summaryrefslogtreecommitdiffstats
path: root/security/yama/yama_lsm.c
blob: 70cd85e3ba307121a135d4f05f033c45326f855f (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
/*
 * Yama Linux Security Module
 *
 * Author: Kees Cook <keescook@chromium.org>
 *
 * Copyright (C) 2010 Canonical, Ltd.
 * Copyright (C) 2011 The Chromium OS Authors.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2, as
 * published by the Free Software Foundation.
 *
 */

#include <linux/security.h>
#include <linux/sysctl.h>
#include <linux/ptrace.h>
#include <linux/prctl.h>
#include <linux/ratelimit.h>

#define YAMA_SCOPE_DISABLED	0
#define YAMA_SCOPE_RELATIONAL	1
#define YAMA_SCOPE_CAPABILITY	2
#define YAMA_SCOPE_NO_ATTACH	3

static int ptrace_scope = YAMA_SCOPE_RELATIONAL;

/* describe a ptrace relationship for potential exception */
struct ptrace_relation {
	struct task_struct *tracer;
	struct task_struct *tracee;
	struct list_head node;
	struct rcu_head rcu;
};

static LIST_HEAD(ptracer_relations);
static DEFINE_SPINLOCK(ptracer_relations_lock);

/**
 * yama_ptracer_add - add/replace an exception for this tracer/tracee pair
 * @tracer: the task_struct of the process doing the ptrace
 * @tracee: the task_struct of the process to be ptraced
 *
 * Each tracee can have, at most, one tracer registered. Each time this
 * is called, the prior registered tracer will be replaced for the tracee.
 *
 * Returns 0 if relationship was added, -ve on error.
 */
static int yama_ptracer_add(struct task_struct *tracer,
			    struct task_struct *tracee)
{
	struct ptrace_relation *relation, *added;

	added = kmalloc(sizeof(*added), GFP_KERNEL);
	if (!added)
		return -ENOMEM;

	added->tracee = tracee;
	added->tracer = tracer;

	spin_lock_bh(&ptracer_relations_lock);
	rcu_read_lock();
	list_for_each_entry_rcu(relation, &ptracer_relations, node) {
		if (relation->tracee == tracee) {
			list_replace_rcu(&relation->node, &added->node);
			kfree_rcu(relation, rcu);
			goto out;
		}
	}

	list_add_rcu(&added->node, &ptracer_relations);

out:
	rcu_read_unlock();
	spin_unlock_bh(&ptracer_relations_lock);
	return 0;
}

/**
 * yama_ptracer_del - remove exceptions related to the given tasks
 * @tracer: remove any relation where tracer task matches
 * @tracee: remove any relation where tracee task matches
 */
static void yama_ptracer_del(struct task_struct *tracer,
			     struct task_struct *tracee)
{
	struct ptrace_relation *relation;

	spin_lock_bh(&ptracer_relations_lock);
	rcu_read_lock();
	list_for_each_entry_rcu(relation, &ptracer_relations, node) {
		if (relation->tracee == tracee ||
		    (tracer && relation->tracer == tracer)) {
			list_del_rcu(&relation->node);
			kfree_rcu(relation, rcu);
		}
	}
	rcu_read_unlock();
	spin_unlock_bh(&ptracer_relations_lock);
}

/**
 * yama_task_free - check for task_pid to remove from exception list
 * @task: task being removed
 */
void yama_task_free(struct task_struct *task)
{
	yama_ptracer_del(task, task);
}

