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