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                         rcu_read_lock();
 254                         tracer = find_task_by_vpid(arg2);
 255                         if (tracer)
 256                                 get_task_struct(tracer);
 257                         else
 258                                 rc = -EINVAL;
 259                         rcu_read_unlock();
 260
 261                         if (tracer) {
 262                                 rc = yama_ptracer_add(tracer, myself);
 263                                 put_task_struct(tracer);
 264                         }
 265                 }
 266
 267                 put_task_struct(myself);
 268                 break;
 269         }
 270
 271         return rc;
 272 }
 273
 274 /**
 275  * task_is_descendant - walk up a process family tree looking for a match
 276  * @parent: the process to compare against while walking up from child
 277  * @child: the process to start from while looking upwards for parent
 278  *
 279  * Returns 1 if child is a descendant of parent, 0 if not.
 280  */
 281 static int task_is_descendant(struct task_struct *parent,
 282                               struct task_struct *child)
 283 {
 284         int rc = 0;
 285         struct task_struct *walker = child;
 286
 287         if (!parent || !child)
 288                 return 0;
 289
 290         rcu_read_lock();
 291         if (!thread_group_leader(parent))
 292                 parent = rcu_dereference(parent->group_leader);
 293         while (walker->pid > 0) {
 294                 if (!thread_group_leader(walker))
 295                         walker = rcu_dereference(walker->group_leader);
 296                 if (walker == parent) {
 297                         rc = 1;
 298                         break;
 299                 }
 300                 walker = rcu_dereference(walker->real_parent);
 301         }
 302         rcu_read_unlock();
 303
 304         return rc;
 305 }
 306
 307 /**
 308  * ptracer_exception_found - tracer registered as exception for this tracee
 309  * @tracer: the task_struct of the process attempting ptrace
 310  * @tracee: the task_struct of the process to be ptraced
 311  *
 312  * Returns 1 if tracer has is ptracer exception ancestor for tracee.
 313  */
 314 static int ptracer_exception_found(struct task_struct *tracer,
 315                                    struct task_struct *tracee)
 316 {
 317         int rc = 0;
 318         struct ptrace_relation *relation;
 319         struct task_struct *parent = NULL;
 320         bool found = false;
 321
 322         rcu_read_lock();
 323         if (!thread_group_leader(tracee))
 324                 tracee = rcu_dereference(tracee->group_leader);
 325         list_for_each_entry_rcu(relation, &ptracer_relations, node) {
 326                 if (relation->invalid)
 327                         continue;
 328                 if (relation->tracee == tracee) {
 329                         parent = relation->tracer;
 330                         found = true;
 331                         break;
 332                 }
 333         }
 334
 335         if (found && (parent == NULL || task_is_descendant(parent, tracer)))
 336                 rc = 1;
 337         rcu_read_unlock();
 338
 339         return rc;
 340 }
 341
 342 /**
 343  * yama_ptrace_access_check - validate PTRACE_ATTACH calls
 344  * @child: task that current task is attempting to ptrace
 345  * @mode: ptrace attach mode
 346  *
 347  * Returns 0 if following the ptrace is allowed, -ve on error.
 348  */
 349 static int yama_ptrace_access_check(struct task_struct *child,
 350                                     unsigned int mode)
 351 {
 352         int rc = 0;
 353
 354         /* require ptrace target be a child of ptracer on attach */
 355         if (mode & PTRACE_MODE_ATTACH) {
 356                 switch (ptrace_scope) {
 357                 case YAMA_SCOPE_DISABLED:
 358                         /* No additional restrictions. */
 359                         break;
 360                 case YAMA_SCOPE_RELATIONAL:
 361                         rcu_read_lock();
 362                         if (!task_is_descendant(current, child) &&
 363                             !ptracer_exception_found(current, child) &&
 364                             !ns_capable(__task_cred(child)->user_ns, CAP_SYS_PTRACE))
 365                                 rc = -EPERM;
 366                         rcu_read_unlock();
 367                         break;
 368                 case YAMA_SCOPE_CAPABILITY:
 369                         rcu_read_lock();
 370                         if (!ns_capable(__task_cred(child)->user_ns, CAP_SYS_PTRACE))
 371                                 rc = -EPERM;
 372                         rcu_read_unlock();
 373                         break;
 374                 case YAMA_SCOPE_NO_ATTACH:
 375                 default:
 376                         rc = -EPERM;
 377                         break;
 378                 }
 379         }
 380
 381         if (rc && (mode & PTRACE_MODE_NOAUDIT) == 0)
 382                 report_access("attach", child, current);
 383
 384         return rc;
 385 }
 386
 387 /**
 388  * yama_ptrace_traceme - validate PTRACE_TRACEME calls
 389  * @parent: task that will become the ptracer of the current task
 390  *
 391  * Returns 0 if following the ptrace is allowed, -ve on error.
