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
28 static int ptrace_scope = YAMA_SCOPE_RELATIONAL;
29
30 /* describe a ptrace relationship for potential exception */
31 struct 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
39 static LIST_HEAD(ptracer_relations);
40 static DEFINE_SPINLOCK(ptracer_relations_lock);
41
42 static void yama_relation_cleanup(struct work_struct *work);
43 static DECLARE_WORK(yama_relation_work, yama_relation_cleanup);
44
45 struct access_report_info {
46 struct callback_head work;
47 const char *access;
48 struct task_struct *target;
49 struct task_struct *agent;
50 };
51
__report_access(struct callback_head * work)52 static 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 */
report_access(const char * access,struct task_struct * target,struct task_struct * agent)75 static 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 */
yama_relation_cleanup(struct work_struct * work)116 static 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 */
yama_ptracer_add(struct task_struct * tracer,struct task_struct * tracee)142 static 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
169 out:
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 */
yama_ptracer_del(struct task_struct * tracer,struct task_struct * tracee)180 static 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 */
yama_task_free(struct task_struct * task)206 static 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 */
yama_task_prctl(int option,unsigned long arg2,unsigned long arg3,unsigned long arg4,unsigned long arg5)222 static 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 */
task_is_descendant(struct task_struct * parent,struct task_struct * child)273 static 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 */
ptracer_exception_found(struct task_struct * tracer,struct task_struct * tracee)306 static 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
342 unlock:
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 */
yama_ptrace_access_check(struct task_struct * child,unsigned int mode)355 static 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 */
yama_ptrace_traceme(struct task_struct * parent)401 static 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
425 static const struct lsm_id yama_lsmid = {
426 .name = "yama",
427 .id = LSM_ID_YAMA,
428 };
429
430 static 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
yama_dointvec_minmax(struct ctl_table * table,int write,void * buffer,size_t * lenp,loff_t * ppos)438 static 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
454 static int max_scope = YAMA_SCOPE_NO_ATTACH;
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("kernel/yama", 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_lsmid);
481 yama_init_sysctl();
482 return 0;
483 }
484
485 DEFINE_LSM(yama) = {
486 .name = "yama",
487 .init = yama_init,
488 };
489