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/lsm_hooks.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 #include <linux/string_helpers.h>
  22 #include <linux/task_work.h>
  23 #include <linux/sched.h>
  24 #include <linux/spinlock.h>
  25
  26 #define YAMA_SCOPE_DISABLED     0
  27 #define YAMA_SCOPE_RELATIONAL   1
  28 #define YAMA_SCOPE_CAPABILITY   2
  29 #define YAMA_SCOPE_NO_ATTACH    3
  30
  31 static int ptrace_scope = YAMA_SCOPE_RELATIONAL;
  32
  33 /* describe a ptrace relationship for potential exception */
  34 struct ptrace_relation {
  35         struct task_struct *tracer;
  36         struct task_struct *tracee;
  37         bool invalid;
  38         struct list_head node;
  39         struct rcu_head rcu;
  40 };
  41
  42 static LIST_HEAD(ptracer_relations);
  43 static DEFINE_SPINLOCK(ptracer_relations_lock);
  44
  45 static void yama_relation_cleanup(struct work_struct *work);
  46 static DECLARE_WORK(yama_relation_work, yama_relation_cleanup);
  47
  48 struct access_report_info {
  49         struct callback_head work;
  50         const char *access;
  51         struct task_struct *target;
  52         struct task_struct *agent;
  53 };
  54
  55 static void __report_access(struct callback_head *work)
  56 {
  57         struct access_report_info *info =
  58                 container_of(work, struct access_report_info, work);
  59         char *target_cmd, *agent_cmd;
  60
  61         target_cmd = kstrdup_quotable_cmdline(info->target, GFP_KERNEL);
  62         agent_cmd = kstrdup_quotable_cmdline(info->agent, GFP_KERNEL);
  63
  64         pr_notice_ratelimited(
  65                 "ptrace %s of \"%s\"[%d] was attempted by \"%s\"[%d]\n",
  66                 info->access, target_cmd, info->target->pid, agent_cmd,
  67                 info->agent->pid);
  68
  69         kfree(agent_cmd);
  70         kfree(target_cmd);
  71
  72         put_task_struct(info->agent);
  73         put_task_struct(info->target);
  74         kfree(info);
  75 }
  76
  77 /* defers execution because cmdline access can sleep */
  78 static void report_access(const char *access, struct task_struct *target,
  79                                 struct task_struct *agent)
  80 {
  81         struct access_report_info *info;
  82         char agent_comm[sizeof(agent->comm)];
  83
  84         assert_spin_locked(&target->alloc_lock); /* for target->comm */
  85
  86         if (current->flags & PF_KTHREAD) {
  87                 /* I don't think kthreads call task_work_run() before exiting.
  88                  * Imagine angry ranting about procfs here.
  89                  */
  90                 pr_notice_ratelimited(
  91                     "ptrace %s of \"%s\"[%d] was attempted by \"%s\"[%d]\n",
  92                     access, target->comm, target->pid,
  93                     get_task_comm(agent_comm, agent), agent->pid);
  94                 return;
  95         }
  96
  97         info = kmalloc(sizeof(*info), GFP_ATOMIC);
  98         if (!info)
  99                 return;
 100         init_task_work(&info->work, __report_access);
 101         get_task_struct(target);
 102         get_task_struct(agent);
 103         info->access = access;
 104         info->target = target;
 105         info->agent = agent;
 106         if (task_work_add(current, &info->work, true) == 0)
 107                 return; /* success */
 108
 109         WARN(1, "report_access called from exiting task");
 110         put_task_struct(target);
 111         put_task_struct(agent);
 112         kfree(info);
 113 }
 114
 115 /**
 116  * yama_relation_cleanup - remove invalid entries from the relation list
 117  *
 118  */
 119 static void yama_relation_cleanup(struct work_struct *work)
 120 {
 121         struct ptrace_relation *relation;
 122
 123         spin_lock(&ptracer_relations_lock);
 124         rcu_read_lock();
 125         list_for_each_entry_rcu(relation, &ptracer_relations, node) {
 126                 if (relation->invalid) {
 127                         list_del_rcu(&relation->node);
 128                         kfree_rcu(relation, rcu);
 129                 }
 130         }
 131         rcu_read_unlock();
 132         spin_unlock(&ptracer_relations_lock);
 133 }
 134
 135 /**
 136  * yama_ptracer_add - add/replace an exception for this tracer/tracee pair
 137  * @tracer: the task_struct of the process doing the ptrace
 138  * @tracee: the task_struct of the process to be ptraced
 139  *
 140  * Each tracee can have, at most, one tracer registered. Each time this
 141  * is called, the prior registered tracer will be replaced for the tracee.
