1/*
2 * Yama Linux Security Module
3 *
4 * Author: Kees Cook <keescook@chromium.org>
5 *
6 * Copyright (C) 2010 Canonical, Ltd.
7 * Copyright (C) 2011 The Chromium OS Authors.
8 *
9 * This program is free software; you can redistribute it and/or modify
10 * it under the terms of the GNU General Public License version 2, as
11 * published by the Free Software Foundation.
12 *
13 */
14
15#include <linux/security.h>
16#include <linux/sysctl.h>
17#include <linux/ptrace.h>
18#include <linux/prctl.h>
19#include <linux/ratelimit.h>
20#include <linux/workqueue.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
44/**
45 * yama_relation_cleanup - remove invalid entries from the relation list
46 *
47 */
48static void yama_relation_cleanup(struct work_struct *work)
49{
50	struct ptrace_relation *relation;
51
52	spin_lock(&ptracer_relations_lock);
53	rcu_read_lock();
54	list_for_each_entry_rcu(relation, &ptracer_relations, node) {
55		if (relation->invalid) {
56			list_del_rcu(&relation->node);
57			kfree_rcu(relation, rcu);
58		}
59	}
60	rcu_read_unlock();
61	spin_unlock(&ptracer_relations_lock);
62}
63
64/**
65 * yama_ptracer_add - add/replace an exception for this tracer/tracee pair
66 * @tracer: the task_struct of the process doing the ptrace
67 * @tracee: the task_struct of the process to be ptraced
68 *
69 * Each tracee can have, at most, one tracer registered. Each time this
70 * is called, the prior registered tracer will be replaced for the tracee.
71 *
72 * Returns 0 if relationship was added, -ve on error.
73 */
74static int yama_ptracer_add(struct task_struct *tracer,
75			    struct task_struct *tracee)
76{
77	struct ptrace_relation *relation, *added;
78
79	added = kmalloc(sizeof(*added), GFP_KERNEL);
80	if (!added)
81		return -ENOMEM;
82
83	added->tracee = tracee;
84	added->tracer = tracer;
85	added->invalid = false;
86
87	spin_lock(&ptracer_relations_lock);
88	rcu_read_lock();
89	list_for_each_entry_rcu(relation, &ptracer_relations, node) {
90		if (relation->invalid)
91			continue;
92		if (relation->tracee == tracee) {
93			list_replace_rcu(&relation->node, &added->node);
94			kfree_rcu(relation, rcu);
95			goto out;
96		}
97	}
98
99	list_add_rcu(&added->node, &ptracer_relations);
100
101out:
102	rcu_read_unlock();
103	spin_unlock(&ptracer_relations_lock);
104	return 0;
105}
106
107/**
108 * yama_ptracer_del - remove exceptions related to the given tasks
109 * @tracer: remove any relation where tracer task matches
110 * @tracee: remove any relation where tracee task matches
111 */
112static void yama_ptracer_del(struct task_struct *tracer,
113			     struct task_struct *tracee)
114{
115	struct ptrace_relation *relation;
116	bool marked = false;
117
118	rcu_read_lock();
119	list_for_each_entry_rcu(relation, &ptracer_relations, node) {
120		if (relation->invalid)
121			continue;
122		if (relation->tracee == tracee ||
123		    (tracer && relation->tracer == tracer)) {
124			relation->invalid = true;
125			marked = true;
126		}
127	}
128	rcu_read_unlock();
129
130	if (marked)
131		schedule_work(&yama_relation_work);
132}
133
134/**
135 * yama_task_free - check for task_pid to remove from exception list
136 * @task: task being removed
137 */
138void yama_task_free(struct task_struct *task)
139{
140	yama_ptracer_del(task, task);
141}
142
143/**
144 * yama_task_prctl - check for Yama-specific prctl operations
145 * @option: operation
146 * @arg2: argument
147 * @arg3: argument
148 * @arg4: argument
149 * @arg5: argument
150 *
151 * Return 0 on success, -ve on error.  -ENOSYS is returned when Yama
152 * does not handle the given option.
153 */
154int yama_task_prctl(int option, unsigned long arg2, unsigned long arg3,
155			   unsigned long arg4, unsigned long arg5)
156{
157	int rc;
158	struct task_struct *myself = current;
159
160	rc = cap_task_prctl(option, arg2, arg3, arg4, arg5);
161	if (rc != -ENOSYS)
162		return rc;
163
164	switch (option) {
165	case PR_SET_PTRACER:
166		/* Since a thread can call prctl(), find the group leader
167		 * before calling _add() or _del() on it, since we want
168		 * process-level granularity of control. The tracer group
169		 * leader checking is handled later when walking the ancestry
170		 * at the time of PTRACE_ATTACH check.
