 0 if relationship was added, -ve on error.
141 */
142static int yama_ptracer_add(struct task_struct *tracer,
143			    struct task_struct *tracee)
144{
145	struct ptrace_relation *relation, *added;
146
147	added = kmalloc(sizeof(*added), GFP_KERNEL);
148	if (!added)
149		return -ENOMEM;
150
151	added->tracee = tracee;
152	added->tracer = tracer;
153	added->invalid = false;
154
155	spin_lock(&ptracer_relations_lock);
156	rcu_read_lock();
157	list_for_each_entry_rcu(relation, &ptracer_relations, node) {
158		if (relation->invalid)
159			continue;
160		if (relation->tracee == tracee) {
161			list_replace_rcu(&relation->node, &added->node);
162			kfree_rcu(relation, rcu);
163			goto out;
164		}
165	}
166
167	list_add_rcu(&added->node, &ptracer_relations);
168
169out:
170	rcu_read_unlock();
171	spin_unlock(&ptracer_relations_lock);
172	return 0;
173}
174
175/**
176 * yama_ptracer_del - remove exceptions related to the given tasks
177 * @tracer: remove any relation where tracer task matches
178 * @tracee: remove any relation where tracee task matches
179 */
180static void yama_ptracer_del(struct task_struct *tracer,
181			     struct task_struct *tracee)
182{
183	struct ptrace_relation *relation;
184	bool marked = false;
185
186	rcu_read_lock();
187	list_for_each_entry_rcu(relation, &ptracer_relations, node) {
188		if (relation->invalid)
189			continue;
190		if (relation->tracee == tracee ||
191		    (tracer && relation->tracer == tracer)) {
192			relation->invalid = true;
193			marked = true;
194		}
195	}
196	rcu_read_unlock();
197
198	if (marked)
199		schedule_work(&yama_relation_work);
200}
201
202/**
203 * yama_task_free - check for task_pid to remove from exception list
204 * @task: task being removed
205 */
206static void yama_task_free(struct task_struct *task)
207{
208	yama_ptracer_del(task, task);
209}
210
211/**
212 * yama_task_prctl - check for Yama-specific prctl operations
213 * @option: operation
214 * @arg2: argument
215 * @arg3: argument
216 * @arg4: argument
217 * @arg5: argument
218 *
219 * Return 0 on success, -ve on error.  -ENOSYS is returned when Yama
220 * does not handle the given option.
221 */
222static int yama_task_prctl(int option, unsigned long arg2, unsigned long arg3,
223			   unsigned long arg4, unsigned long arg5)
224{
225	int rc = -ENOSYS;
226	struct task_struct *myself = current;
227
228	switch (option) {
229	case PR_SET_PTRACER:
230		/* Since a thread can call prctl(), find the group leader
231		 * before calling _add() or _del() on it, since we want
232		 * process-level granularity of control. The tracer group
233		 * leader checking is handled later when walking the ancestry
234		 * at the time of PTRACE_ATTACH check.
235		 */
236		rcu_read_lock();
237		if (!thread_group_leader(myself))
238			myself = rcu_dereference(myself->group_leader);
239		get_task_struct(myself);
240		rcu_read_unlock();
241
242		if (arg2 == 0) {
243			yama_ptracer_del(NULL, myself);
244			rc = 0;
245		} else if (arg2 == PR_SET_PTRACER_ANY || (int)arg2 == -1) {
246			rc = yama_ptracer_add(NULL, myself);
247		} else {
248			struct task_struct *tracer;
249
250			tracer = find_get_task_by_vpid(arg2);
251			if (!tracer) {
252				rc = -EINVAL;
253			} else {
254				rc = yama_ptracer_add(tracer, myself);
255				put_task_struct(tracer);
256			}
257		}
258
259		put_task_struct(myself);
260		break;
261	}
262
263	return rc;
264}
265
266/**
267 * task_is_descendant - walk up a process family tree looking for a match
268 * @parent: the process to compare against while walking up from child
269 * @child: the process to start from while looking upwards for parent
270 *
271 * Returns 1 if child is a descendant of parent, 0 if not.
272 */
273static int task_is_descendant(struct task_struct *parent,
274			      struct task_struct *child)
275{
276	int rc = 0;
277	struct task_struct *walker = child;
278
279	if (!parent || !child)
280		return 0;
281
282	rcu_read_lock();
283	if (!thread_group_leader(parent))
284		parent = rcu_dereference(parent->group_leader);
285	while (walker->pid > 0) {
286		if (!thread_group_leader(walker))
287			walker = rcu_dereference(walker->group_leader);
288		if (walker == parent) {
289			rc = 1;
290			break;
291		}
292		walker = rcu_dereference(walker->real_parent);
293	}
294	rcu_read_unlock();
295
296	return rc;
297}
298
299/**
300 * ptracer_exception_found - tracer registered as exception for this tracee
301 * @tracer: the task_struct of the process attempting ptrace
302 * @tracee: the task_struct of the process to be ptraced
303 *
304 * Returns 1 if tracer has a ptracer exception ancestor for tracee.
305 */
306static int ptracer_exception_found(struct task_struct *tracer,
307				   struct task_struct *tracee)
308{
309	int rc = 0;
310	struct ptrace_relation *relation;
311	struct task_struct *parent = NULL;
312	bool found = false;
313
314	rcu_read_lock();
315
316	/*
317	 * If there's already an active tracing relationship, then make an
318	 * exception for the sake of other accesses, like process_vm_rw().