 142  *
 143  * Returns 0 if relationship was added, -ve on error.
 144  */
 145 static int yama_ptracer_add(struct task_struct *tracer,
 146                             struct task_struct *tracee)
 147 {
 148         struct ptrace_relation *relation, *added;
 149
 150         added = kmalloc(sizeof(*added), GFP_KERNEL);
 151         if (!added)
 152                 return -ENOMEM;
 153
 154         added->tracee = tracee;
 155         added->tracer = tracer;
 156         added->invalid = false;
 157
 158         spin_lock(&ptracer_relations_lock);
 159         rcu_read_lock();
 160         list_for_each_entry_rcu(relation, &ptracer_relations, node) {
 161                 if (relation->invalid)
 162                         continue;
 163                 if (relation->tracee == tracee) {
 164                         list_replace_rcu(&relation->node, &added->node);
 165                         kfree_rcu(relation, rcu);
 166                         goto out;
 167                 }
 168         }
 169
 170         list_add_rcu(&added->node, &ptracer_relations);
 171
 172 out:
 173         rcu_read_unlock();
 174         spin_unlock(&ptracer_relations_lock);
 175         return 0;
 176 }
 177
 178 /**
 179  * yama_ptracer_del - remove exceptions related to the given tasks
 180  * @tracer: remove any relation where tracer task matches
 181  * @tracee: remove any relation where tracee task matches
 182  */
 183 static void yama_ptracer_del(struct task_struct *tracer,
 184                              struct task_struct *tracee)
 185 {
 186         struct ptrace_relation *relation;
 187         bool marked = false;
 188
 189         rcu_read_lock();
 190         list_for_each_entry_rcu(relation, &ptracer_relations, node) {
 191                 if (relation->invalid)
 192                         continue;
 193                 if (relation->tracee == tracee ||
 194                     (tracer && relation->tracer == tracer)) {
 195                         relation->invalid = true;
 196                         marked = true;
 197                 }
 198         }
 199         rcu_read_unlock();
 200
 201         if (marked)
 202                 schedule_work(&yama_relation_work);
 203 }
 204
 205 /**
 206  * yama_task_free - check for task_pid to remove from exception list
 207  * @task: task being removed
 208  */
 209 void yama_task_free(struct task_struct *task)
 210 {
 211         yama_ptracer_del(task, task);
 212 }
 213
 214 /**
 215  * yama_task_prctl - check for Yama-specific prctl operations
 216  * @option: operation
 217  * @arg2: argument
 218  * @arg3: argument
 219  * @arg4: argument
 220  * @arg5: argument
 221  *
 222  * Return 0 on success, -ve on error.  -ENOSYS is returned when Yama
 223  * does not handle the given option.
 224  */
 225 int yama_task_prctl(int option, unsigned long arg2, unsigned long arg3,
 226                            unsigned long arg4, unsigned long arg5)
 227 {
 228         int rc = -ENOSYS;
 229         struct task_struct *myself = current;
 230
 231         switch (option) {
 232         case PR_SET_PTRACER:
 233                 /* Since a thread can call prctl(), find the group leader
 234                  * before calling _add() or _del() on it, since we want
 235                  * process-level granularity of control. The tracer group
 236                  * leader checking is handled later when walking the ancestry
 237                  * at the time of PTRACE_ATTACH check.
 238                  */
 239                 rcu_read_lock();
 240                 if (!thread_group_leader(myself))
 241                         myself = rcu_dereference(myself->group_leader);
 242                 get_task_struct(myself);
 243                 rcu_read_unlock();
 244
 245                 if (arg2 == 0) {
 246                         yama_ptracer_del(NULL, myself);
 247                         rc = 0;
 248                 } else if (arg2 == PR_SET_PTRACER_ANY || (int)arg2 == -1) {
 249                         rc = yama_ptracer_add(NULL, myself);
 250                 } else {
 251                         struct task_struct *tracer;
 252
 253                         tracer = find_get_task_by_vpid(arg2);
 254                         if (!tracer) {
 255                                 rc = -EINVAL;
 256                         } else {
 257                                 rc = yama_ptracer_add(tracer, myself);
 258                                 put_task_struct(tracer);
 259                         }
 260                 }
 261
 262                 put_task_struct(myself);
 263                 break;
 264         }
 265
 266         return rc;
 267 }
 268
 269 /**
 270  * task_is_descendant - walk up a process family tree looking for a match
 271  * @parent: the process to compare against while walking up from child
 272  * @child: the process to start from while looking upwards for parent
 273  *
 274  * Returns 1 if child is a descendant of parent, 0 if not.
