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