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
  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 = -ENOSYS;
 158         struct task_struct *myself = current;
 159
 160         switch (option) {
 161         case PR_SET_PTRACER:
 162                 /* Since a thread can call prctl(), find the group leader
 163                  * before calling _add() or _del() on it, since we want
 164                  * process-level granularity of control. The tracer group
 165                  * leader checking is handled later when walking the ancestry
 166                  * at the time of PTRACE_ATTACH check.
 167                  */
 168                 rcu_read_lock();
 169                 if (!thread_group_leader(myself))
 170                         myself = rcu_dereference(myself->group_leader);
 171                 get_task_struct(myself);
 172                 rcu_read_unlock();
 173
 174                 if (arg2 == 0) {
 175                         yama_ptracer_del(NULL, myself);
 176                         rc = 0;
 177                 } else if (arg2 == PR_SET_PTRACER_ANY || (int)arg2 == -1) {
 178                         rc = yama_ptracer_add(NULL, myself);
 179                 } else {
 180                         struct task_struct *tracer;
 181
 182                         rcu_read_lock();
 183                         tracer = find_task_by_vpid(arg2);
 184                         if (tracer)
 185                                 get_task_struct(tracer);
 186                         else
 187                                 rc = -EINVAL;
 188                         rcu_read_unlock();
 189
 190                         if (tracer) {
 191                                 rc = yama_ptracer_add(tracer, myself);
 192                                 put_task_struct(tracer);
 193                         }
 194                 }
 195
 196                 put_task_struct(myself);
 197                 break;
 198         }
 199
 200         return rc;
 201 }
 202
 203 /**
 204  * task_is_descendant - walk up a process family tree looking for a match
 205  * @parent: the process to compare against while walking up from child
 206  * @child: the process to start from while looking upwards for parent
 207  *
 208  * Returns 1 if child is a descendant of parent, 0 if not.
 209  */
 210 static int task_is_descendant(struct task_struct *parent,
 211                               struct task_struct *child)
 212 {
 213         int rc = 0;
 214         struct task_struct *walker = child;
 215
 216         if (!parent || !child)
 217                 return 0;
 218
 219         rcu_read_lock();
 220         if (!thread_group_leader(parent))
 221                 parent = rcu_dereference(parent->group_leader);
 222         while (walker->pid > 0) {
 223                 if (!thread_group_leader(walker))
 224                         walker = rcu_dereference(walker->group_leader);
 225                 if (walker == parent) {
 226                         rc = 1;
 227                         break;
 228                 }
 229                 walker = rcu_dereference(walker->real_parent);
 230         }
 231         rcu_read_unlock();
 232
 233         return rc;
 234 }
 235
 236 /**
 237  * ptracer_exception_found - tracer registered as exception for this tracee
 238  * @tracer: the task_struct of the process attempting ptrace
 239  * @tracee: the task_struct of the process to be ptraced
 240  *
 241  * Returns 1 if tracer has is ptracer exception ancestor for tracee.
 242  */
 243 static int ptracer_exception_found(struct task_struct *tracer,
 244                                    struct task_struct *tracee)
 245 {
 246         int rc = 0;
 247         struct ptrace_relation *relation;
 248         struct task_struct *parent = NULL;
 249         bool found = false;
 250
 251         rcu_read_lock();
 252         if (!thread_group_leader(tracee))
 253                 tracee = rcu_dereference(tracee->group_leader);
 254         list_for_each_entry_rcu(relation, &ptracer_relations, node) {
 255                 if (relation->invalid)
 256                         continue;
 257                 if (relation->tracee == tracee) {
 258                         parent = relation->tracer;
 259                         found = true;
 260                         break;
 261                 }
 262         }
 263
 264         if (found && (parent == NULL || task_is_descendant(parent, tracer)))
 265                 rc = 1;
 266         rcu_read_unlock();
 267
 268         return rc;
 269 }
 270
 271 /**
 272  * yama_ptrace_access_check - validate PTRACE_ATTACH calls
 273  * @child: task that current task is attempting to ptrace
 274  * @mode: ptrace attach mode
 275  *
 276  * Returns 0 if following the ptrace is allowed, -ve on error.
