security/device_cgroup.c

   1 /*
   2  * device_cgroup.c - device cgroup subsystem
   3  *
   4  * Copyright 2007 IBM Corp
   5  */
   6
   7 #include <linux/device_cgroup.h>
   8 #include <linux/cgroup.h>
   9 #include <linux/ctype.h>
  10 #include <linux/list.h>
  11 #include <linux/uaccess.h>
  12 #include <linux/seq_file.h>
  13 #include <linux/slab.h>
  14 #include <linux/rcupdate.h>
  15 #include <linux/mutex.h>
  16
  17 #define ACC_MKNOD 1
  18 #define ACC_READ  2
  19 #define ACC_WRITE 4
  20 #define ACC_MASK (ACC_MKNOD | ACC_READ | ACC_WRITE)
  21
  22 #define DEV_BLOCK 1
  23 #define DEV_CHAR  2
  24 #define DEV_ALL   4  /* this represents all devices */
  25
  26 static DEFINE_MUTEX(devcgroup_mutex);
  27
  28 enum devcg_behavior {
  29         DEVCG_DEFAULT_NONE,
  30         DEVCG_DEFAULT_ALLOW,
  31         DEVCG_DEFAULT_DENY,
  32 };
  33
  34 /*
  35  * exception list locking rules:
  36  * hold devcgroup_mutex for update/read.
  37  * hold rcu_read_lock() for read.
  38  */
  39
  40 struct dev_exception_item {
  41         u32 major, minor;
  42         short type;
  43         short access;
  44         struct list_head list;
  45         struct rcu_head rcu;
  46 };
  47
  48 struct dev_cgroup {
  49         struct cgroup_subsys_state css;
  50         struct list_head exceptions;
  51         enum devcg_behavior behavior;
  52 };
  53
  54 static inline struct dev_cgroup *css_to_devcgroup(struct cgroup_subsys_state *s)
  55 {
  56         return s ? container_of(s, struct dev_cgroup, css) : NULL;
  57 }
  58
  59 static inline struct dev_cgroup *task_devcgroup(struct task_struct *task)
  60 {
  61         return css_to_devcgroup(task_css(task, devices_cgrp_id));
  62 }
  63
  64 /*
  65  * called under devcgroup_mutex
  66  */
  67 static int dev_exceptions_copy(struct list_head *dest, struct list_head *orig)
  68 {
  69         struct dev_exception_item *ex, *tmp, *new;
  70
  71         lockdep_assert_held(&devcgroup_mutex);
  72
  73         list_for_each_entry(ex, orig, list) {
  74                 new = kmemdup(ex, sizeof(*ex), GFP_KERNEL);
  75                 if (!new)
  76                         goto free_and_exit;
  77                 list_add_tail(&new->list, dest);
  78         }
  79
  80         return 0;
  81
  82 free_and_exit:
  83         list_for_each_entry_safe(ex, tmp, dest, list) {
  84                 list_del(&ex->list);
  85                 kfree(ex);
  86         }
  87         return -ENOMEM;
  88 }
  89
  90 /*
  91  * called under devcgroup_mutex
  92  */
  93 static int dev_exception_add(struct dev_cgroup *dev_cgroup,
  94                              struct dev_exception_item *ex)
  95 {
  96         struct dev_exception_item *excopy, *walk;
  97
  98         lockdep_assert_held(&devcgroup_mutex);
  99
 100         excopy = kmemdup(ex, sizeof(*ex), GFP_KERNEL);
 101         if (!excopy)
 102                 return -ENOMEM;
 103
 104         list_for_each_entry(walk, &dev_cgroup->exceptions, list) {
 105                 if (walk->type != ex->type)
 106                         continue;
 107                 if (walk->major != ex->major)
 108                         continue;
 109                 if (walk->minor != ex->minor)
 110                         continue;
 111
 112                 walk->access |= ex->access;
 113                 kfree(excopy);
