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 && (mode & PTRACE_MODE_NOAUDIT) == 0) {
 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);