 392  */
 393 int yama_ptrace_traceme(struct task_struct *parent)
 394 {
 395         int rc = 0;
 396
 397         /* Only disallow PTRACE_TRACEME on more aggressive settings. */
 398         switch (ptrace_scope) {
 399         case YAMA_SCOPE_CAPABILITY:
 400                 if (!has_ns_capability(parent, current_user_ns(), CAP_SYS_PTRACE))
 401                         rc = -EPERM;
 402                 break;
 403         case YAMA_SCOPE_NO_ATTACH:
 404                 rc = -EPERM;
 405                 break;
 406         }
 407
 408         if (rc) {
 409                 task_lock(current);
 410                 report_access("traceme", current, parent);
 411                 task_unlock(current);
 412         }
 413
 414         return rc;
 415 }
 416
 417 static struct security_hook_list yama_hooks[] = {
 418         LSM_HOOK_INIT(ptrace_access_check, yama_ptrace_access_check),
 419         LSM_HOOK_INIT(ptrace_traceme, yama_ptrace_traceme),
 420         LSM_HOOK_INIT(task_prctl, yama_task_prctl),
 421         LSM_HOOK_INIT(task_free, yama_task_free),
 422 };
 423
 424 #ifdef CONFIG_SYSCTL
 425 static int yama_dointvec_minmax(struct ctl_table *table, int write,
 426                                 void __user *buffer, size_t *lenp, loff_t *ppos)
 427 {
 428         struct ctl_table table_copy;
 429
 430         if (write && !capable(CAP_SYS_PTRACE))
 431                 return -EPERM;
 432
 433         /* Lock the max value if it ever gets set. */
 434         table_copy = *table;
 435         if (*(int *)table_copy.data == *(int *)table_copy.extra2)
 436                 table_copy.extra1 = table_copy.extra2;
 437
 438         return proc_dointvec_minmax(&table_copy, write, buffer, lenp, ppos);
 439 }
 440
 441 static int zero;
 442 static int max_scope = YAMA_SCOPE_NO_ATTACH;
 443
 444 struct ctl_path yama_sysctl_path[] = {
 445         { .procname = "kernel", },
 446         { .procname = "yama", },
 447         { }
 448 };
 449
 450 static struct ctl_table yama_sysctl_table[] = {
 451         {
 452                 .procname       = "ptrace_scope",
 453                 .data           = &ptrace_scope,
 454                 .maxlen         = sizeof(int),
 455                 .mode           = 0644,
 456                 .proc_handler   = yama_dointvec_minmax,
 457                 .extra1         = &zero,
 458                 .extra2         = &max_scope,
 459         },
 460         { }
 461 };
 462 static void __init yama_init_sysctl(void)
 463 {
 464         if (!register_sysctl_paths(yama_sysctl_path, yama_sysctl_table))
 465                 panic("Yama: sysctl registration failed.\n");
 466 }
 467 #else
 468 static inline void yama_init_sysctl(void) { }
 469 #endif /* CONFIG_SYSCTL */
 470
 471 void __init yama_add_hooks(void)
 472 {
 473         pr_info("Yama: becoming mindful.\n");
 474         security_add_hooks(yama_hooks, ARRAY_SIZE(yama_hooks));
 475         yama_init_sysctl();
 476 }