 114                 excopy = NULL;
 115         }
 116
 117         if (excopy != NULL)
 118                 list_add_tail_rcu(&excopy->list, &dev_cgroup->exceptions);
 119         return 0;
 120 }
 121
 122 /*
 123  * called under devcgroup_mutex
 124  */
 125 static void dev_exception_rm(struct dev_cgroup *dev_cgroup,
 126                              struct dev_exception_item *ex)
 127 {
 128         struct dev_exception_item *walk, *tmp;
 129
 130         lockdep_assert_held(&devcgroup_mutex);
 131
 132         list_for_each_entry_safe(walk, tmp, &dev_cgroup->exceptions, list) {
 133                 if (walk->type != ex->type)
 134                         continue;
 135                 if (walk->major != ex->major)
 136                         continue;
 137                 if (walk->minor != ex->minor)
 138                         continue;
 139
 140                 walk->access &= ~ex->access;
 141                 if (!walk->access) {
 142                         list_del_rcu(&walk->list);
 143                         kfree_rcu(walk, rcu);
 144                 }
 145         }
 146 }
 147
 148 static void __dev_exception_clean(struct dev_cgroup *dev_cgroup)
 149 {
 150         struct dev_exception_item *ex, *tmp;
 151
 152         list_for_each_entry_safe(ex, tmp, &dev_cgroup->exceptions, list) {
 153                 list_del_rcu(&ex->list);
 154                 kfree_rcu(ex, rcu);
 155         }
 156 }
 157
 158 /**
 159  * dev_exception_clean - frees all entries of the exception list
 160  * @dev_cgroup: dev_cgroup with the exception list to be cleaned
 161  *
 162  * called under devcgroup_mutex
 163  */
 164 static void dev_exception_clean(struct dev_cgroup *dev_cgroup)
 165 {
 166         lockdep_assert_held(&devcgroup_mutex);
 167
 168         __dev_exception_clean(dev_cgroup);
 169 }
 170
 171 static inline bool is_devcg_online(const struct dev_cgroup *devcg)
 172 {
 173         return (devcg->behavior != DEVCG_DEFAULT_NONE);
 174 }
 175
 176 /**
 177  * devcgroup_online - initializes devcgroup's behavior and exceptions based on
 178  *                    parent's
 179  * @css: css getting online
 180  * returns 0 in case of success, error code otherwise
 181  */
 182 static int devcgroup_online(struct cgroup_subsys_state *css)
 183 {
 184         struct dev_cgroup *dev_cgroup = css_to_devcgroup(css);
 185         struct dev_cgroup *parent_dev_cgroup = css_to_devcgroup(css_parent(css));
 186         int ret = 0;
 187
 188         mutex_lock(&devcgroup_mutex);
 189
 190         if (parent_dev_cgroup == NULL)
 191                 dev_cgroup->behavior = DEVCG_DEFAULT_ALLOW;
 192         else {
 193                 ret = dev_exceptions_copy(&dev_cgroup->exceptions,
 194                                           &parent_dev_cgroup->exceptions);
 195                 if (!ret)
 196                         dev_cgroup->behavior = parent_dev_cgroup->behavior;
 197         }
 198         mutex_unlock(&devcgroup_mutex);
 199
 200         return ret;
 201 }
 202
 203 static void devcgroup_offline(struct cgroup_subsys_state *css)
 204 {
 205         struct dev_cgroup *dev_cgroup = css_to_devcgroup(css);
 206
 207         mutex_lock(&devcgroup_mutex);
 208         dev_cgroup->behavior = DEVCG_DEFAULT_NONE;
 209         mutex_unlock(&devcgroup_mutex);
 210 }
 211
 212 /*
 213  * called from kernel/cgroup.c with cgroup_lock() held.
