security/yama/yama_lsm.c

   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
  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 */
  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  */
 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  */
 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  */
 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  */
 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  */
 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  */
 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  */
 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  */
 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  */
 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
 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 };
 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
 474 static inline void yama_init_sysctl(void) { }
 475 #endif /* CONFIG_SYSCTL */
 476
 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 };