319	 */
320	parent = ptrace_parent(tracee);
321	if (parent != NULL && same_thread_group(parent, tracer)) {
322		rc = 1;
323		goto unlock;
324	}
325
326	/* Look for a PR_SET_PTRACER relationship. */
327	if (!thread_group_leader(tracee))
328		tracee = rcu_dereference(tracee->group_leader);
329	list_for_each_entry_rcu(relation, &ptracer_relations, node) {
330		if (relation->invalid)
331			continue;
332		if (relation->tracee == tracee) {
333			parent = relation->tracer;
334			found = true;
335			break;
336		}
337	}
338
339	if (found && (parent == NULL || task_is_descendant(parent, tracer)))
340		rc = 1;
341
342unlock:
343	rcu_read_unlock();
344
345	return rc;
346}
347
348/**
349 * yama_ptrace_access_check - validate PTRACE_ATTACH calls
350 * @child: task that current task is attempting to ptrace
351 * @mode: ptrace attach mode
352 *
353 * Returns 0 if following the ptrace is allowed, -ve on error.
354 */
355static int yama_ptrace_access_check(struct task_struct *child,
356				    unsigned int mode)
357{
358	int rc = 0;
359
360	/* require ptrace target be a child of ptracer on attach */
361	if (mode & PTRACE_MODE_ATTACH) {
362		switch (ptrace_scope) {
363		case YAMA_SCOPE_DISABLED:
364			/* No additional restrictions. */
365			break;
366		case YAMA_SCOPE_RELATIONAL:
367			rcu_read_lock();
368			if (!pid_alive(child))
369				rc = -EPERM;
370			if (!rc && !task_is_descendant(current, child) &&
371			    !ptracer_exception_found(current, child) &&
372			    !ns_capable(__task_cred(child)->user_ns, CAP_SYS_PTRACE))
373				rc = -EPERM;
374			rcu_read_unlock();
375			break;
376		case YAMA_SCOPE_CAPABILITY:
377			rcu_read_lock();
378			if (!ns_capable(__task_cred(child)->user_ns, CAP_SYS_PTRACE))
379				rc = -EPERM;
380			rcu_read_unlock();
381			break;
382		case YAMA_SCOPE_NO_ATTACH:
383		default:
384			rc = -EPERM;
385			break;
386		}
387	}
388
389	if (rc && (mode & PTRACE_MODE_NOAUDIT) == 0)
390		report_access("attach", child, current);
391
392	return rc;
393}
394
395/**
396 * yama_ptrace_traceme - validate PTRACE_TRACEME calls
397 * @parent: task that will become the ptracer of the current task
398 *
399 * Returns 0 if following the ptrace is allowed, -ve on error.
400 */
401static int yama_ptrace_traceme(struct task_struct *parent)
402{
403	int rc = 0;
404
405	/* Only disallow PTRACE_TRACEME on more aggressive settings. */
406	switch (ptrace_scope) {
407	case YAMA_SCOPE_CAPABILITY:
408		if (!has_ns_capability(parent, current_user_ns(), CAP_SYS_PTRACE))
409			rc = -EPERM;
410		break;
411	case YAMA_SCOPE_NO_ATTACH:
412		rc = -EPERM;
413		break;
414	}
415
416	if (rc) {
417		task_lock(current);
418		report_access("traceme", current, parent);
419		task_unlock(current);
420	}
421
422	return rc;
423}
424
425static const struct lsm_id yama_lsmid = {
426	.name = "yama",
427	.id = LSM_ID_YAMA,
428};
429
430static struct security_hook_list yama_hooks[] __ro_after_init = {
431	LSM_HOOK_INIT(ptrace_access_check, yama_ptrace_access_check),
432	LSM_HOOK_INIT(ptrace_traceme, yama_ptrace_traceme),
433	LSM_HOOK_INIT(task_prctl, yama_task_prctl),
434	LSM_HOOK_INIT(task_free, yama_task_free),
435};
436
437#ifdef CONFIG_SYSCTL
438static int yama_dointvec_minmax(struct ctl_table *table, int write,
439				void *buffer, size_t *lenp, loff_t *ppos)
440{
441	struct ctl_table table_copy;
442
443	if (write && !capable(CAP_SYS_PTRACE))
444		return -EPERM;
445
446	/* Lock the max value if it ever gets set. */
447	table_copy = *table;
448	if (*(int *)table_copy.data == *(int *)table_copy.extra2)
449		table_copy.extra1 = table_copy.extra2;
450
451	return proc_dointvec_minmax(&table_copy, write, buffer, lenp, ppos);
452}
453
454static int max_scope = YAMA_SCOPE_NO_ATTACH;
455
456static struct ctl_table yama_sysctl_table[] = {
457	{
458		.procname       = "ptrace_scope",
459		.data           = &ptrace_scope,
460		.maxlen         = sizeof(int),
461		.mode           = 0644,
462		.proc_handler   = yama_dointvec_minmax,
463		.extra1         = SYSCTL_ZERO,
464		.extra2         = &max_scope,
465	},
466	{ }
467};
468static void __init yama_init_sysctl(void)
469{
470	if (!register_sysctl("kernel/yama", yama_sysctl_table))
471		panic("Yama: sysctl registration failed.\n");
472}
473#else
474static inline void yama_init_sysctl(void) { }
475#endif /* CONFIG_SYSCTL */
476
477static int __init yama_init(void)
478{
479	pr_info("Yama: becoming mindful.\n");
480	security_add_hooks(yama_hooks, ARRAY_SIZE(yama_hooks), &yama_lsmid);
481	yama_init_sysctl();
482	return 0;
483}
484
485DEFINE_LSM(yama) = {
486	.name = "yama",
487	.init = yama_init,
488};
489