 214  */
 215 static struct cgroup_subsys_state *
 216 devcgroup_css_alloc(struct cgroup_subsys_state *parent_css)
 217 {
 218         struct dev_cgroup *dev_cgroup;
 219
 220         dev_cgroup = kzalloc(sizeof(*dev_cgroup), GFP_KERNEL);
 221         if (!dev_cgroup)
 222                 return ERR_PTR(-ENOMEM);
 223         INIT_LIST_HEAD(&dev_cgroup->exceptions);
 224         dev_cgroup->behavior = DEVCG_DEFAULT_NONE;
 225
 226         return &dev_cgroup->css;
 227 }
 228
 229 static void devcgroup_css_free(struct cgroup_subsys_state *css)
 230 {
 231         struct dev_cgroup *dev_cgroup = css_to_devcgroup(css);
 232
 233         __dev_exception_clean(dev_cgroup);
 234         kfree(dev_cgroup);
 235 }
 236
 237 #define DEVCG_ALLOW 1
 238 #define DEVCG_DENY 2
 239 #define DEVCG_LIST 3
 240
 241 #define MAJMINLEN 13
 242 #define ACCLEN 4
 243
 244 static void set_access(char *acc, short access)
 245 {
 246         int idx = 0;
 247         memset(acc, 0, ACCLEN);
 248         if (access & ACC_READ)
 249                 acc[idx++] = 'r';
 250         if (access & ACC_WRITE)
 251                 acc[idx++] = 'w';
 252         if (access & ACC_MKNOD)
 253                 acc[idx++] = 'm';
 254 }
 255
 256 static char type_to_char(short type)
 257 {
 258         if (type == DEV_ALL)
 259                 return 'a';
 260         if (type == DEV_CHAR)
 261                 return 'c';
 262         if (type == DEV_BLOCK)
 263                 return 'b';
 264         return 'X';
 265 }
 266
 267 static void set_majmin(char *str, unsigned m)
 268 {
 269         if (m == ~0)
 270                 strcpy(str, "*");
 271         else
 272                 sprintf(str, "%u", m);
 273 }
 274
 275 static int devcgroup_seq_show(struct seq_file *m, void *v)
 276 {
 277         struct dev_cgroup *devcgroup = css_to_devcgroup(seq_css(m));
 278         struct dev_exception_item *ex;
 279         char maj[MAJMINLEN], min[MAJMINLEN], acc[ACCLEN];
 280
 281         rcu_read_lock();
 282         /*
 283          * To preserve the compatibility:
 284          * - Only show the "all devices" when the default policy is to allow
 285          * - List the exceptions in case the default policy is to deny
 286          * This way, the file remains as a "whitelist of devices"
 287          */
 288         if (devcgroup->behavior == DEVCG_DEFAULT_ALLOW) {
 289                 set_access(acc, ACC_MASK);
 290                 set_majmin(maj, ~0);
 291                 set_majmin(min, ~0);
 292                 seq_printf(m, "%c %s:%s %s\n", type_to_char(DEV_ALL),
 293                            maj, min, acc);
 294         } else {
 295                 list_for_each_entry_rcu(ex, &devcgroup->exceptions, list) {
 296                         set_access(acc, ex->access);
 297                         set_majmin(maj, ex->major);
 298                         set_majmin(min, ex->minor);
 299                         seq_printf(m, "%c %s:%s %s\n", type_to_char(ex->type),
 300                                    maj, min, acc);
 301                 }
 302         }
 303         rcu_read_unlock();
 304
 305         return 0;
 306 }
 307
 308 /**
 309  * match_exception      - iterates the exception list trying to find a complete match
 310  * @exceptions: list of exceptions
 311  * @type: device type (DEV_BLOCK or DEV_CHAR)
 312  * @major: device file major number, ~0 to match all
 313  * @minor: device file minor number, ~0 to match all
 314  * @access: permission mask (ACC_READ, ACC_WRITE, ACC_MKNOD)
 315  *
 316  * It is considered a complete match if an exception is found that will
 317  * contain the entire range of provided parameters.
 318  *
 319  * Return: true in case it matches an exception completely
 320  */
 321 static bool match_exception(struct list_head *exceptions, short type,
 322                             u32 major, u32 minor, short access)
 323 {
 324         struct dev_exception_item *ex;
 325
 326         list_for_each_entry_rcu(ex, exceptions, list) {
 327                 if ((type & DEV_BLOCK) && !(ex->type & DEV_BLOCK))
 328                         continue;
 329                 if ((type & DEV_CHAR) && !(ex->type & DEV_CHAR))
 330                         continue;
 331                 if (ex->major != ~0 && ex->major != major)
 332                         continue;
 333                 if (ex->minor != ~0 && ex->minor != minor)
 334                         continue;
 335                 /* provided access cannot have more than the exception rule */
 336                 if (access & (~ex->access))
 337                         continue;
 338                 return true;
 339         }
 340         return false;
 341 }
 342
 343 /**
 344  * match_exception_partial - iterates the exception list trying to find a partial match
 345  * @exceptions: list of exceptions
 346  * @type: device type (DEV_BLOCK or DEV_CHAR)
 347  * @major: device file major number, ~0 to match all
 348  * @minor: device file minor number, ~0 to match all
 349  * @access: permission mask (ACC_READ, ACC_WRITE, ACC_MKNOD)
 350  *
 351  * It is considered a partial match if an exception's range is found to
 352  * contain *any* of the devices specified by provided parameters. This is
 353  * used to make sure no extra access is being granted that is forbidden by
 354  * any of the exception list.