 277  */
 278 static int yama_ptrace_access_check(struct task_struct *child,
 279                                     unsigned int mode)
 280 {
 281         int rc = 0;
 282
 283         /* require ptrace target be a child of ptracer on attach */
 284         if (mode & PTRACE_MODE_ATTACH) {
 285                 switch (ptrace_scope) {
 286                 case YAMA_SCOPE_DISABLED:
 287                         /* No additional restrictions. */
 288                         break;
 289                 case YAMA_SCOPE_RELATIONAL:
 290                         rcu_read_lock();
 291                         if (!task_is_descendant(current, child) &&
 292                             !ptracer_exception_found(current, child) &&
 293                             !ns_capable(__task_cred(child)->user_ns, CAP_SYS_PTRACE))
 294                                 rc = -EPERM;
 295                         rcu_read_unlock();
 296                         break;
 297                 case YAMA_SCOPE_CAPABILITY:
 298                         rcu_read_lock();
 299                         if (!ns_capable(__task_cred(child)->user_ns, CAP_SYS_PTRACE))
 300                                 rc = -EPERM;
 301                         rcu_read_unlock();
 302                         break;
 303                 case YAMA_SCOPE_NO_ATTACH:
 304                 default:
 305                         rc = -EPERM;
 306                         break;
 307                 }
 308         }
 309
 310         if (rc && (mode & PTRACE_MODE_NOAUDIT) == 0) {
 311                 printk_ratelimited(KERN_NOTICE
 312                         "ptrace of pid %d was attempted by: %s (pid %d)\n",
 313                         child->pid, current->comm, current->pid);
 314         }
 315
 316         return rc;
 317 }
 318
 319 /**
 320  * yama_ptrace_traceme - validate PTRACE_TRACEME calls
 321  * @parent: task that will become the ptracer of the current task
 322  *
 323  * Returns 0 if following the ptrace is allowed, -ve on error.
 324  */
 325 int yama_ptrace_traceme(struct task_struct *parent)
 326 {
 327         int rc = 0;
 328
 329         /* Only disallow PTRACE_TRACEME on more aggressive settings. */
 330         switch (ptrace_scope) {
 331         case YAMA_SCOPE_CAPABILITY:
 332                 if (!has_ns_capability(parent, current_user_ns(), CAP_SYS_PTRACE))
 333                         rc = -EPERM;
 334                 break;
 335         case YAMA_SCOPE_NO_ATTACH:
 336                 rc = -EPERM;
 337                 break;
 338         }
 339
 340         if (rc) {
 341                 printk_ratelimited(KERN_NOTICE
 342                         "ptraceme of pid %d was attempted by: %s (pid %d)\n",
 343                         current->pid, parent->comm, parent->pid);
 344         }
 345
 346         return rc;
 347 }
 348
 349 static struct security_hook_list yama_hooks[] = {
 350         LSM_HOOK_INIT(ptrace_access_check, yama_ptrace_access_check),
 351         LSM_HOOK_INIT(ptrace_traceme, yama_ptrace_traceme),
 352         LSM_HOOK_INIT(task_prctl, yama_task_prctl),
 353         LSM_HOOK_INIT(task_free, yama_task_free),
 354 };
 355
 356 #ifdef CONFIG_SYSCTL
 357 static int yama_dointvec_minmax(struct ctl_table *table, int write,
 358                                 void __user *buffer, size_t *lenp, loff_t *ppos)
 359 {
 360         struct ctl_table table_copy;
 361
 362         if (write && !capable(CAP_SYS_PTRACE))
 363                 return -EPERM;
 364
 365         /* Lock the max value if it ever gets set. */
 366         table_copy = *table;
 367         if (*(int *)table_copy.data == *(int *)table_copy.extra2)
 368                 table_copy.extra1 = table_copy.extra2;
 369
 370         return proc_dointvec_minmax(&table_copy, write, buffer, lenp, ppos);
 371 }
 372
 373 static int zero;
 374 static int max_scope = YAMA_SCOPE_NO_ATTACH;
 375
 376 struct ctl_path yama_sysctl_path[] = {
 377         { .procname = "kernel", },
 378         { .procname = "yama", },
 379         { }
 380 };
 381
 382 static struct ctl_table yama_sysctl_table[] = {
 383         {
 384                 .procname       = "ptrace_scope",
 385                 .data           = &ptrace_scope,
 386                 .maxlen         = sizeof(int),
 387                 .mode           = 0644,
 388                 .proc_handler   = yama_dointvec_minmax,
 389                 .extra1         = &zero,
 390                 .extra2         = &max_scope,
 391         },
 392         { }
 393 };
 394 static void __init yama_init_sysctl(void)
 395 {
 396         if (!register_sysctl_paths(yama_sysctl_path, yama_sysctl_table))
 397                 panic("Yama: sysctl registration failed.\n");
 398 }
 399 #else
 400 static inline void yama_init_sysctl(void) { }
 401 #endif /* CONFIG_SYSCTL */
 402
 403 void __init yama_add_hooks(void)
 404 {
 405         pr_info("Yama: becoming mindful.\n");
 406         security_add_hooks(yama_hooks, ARRAY_SIZE(yama_hooks));
 407         yama_init_sysctl();
 408 }