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
27 static int ptrace_scope = YAMA_SCOPE_RELATIONAL;
28
29 /* describe a ptrace relationship for potential exception */
30 struct 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
38 static LIST_HEAD(ptracer_relations);
39 static DEFINE_SPINLOCK(ptracer_relations_lock);
40
41 static void yama_relation_cleanup(struct work_struct *work);
42 static DECLARE_WORK(yama_relation_work, yama_relation_cleanup);
43
44 struct access_report_info {
45 struct callback_head work;
46 const char *access;
47 struct task_struct *target;
48 struct task_struct *agent;
49 };
50
__report_access(struct callback_head * work)51 static 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 */
report_access(const char * access,struct task_struct * target,struct task_struct * agent)74 static 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, true) == 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 */
yama_relation_cleanup(struct work_struct * work)115 static 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 */
yama_ptracer_add(struct task_struct * tracer,struct task_struct * tracee)141 static 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
168 out:
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 */
yama_ptracer_del(struct task_struct * tracer,struct task_struct * tracee)179 static 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 */
yama_task_free(struct task_struct * task)205 static 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 */
yama_task_prctl(int option,unsigned long arg2,unsigned long arg3,unsigned long arg4,unsigned long arg5)221 static 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 */
task_is_descendant(struct task_struct * parent,struct task_struct * child)272 static 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 */
ptracer_exception_found(struct task_struct * tracer,struct task_struct * tracee)305 static 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
341 unlock:
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 */
yama_ptrace_access_check(struct task_struct * child,unsigned int mode)354 static 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 */
yama_ptrace_traceme(struct task_struct * parent)400 static 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
424 static struct security_hook_list yama_hooks[] __lsm_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
yama_dointvec_minmax(struct ctl_table * table,int write,void __user * buffer,size_t * lenp,loff_t * ppos)432 static int yama_dointvec_minmax(struct ctl_table *table, int write,
433 void __user *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
448 static int max_scope = YAMA_SCOPE_NO_ATTACH;
449
450 static struct ctl_path yama_sysctl_path[] = {
451 { .procname = "kernel", },
452 { .procname = "yama", },
453 { }
454 };
455
456 static 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 };
yama_init_sysctl(void)468 static void __init yama_init_sysctl(void)
469 {
470 if (!register_sysctl_paths(yama_sysctl_path, yama_sysctl_table))
471 panic("Yama: sysctl registration failed.\n");
472 }
473 #else
yama_init_sysctl(void)474 static inline void yama_init_sysctl(void) { }
475 #endif /* CONFIG_SYSCTL */
476
yama_init(void)477 static int __init yama_init(void)
478 {
479 pr_info("Yama: becoming mindful.\n");
480 security_add_hooks(yama_hooks, ARRAY_SIZE(yama_hooks), "yama");
481 yama_init_sysctl();
482 return 0;
483 }
484
485 DEFINE_LSM(yama) = {
486 .name = "yama",
487 .init = yama_init,
488 };
489