 355  *
 356  * Return: true in case the provided range mat matches an exception completely
 357  */
 358 static bool match_exception_partial(struct list_head *exceptions, short type,
 359                                     u32 major, u32 minor, short access)
 360 {
 361         struct dev_exception_item *ex;
 362
 363         list_for_each_entry_rcu(ex, exceptions, list) {
 364                 if ((type & DEV_BLOCK) && !(ex->type & DEV_BLOCK))
 365                         continue;
 366                 if ((type & DEV_CHAR) && !(ex->type & DEV_CHAR))
 367                         continue;
 368                 /*
 369                  * We must be sure that both the exception and the provided
 370                  * range aren't masking all devices
 371                  */
 372                 if (ex->major != ~0 && major != ~0 && ex->major != major)
 373                         continue;
 374                 if (ex->minor != ~0 && minor != ~0 && ex->minor != minor)
 375                         continue;
 376                 /*
 377                  * In order to make sure the provided range isn't matching
 378                  * an exception, all its access bits shouldn't match the
 379                  * exception's access bits
 380                  */
 381                 if (!(access & ex->access))
 382                         continue;
 383                 return true;
 384         }
 385         return false;
 386 }
 387
 388 /**
 389  * verify_new_ex - verifies if a new exception is allowed by parent cgroup's permissions
 390  * @dev_cgroup: dev cgroup to be tested against
 391  * @refex: new exception
 392  * @behavior: behavior of the exception's dev_cgroup
 393  *
 394  * This is used to make sure a child cgroup won't have more privileges
 395  * than its parent
 396  */
 397 static bool verify_new_ex(struct dev_cgroup *dev_cgroup,
 398                           struct dev_exception_item *refex,
 399                           enum devcg_behavior behavior)
 400 {
 401         bool match = false;
 402
 403         rcu_lockdep_assert(rcu_read_lock_held() ||
 404                            lockdep_is_held(&devcgroup_mutex),
 405                            "device_cgroup:verify_new_ex called without proper synchronization");
 406
 407         if (dev_cgroup->behavior == DEVCG_DEFAULT_ALLOW) {
 408                 if (behavior == DEVCG_DEFAULT_ALLOW) {
 409                         /*
 410                          * new exception in the child doesn't matter, only
 411                          * adding extra restrictions
 412                          */
 413                         return true;
 414                 } else {
 415                         /*
 416                          * new exception in the child will add more devices
 417                          * that can be acessed, so it can't match any of
 418                          * parent's exceptions, even slightly
 419                          */
 420                         match = match_exception_partial(&dev_cgroup->exceptions,
 421                                                         refex->type,
 422                                                         refex->major,
 423                                                         refex->minor,
 424                                                         refex->access);
 425
 426                         if (match)
 427                                 return false;
 428                         return true;
 429                 }
 430         } else {
 431                 /*
 432                  * Only behavior == DEVCG_DEFAULT_DENY allowed here, therefore
 433                  * the new exception will add access to more devices and must
 434                  * be contained completely in an parent's exception to be
 435                  * allowed
 436                  */
 437                 match = match_exception(&dev_cgroup->exceptions, refex->type,
 438                                         refex->major, refex->minor,
 439                                         refex->access);
 440
 441                 if (match)
 442                         /* parent has an exception that matches the proposed */
 443                         return true;
 444                 else
 445                         return false;
 446         }
 447         return false;
 448 }
 449
 450 /*
 451  * parent_has_perm:
 452  * when adding a new allow rule to a device exception list, the rule
 453  * must be allowed in the parent device
 454  */
 455 static int parent_has_perm(struct dev_cgroup *childcg,
 456                                   struct dev_exception_item *ex)
 457 {
 458         struct dev_cgroup *parent = css_to_devcgroup(css_parent(&childcg->css));
 459
 460         if (!parent)
 461                 return 1;
 462         return verify_new_ex(parent, ex, childcg->behavior);
 463 }
 464
 465 /**
 466  * may_allow_all - checks if it's possible to change the behavior to
 467  *                 allow based on parent's rules.