 275  */
 276 static int task_is_descendant(struct task_struct *parent,
 277                               struct task_struct *child)
 278 {
 279         int rc = 0;
 280         struct task_struct *walker = child;
 281
 282         if (!parent || !child)
 283                 return 0;
 284
 285         rcu_read_lock();
 286         if (!thread_group_leader(parent))
 287                 parent = rcu_dereference(parent->group_leader);
 288         while (walker->pid > 0) {
 289                 if (!thread_group_leader(walker))
 290                         walker = rcu_dereference(walker->group_leader);
 291                 if (walker == parent) {
 292                         rc = 1;
 293                         break;
 294                 }
 295                 walker = rcu_dereference(walker->real_parent);
 296         }
 297         rcu_read_unlock();
 298
 299         return rc;
 300 }
 301
 302 /**
 303  * ptracer_exception_found - tracer registered as exception for this tracee
 304  * @tracer: the task_struct of the process attempting ptrace
 305  * @tracee: the task_struct of the process to be ptraced
 306  *
 307  * Returns 1 if tracer has a ptracer exception ancestor for tracee.
 308  */
 309 static int ptracer_exception_found(struct task_struct *tracer,
 310                                    struct task_struct *tracee)
 311 {
 312         int rc = 0;
 313         struct ptrace_relation *relation;
 314         struct task_struct *parent = NULL;
 315         bool found = false;
 316
 317         rcu_read_lock();
 318
 319         /*
 320          * If there's already an active tracing relationship, then make an
 321          * exception for the sake of other accesses, like process_vm_rw().
 322          */
 323         parent = ptrace_parent(tracee);
 324         if (parent != NULL && same_thread_group(parent, tracer)) {
 325                 rc = 1;
 326                 goto unlock;
 327         }
 328
 329         /* Look for a PR_SET_PTRACER relationship. */
 330         if (!thread_group_leader(tracee))
 331                 tracee = rcu_dereference(tracee->group_leader);
 332         list_for_each_entry_rcu(relation, &ptracer_relations, node) {
 333                 if (relation->invalid)
 334                         continue;
 335                 if (relation->tracee == tracee) {
 336                         parent = relation->tracer;
 337                         found = true;
 338                         break;
 339                 }
 340         }
 341
 342         if (found && (parent == NULL || task_is_descendant(parent, tracer)))
 343                 rc = 1;
 344
 345 unlock:
 346         rcu_read_unlock();
 347
 348         return rc;
 349 }
 350
 351 /**
 352  * yama_ptrace_access_check - validate PTRACE_ATTACH calls
 353  * @child: task that current task is attempting to ptrace
 354  * @mode: ptrace attach mode
 355  *
 356  * Returns 0 if following the ptrace is allowed, -ve on error.
 357  */
 358 static int yama_ptrace_access_check(struct task_struct *child,
 359                                     unsigned int mode)
 360 {
 361         int rc = 0;
 362
 363         /* require ptrace target be a child of ptracer on attach */
 364         if (mode & PTRACE_MODE_ATTACH) {
 365                 switch (ptrace_scope) {
 366                 case YAMA_SCOPE_DISABLED:
 367                         /* No additional restrictions. */
 368                         break;
 369                 case YAMA_SCOPE_RELATIONAL:
 370                         rcu_read_lock();
 371                         if (!pid_alive(child))
 372                                 rc = -EPERM;
 373                         if (!rc && !task_is_descendant(current, child) &&
 374                             !ptracer_exception_found(current, child) &&
 375                             !ns_capable(__task_cred(child)->user_ns, CAP_SYS_PTRACE))
 376                                 rc = -EPERM;
 377                         rcu_read_unlock();
 378                         break;
 379                 case YAMA_SCOPE_CAPABILITY:
 380                         rcu_read_lock();
 381                         if (!ns_capable(__task_cred(child)->user_ns, CAP_SYS_PTRACE))
 382                                 rc = -EPERM;
 383                         rcu_read_unlock();
 384                         break;
 385                 case YAMA_SCOPE_NO_ATTACH:
 386                 default:
 387                         rc = -EPERM;
 388                         break;
 389                 }
 390         }
 391
 392         if (rc && (mode & PTRACE_MODE_NOAUDIT) == 0)
 393                 report_access("attach", child, current);
 394
 395         return rc;
 396 }
 397
 398 /**
 399  * yama_ptrace_traceme - validate PTRACE_TRACEME calls
 400  * @parent: task that will become the ptracer of the current task
 401  *
 402  * Returns 0 if following the ptrace is allowed, -ve on error.