171		 */
172		rcu_read_lock();
173		if (!thread_group_leader(myself))
174			myself = rcu_dereference(myself->group_leader);
175		get_task_struct(myself);
176		rcu_read_unlock();
177
178		if (arg2 == 0) {
179			yama_ptracer_del(NULL, myself);
180			rc = 0;
181		} else if (arg2 == PR_SET_PTRACER_ANY || (int)arg2 == -1) {
182			rc = yama_ptracer_add(NULL, myself);
183		} else {
184			struct task_struct *tracer;
185
186			rcu_read_lock();
187			tracer = find_task_by_vpid(arg2);
188			if (tracer)
189				get_task_struct(tracer);
190			else
191				rc = -EINVAL;
192			rcu_read_unlock();
193
194			if (tracer) {
195				rc = yama_ptracer_add(tracer, myself);
196				put_task_struct(tracer);
197			}
198		}
199
200		put_task_struct(myself);
201		break;
202	}
203
204	return rc;
205}
206
207/**
208 * task_is_descendant - walk up a process family tree looking for a match
209 * @parent: the process to compare against while walking up from child
210 * @child: the process to start from while looking upwards for parent
211 *
212 * Returns 1 if child is a descendant of parent, 0 if not.
213 */
214static int task_is_descendant(struct task_struct *parent,
215			      struct task_struct *child)
216{
217	int rc = 0;
218	struct task_struct *walker = child;
219
220	if (!parent || !child)
221		return 0;
222
223	rcu_read_lock();
224	if (!thread_group_leader(parent))
225		parent = rcu_dereference(parent->group_leader);
226	while (walker->pid > 0) {
227		if (!thread_group_leader(walker))
228			walker = rcu_dereference(walker->group_leader);
229		if (walker == parent) {
230			rc = 1;
231			break;
232		}
233		walker = rcu_dereference(walker->real_parent);
234	}
235	rcu_read_unlock();
236
237	return rc;
238}
239
240/**
241 * ptracer_exception_found - tracer registered as exception for this tracee
242 * @tracer: the task_struct of the process attempting ptrace
243 * @tracee: the task_struct of the process to be ptraced
244 *
245 * Returns 1 if tracer has is ptracer exception ancestor for tracee.
246 */
247static int ptracer_exception_found(struct task_struct *tracer,
248				   struct task_struct *tracee)
249{
250	int rc = 0;
251	struct ptrace_relation *relation;
252	struct task_struct *parent = NULL;
253	bool found = false;
254
255	rcu_read_lock();
256	if (!thread_group_leader(tracee))
257		tracee = rcu_dereference(tracee->group_leader);
258	list_for_each_entry_rcu(relation, &ptracer_relations, node) {
259		if (relation->invalid)
260			continue;
261		if (relation->tracee == tracee) {
262			parent = relation->tracer;
263			found = true;
264			break;
265		}
266	}
267
268	if (found && (parent == NULL || task_is_descendant(parent, tracer)))
269		rc = 1;
270	rcu_read_unlock();
271
272	return rc;
273}
274
275/**
276 * yama_ptrace_access_check - validate PTRACE_ATTACH calls
277 * @child: task that current task is attempting to ptrace
278 * @mode: ptrace attach mode
279 *
280 * Returns 0 if following the ptrace is allowed, -ve on error.
281 */
282int yama_ptrace_access_check(struct task_struct *child,
283				    unsigned int mode)
284{
285	int rc;
286
287	/* If standard caps disallows it, so does Yama.  We should
288	 * only tighten restrictions further.
