yama: Better permission check for ptraceme
[linux-3.10.git] / security / yama / yama_lsm.c
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
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 /**
45  * yama_relation_cleanup - remove invalid entries from the relation list
46  *
47  */
48 static 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  */
74 static 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
101 out:
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  */
112 static 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  */
138 void 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  */
154 int 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  */
214 static 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  */
247 static 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  */
282 int 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) {
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  */
336 int 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
368 static 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
379 static int yama_dointvec_minmax(struct ctl_table *table, int write,
380                                 void __user *buffer, size_t *lenp, loff_t *ppos)
381 {
382         int rc;
383
384         if (write && !capable(CAP_SYS_PTRACE))
385                 return -EPERM;
386
387         rc = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
388         if (rc)
389                 return rc;
390
391         /* Lock the max value if it ever gets set. */
392         if (write && *(int *)table->data == *(int *)table->extra2)
393                 table->extra1 = table->extra2;
394
395         return rc;
396 }
397
398 static int zero;
399 static int max_scope = YAMA_SCOPE_NO_ATTACH;
400
401 struct ctl_path yama_sysctl_path[] = {
402         { .procname = "kernel", },
403         { .procname = "yama", },
404         { }
405 };
406
407 static struct ctl_table yama_sysctl_table[] = {
408         {
409                 .procname       = "ptrace_scope",
410                 .data           = &ptrace_scope,
411                 .maxlen         = sizeof(int),
412                 .mode           = 0644,
413                 .proc_handler   = yama_dointvec_minmax,
414                 .extra1         = &zero,
415                 .extra2         = &max_scope,
416         },
417         { }
418 };
419 #endif /* CONFIG_SYSCTL */
420
421 static __init int yama_init(void)
422 {
423 #ifndef CONFIG_SECURITY_YAMA_STACKED
424         if (!security_module_enable(&yama_ops))
425                 return 0;
426 #endif
427
428         printk(KERN_INFO "Yama: becoming mindful.\n");
429
430 #ifndef CONFIG_SECURITY_YAMA_STACKED
431         if (register_security(&yama_ops))
432                 panic("Yama: kernel registration failed.\n");
433 #endif
434
435 #ifdef CONFIG_SYSCTL
436         if (!register_sysctl_paths(yama_sysctl_path, yama_sysctl_table))
437                 panic("Yama: sysctl registration failed.\n");
438 #endif
439
440         return 0;
441 }
442
443 security_initcall(yama_init);