/**
 * yama_task_prctl - check for Yama-specific prctl operations
 * @option: operation
 * @arg2: argument
 * @arg3: argument
 * @arg4: argument
 * @arg5: argument
 *
 * Return 0 on success, -ve on error.  -ENOSYS is returned when Yama
 * does not handle the given option.
 */
int yama_task_prctl(int option, unsigned long arg2, unsigned long arg3,
			   unsigned long arg4, unsigned long arg5)
{
	int rc;
	struct task_struct *myself = current;

	rc = cap_task_prctl(option, arg2, arg3, arg4, arg5);
	if (rc != -ENOSYS)
		return rc;

	switch (option) {
	case PR_SET_PTRACER:
		/* Since a thread can call prctl(), find the group leader
		 * before calling _add() or _del() on it, since we want
		 * process-level granularity of control. The tracer group
		 * leader checking is handled later when walking the ancestry
		 * at the time of PTRACE_ATTACH check.
		 */
		rcu_read_lock();
		if (!thread_group_leader(myself))
			myself = rcu_dereference(myself->group_leader);
		get_task_struct(myself);
		rcu_read_unlock();

		if (arg2 == 0) {
			yama_ptracer_del(NULL, myself);
			rc = 0;
		} else if (arg2 == PR_SET_PTRACER_ANY || (int)arg2 == -1) {
			rc = yama_ptracer_add(NULL, myself);
		} else {
			struct task_struct *tracer;

			rcu_read_lock();
			tracer = find_task_by_vpid(arg2);
			if (tracer)
				get_task_struct(tracer);
			else
				rc = -EINVAL;
			rcu_read_unlock();

			if (tracer) {
				rc = yama_ptracer_add(tracer, myself);
				put_task_struct(tracer);
			}
		}

		put_task_struct(myself);
		break;
	}

	return rc;
}

/**
 * task_is_descendant - walk up a process family tree looking for a match
 * @parent: the process to compare against while walking up from child
 * @child: the process to start from while looking upwards for parent
 *
 * Returns 1 if child is a descendant of parent, 0 if not.
 */
static int task_is_descendant(struct task_struct *parent,
			      struct task_struct *child)
{
	int rc = 0;
	struct task_struct *walker = child;

	if (!parent || !child)
		return 0;

	rcu_read_lock();
	if (!thread_group_leader(parent))
		parent = rcu_dereference(parent->group_leader);
	while (walker->pid > 0) {
		if (!thread_group_leader(walker))
			walker = rcu_dereference(walker->group_leader);
		if (walker == parent) {
			rc = 1;
			break;
		}
		walker = rcu_dereference(walker->real_parent);
	}
	rcu_read_unlock();

	return rc;
}

/**
 * ptracer_exception_found - tracer registered as exception for this tracee
 * @tracer: the task_struct of the process attempting ptrace
 * @tracee: the task_struct of the process to be ptraced
 *
 * Returns 1 if tracer has is ptracer exception ancestor for tracee.
 */
static int ptracer_exception_found(struct task_struct *tracer,
				   struct task_struct *tracee)
{
	int rc = 0;
	struct ptrace_relation *relation;
	struct task_struct *parent = NULL;
	bool found = false;

	rcu_read_lock();
	if (!thread_group_leader(tracee))
		tracee = rcu_dereference(tracee->group_leader);
	list_for_each_entry_rcu(relation, &ptracer_relations, node)
		if (relation->tracee == tracee) {
			parent = relation->tracer;
			found = true;
			break;
		}

	if (found && (parent == NULL || task_is_descendant(parent, tracer)))
		rc = 1;
	rcu_read_unlock();

	return rc;
}

/**
 * yama_ptrace_access_check - validate PTRACE_ATTACH calls
 * @child: task that current task is attempting to ptrace
 * @mode: ptrace attach mode
 *
 * Returns 0 if following the ptrace is allowed, -ve on error.
 */
int yama_ptrace_access_check(struct task_struct *child,
				    unsigned int mode)
{
	int rc;

	/* If standard caps disallows it, so does Yama.  We should
	 * only tighten restrictions further.
	 */
	rc = cap_ptrace_access_check(child, mode);
	if (rc)
		return rc;