289	 */
290	rc = cap_ptrace_access_check(child, mode);
291	if (rc)
292		return rc;
293
294	/* require ptrace target be a child of ptracer on attach */
295	if (mode & PTRACE_MODE_ATTACH) {
296		switch (ptrace_scope) {
297		case YAMA_SCOPE_DISABLED:
298			/* No additional restrictions. */
299			break;
300		case YAMA_SCOPE_RELATIONAL:
301			rcu_read_lock();
302			if (!task_is_descendant(current, child) &&
303			    !ptracer_exception_found(current, child) &&
304			    !ns_capable(__task_cred(child)->user_ns, CAP_SYS_PTRACE))
305				rc = -EPERM;
306			rcu_read_unlock();
307			break;
308		case YAMA_SCOPE_CAPABILITY:
309			rcu_read_lock();
310			if (!ns_capable(__task_cred(child)->user_ns, CAP_SYS_PTRACE))
311				rc = -EPERM;
312			rcu_read_unlock();
313			break;
314		case YAMA_SCOPE_NO_ATTACH:
315		default:
316			rc = -EPERM;
317			break;
318		}
319	}
320
321	if (rc && (mode & PTRACE_MODE_NOAUDIT) == 0) {
322		printk_ratelimited(KERN_NOTICE
323			"ptrace of pid %d was attempted by: %s (pid %d)\n",
324			child->pid, current->comm, current->pid);
325	}
326
327	return rc;
328}
329
330/**
331 * yama_ptrace_traceme - validate PTRACE_TRACEME calls
332 * @parent: task that will become the ptracer of the current task
333 *
334 * Returns 0 if following the ptrace is allowed, -ve on error.
335 */
336int yama_ptrace_traceme(struct task_struct *parent)
337{
338	int rc;
339
340	/* If standard caps disallows it, so does Yama.  We should
341	 * only tighten restrictions further.
342	 */
343	rc = cap_ptrace_traceme(parent);
344	if (rc)
345		return rc;
346
347	/* Only disallow PTRACE_TRACEME on more aggressive settings. */
348	switch (ptrace_scope) {
349	case YAMA_SCOPE_CAPABILITY:
350		if (!has_ns_capability(parent, current_user_ns(), CAP_SYS_PTRACE))
351			rc = -EPERM;
352		break;
353	case YAMA_SCOPE_NO_ATTACH:
354		rc = -EPERM;
355		break;
356	}
357
358	if (rc) {
359		printk_ratelimited(KERN_NOTICE
360			"ptraceme of pid %d was attempted by: %s (pid %d)\n",
361			current->pid, parent->comm, parent->pid);
362	}
363
364	return rc;
365}
366
367#ifndef CONFIG_SECURITY_YAMA_STACKED
368static struct security_operations yama_ops = {
369	.name =			"yama",
370
371	.ptrace_access_check =	yama_ptrace_access_check,
372	.ptrace_traceme =	yama_ptrace_traceme,
373	.task_prctl =		yama_task_prctl,
374	.task_free =		yama_task_free,
375};
376#endif
377
378#ifdef CONFIG_SYSCTL
379static int yama_dointvec_minmax(struct ctl_table *table, int write,
380				void __user *buffer, size_t *lenp, loff_t *ppos)
381{
382	struct ctl_table table_copy;
383
384	if (write && !capable(CAP_SYS_PTRACE))
385		return -EPERM;
386
387	/* Lock the max value if it ever gets set. */
388	table_copy = *table;
389	if (*(int *)table_copy.data == *(int *)table_copy.extra2)
390		table_copy.extra1 = table_copy.extra2;
391
392	return proc_dointvec_minmax(&table_copy, write, buffer, lenp, ppos);
393}
394
395static int zero;
396static int max_scope = YAMA_SCOPE_NO_ATTACH;
397
398struct ctl_path yama_sysctl_path[] = {
399	{ .procname = "kernel", },
400	{ .procname = "yama", },
401	{ }
402};
403
404static struct ctl_table yama_sysctl_table[] = {
405	{
406		.procname       = "ptrace_scope",
407		.data           = &ptrace_scope,
408		.maxlen         = sizeof(int),
409		.mode           = 0644,
410		.proc_handler   = yama_dointvec_minmax,
411		.extra1         = &zero,
412		.extra2         = &max_scope,
413	},
414	{ }
415};
416#endif /* CONFIG_SYSCTL */
417
418static __init int yama_init(void)
419{
420#ifndef CONFIG_SECURITY_YAMA_STACKED
421	if (!security_module_enable(&yama_ops))
422		return 0;
423#endif
424
425	printk(KERN_INFO "Yama: becoming mindful.\n");
426
427#ifndef CONFIG_SECURITY_YAMA_STACKED
428	if (register_security(&yama_ops))
429		panic("Yama: kernel registration failed.\n");
430#endif
431
432#ifdef CONFIG_SYSCTL
433	if (!register_sysctl_paths(yama_sysctl_path, yama_sysctl_table))
434		panic("Yama: sysctl registration failed.\n");
435#endif
436
437	return 0;
438}
439
440security_initcall(yama_init);
441