 403  */
 404 int yama_ptrace_traceme(struct task_struct *parent)
 405 {
 406         int rc = 0;
 407
 408         /* Only disallow PTRACE_TRACEME on more aggressive settings. */
 409         switch (ptrace_scope) {
 410         case YAMA_SCOPE_CAPABILITY:
 411                 if (!has_ns_capability(parent, current_user_ns(), CAP_SYS_PTRACE))
 412                         rc = -EPERM;
 413                 break;
 414         case YAMA_SCOPE_NO_ATTACH:
 415                 rc = -EPERM;
 416                 break;
 417         }
 418
 419         if (rc) {
 420                 task_lock(current);
 421                 report_access("traceme", current, parent);
 422                 task_unlock(current);
 423         }
 424
 425         return rc;
 426 }
 427
 428 static struct security_hook_list yama_hooks[] __lsm_ro_after_init = {
 429         LSM_HOOK_INIT(ptrace_access_check, yama_ptrace_access_check),
 430         LSM_HOOK_INIT(ptrace_traceme, yama_ptrace_traceme),
 431         LSM_HOOK_INIT(task_prctl, yama_task_prctl),
 432         LSM_HOOK_INIT(task_free, yama_task_free),
 433 };
 434
 435 #ifdef CONFIG_SYSCTL
 436 static int yama_dointvec_minmax(struct ctl_table *table, int write,
 437                                 void __user *buffer, size_t *lenp, loff_t *ppos)
 438 {
 439         struct ctl_table table_copy;
 440
 441         if (write && !capable(CAP_SYS_PTRACE))
 442                 return -EPERM;
 443
 444         /* Lock the max value if it ever gets set. */
 445         table_copy = *table;
 446         if (*(int *)table_copy.data == *(int *)table_copy.extra2)
 447                 table_copy.extra1 = table_copy.extra2;
 448
 449         return proc_dointvec_minmax(&table_copy, write, buffer, lenp, ppos);
 450 }
 451
 452 static int zero;
 453 static int max_scope = YAMA_SCOPE_NO_ATTACH;
 454
 455 struct ctl_path yama_sysctl_path[] = {
 456         { .procname = "kernel", },
 457         { .procname = "yama", },
 458         { }
 459 };
 460
 461 static struct ctl_table yama_sysctl_table[] = {
 462         {
 463                 .procname       = "ptrace_scope",
 464                 .data           = &ptrace_scope,
 465                 .maxlen         = sizeof(int),
 466                 .mode           = 0644,
 467                 .proc_handler   = yama_dointvec_minmax,
 468                 .extra1         = &zero,
 469                 .extra2         = &max_scope,
 470         },
 471         { }
 472 };
 473 static void __init yama_init_sysctl(void)
 474 {
 475         if (!register_sysctl_paths(yama_sysctl_path, yama_sysctl_table))
 476                 panic("Yama: sysctl registration failed.\n");
 477 }
 478 #else
 479 static inline void yama_init_sysctl(void) { }
 480 #endif /* CONFIG_SYSCTL */
 481
 482 static int __init yama_init(void)
 483 {
 484         pr_info("Yama: becoming mindful.\n");
 485         security_add_hooks(yama_hooks, ARRAY_SIZE(yama_hooks), "yama");
 486         yama_init_sysctl();
 487         return 0;
 488 }
 489
 490 DEFINE_LSM(yama) = {
 491         .name = "yama",
 492         .init = yama_init,
 493 };