	/* require ptrace target be a child of ptracer on attach */
	if (mode == PTRACE_MODE_ATTACH) {
		switch (ptrace_scope) {
		case YAMA_SCOPE_DISABLED:
			/* No additional restrictions. */
			break;
		case YAMA_SCOPE_RELATIONAL:
			if (!task_is_descendant(current, child) &&
			    !ptracer_exception_found(current, child) &&
			    !ns_capable(task_user_ns(child), CAP_SYS_PTRACE))
				rc = -EPERM;
			break;
		case YAMA_SCOPE_CAPABILITY:
			if (!ns_capable(task_user_ns(child), CAP_SYS_PTRACE))
				rc = -EPERM;
			break;
		case YAMA_SCOPE_NO_ATTACH:
		default:
			rc = -EPERM;
			break;
		}
	}

	if (rc) {
		printk_ratelimited(KERN_NOTICE
			"ptrace of pid %d was attempted by: %s (pid %d)\n",
			child->pid, current->comm, current->pid);
	}

	return rc;
}

/**
 * yama_ptrace_traceme - validate PTRACE_TRACEME calls
 * @parent: task that will become the ptracer of the current task
 *
 * Returns 0 if following the ptrace is allowed, -ve on error.
 */
int yama_ptrace_traceme(struct task_struct *parent)
{
	int rc;

	/* If standard caps disallows it, so does Yama.  We should
	 * only tighten restrictions further.
	 */
	rc = cap_ptrace_traceme(parent);
	if (rc)
		return rc;

	/* Only disallow PTRACE_TRACEME on more aggressive settings. */
	switch (ptrace_scope) {
	case YAMA_SCOPE_CAPABILITY:
		if (!ns_capable(task_user_ns(parent), CAP_SYS_PTRACE))
			rc = -EPERM;
		break;
	case YAMA_SCOPE_NO_ATTACH:
		rc = -EPERM;
		break;
	}

	if (rc) {
		printk_ratelimited(KERN_NOTICE
			"ptraceme of pid %d was attempted by: %s (pid %d)\n",
			current->pid, parent->comm, parent->pid);
	}

	return rc;
}

#ifndef CONFIG_SECURITY_YAMA_STACKED
static struct security_operations yama_ops = {
	.name =			"yama",

	.ptrace_access_check =	yama_ptrace_access_check,
	.ptrace_traceme =	yama_ptrace_traceme,
	.task_prctl =		yama_task_prctl,
	.task_free =		yama_task_free,
};
#endif

#ifdef CONFIG_SYSCTL
static int yama_dointvec_minmax(struct ctl_table *table, int write,
				void __user *buffer, size_t *lenp, loff_t *ppos)
{
	int rc;

	if (write && !capable(CAP_SYS_PTRACE))
		return -EPERM;

	rc = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
	if (rc)
		return rc;

	/* Lock the max value if it ever gets set. */
	if (write && *(int *)table->data == *(int *)table->extra2)
		table->extra1 = table->extra2;

	return rc;
}

static int zero;
static int max_scope = YAMA_SCOPE_NO_ATTACH;

struct ctl_path yama_sysctl_path[] = {
	{ .procname = "kernel", },
	{ .procname = "yama", },
	{ }
};

static struct ctl_table yama_sysctl_table[] = {
	{
		.procname       = "ptrace_scope",
		.data           = &ptrace_scope,
		.maxlen         = sizeof(int),
		.mode           = 0644,
		.proc_handler   = yama_dointvec_minmax,
		.extra1         = &zero,
		.extra2         = &max_scope,
	},
	{ }
};
#endif /* CONFIG_SYSCTL */

static __init int yama_init(void)
{
#ifndef CONFIG_SECURITY_YAMA_STACKED
	if (!security_module_enable(&yama_ops))
		return 0;
#endif

	printk(KERN_INFO "Yama: becoming mindful.\n");

#ifndef CONFIG_SECURITY_YAMA_STACKED
	if (register_security(&yama_ops))
		panic("Yama: kernel registration failed.\n");
#endif

#ifdef CONFIG_SYSCTL
	if (!register_sysctl_paths(yama_sysctl_path, yama_sysctl_table))
		panic("Yama: sysctl registration failed.\n");
#endif

	return 0;
}

security_initcall(yama_init);