 468  * @parent: device cgroup's parent
 469  * returns: != 0 in case it's allowed, 0 otherwise
 470  */
 471 static inline int may_allow_all(struct dev_cgroup *parent)
 472 {
 473         if (!parent)
 474                 return 1;
 475         return parent->behavior == DEVCG_DEFAULT_ALLOW;
 476 }
 477
 478 /**
 479  * revalidate_active_exceptions - walks through the active exception list and
 480  *                                revalidates the exceptions based on parent's
 481  *                                behavior and exceptions. The exceptions that
 482  *                                are no longer valid will be removed.
 483  *                                Called with devcgroup_mutex held.
 484  * @devcg: cgroup which exceptions will be checked
 485  *
 486  * This is one of the three key functions for hierarchy implementation.
 487  * This function is responsible for re-evaluating all the cgroup's active
 488  * exceptions due to a parent's exception change.
 489  * Refer to Documentation/cgroups/devices.txt for more details.
 490  */
 491 static void revalidate_active_exceptions(struct dev_cgroup *devcg)
 492 {
 493         struct dev_exception_item *ex;
 494         struct list_head *this, *tmp;
 495
 496         list_for_each_safe(this, tmp, &devcg->exceptions) {
 497                 ex = container_of(this, struct dev_exception_item, list);
 498                 if (!parent_has_perm(devcg, ex))
 499                         dev_exception_rm(devcg, ex);
 500         }
 501 }
 502
 503 /**
 504  * propagate_exception - propagates a new exception to the children
 505  * @devcg_root: device cgroup that added a new exception
 506  * @ex: new exception to be propagated
 507  *
 508  * returns: 0 in case of success, != 0 in case of error
 509  */
 510 static int propagate_exception(struct dev_cgroup *devcg_root,
 511                                struct dev_exception_item *ex)
 512 {
 513         struct cgroup_subsys_state *pos;
 514         int rc = 0;
 515
 516         rcu_read_lock();
 517
 518         css_for_each_descendant_pre(pos, &devcg_root->css) {
 519                 struct dev_cgroup *devcg = css_to_devcgroup(pos);
 520
 521                 /*
 522                  * Because devcgroup_mutex is held, no devcg will become
 523                  * online or offline during the tree walk (see on/offline
 524                  * methods), and online ones are safe to access outside RCU
 525                  * read lock without bumping refcnt.
 526                  */
 527                 if (pos == &devcg_root->css || !is_devcg_online(devcg))
 528                         continue;
 529
 530                 rcu_read_unlock();
 531
 532                 /*
 533                  * in case both root's behavior and devcg is allow, a new
 534                  * restriction means adding to the exception list
 535                  */
 536                 if (devcg_root->behavior == DEVCG_DEFAULT_ALLOW &&
 537                     devcg->behavior == DEVCG_DEFAULT_ALLOW) {
 538                         rc = dev_exception_add(devcg, ex);
 539                         if (rc)
 540                                 break;
 541                 } else {
 542                         /*
 543                          * in the other possible cases:
 544                          * root's behavior: allow, devcg's: deny
 545                          * root's behavior: deny, devcg's: deny
 546                          * the exception will be removed
 547                          */
 548                         dev_exception_rm(devcg, ex);
 549                 }
 550                 revalidate_active_exceptions(devcg);
 551
 552                 rcu_read_lock();
 553         }
 554
 555         rcu_read_unlock();
 556         return rc;
 557 }
 558
 559 static inline bool has_children(struct dev_cgroup *devcgroup)
 560 {
 561         struct cgroup *cgrp = devcgroup->css.cgroup;
 562
 563         return !list_empty(&cgrp->children);
 564 }
 565
 566 /*
 567  * Modify the exception list using allow/deny rules.
 568  * CAP_SYS_ADMIN is needed for this.  It's at least separate from CAP_MKNOD
 569  * so we can give a container CAP_MKNOD to let it create devices but not
 570  * modify the exception list.
 571  * It seems likely we'll want to add a CAP_CONTAINER capability to allow
 572  * us to also grant CAP_SYS_ADMIN to containers without giving away the
 573  * device exception list controls, but for now we'll stick with CAP_SYS_ADMIN
 574  *
 575  * Taking rules away is always allowed (given CAP_SYS_ADMIN).  Granting
 576  * new access is only allowed if you're in the top-level cgroup, or your
 577  * parent cgroup has the access you're asking for.
 578  */
 579 static int devcgroup_update_access(struct dev_cgroup *devcgroup,
 580                                    int filetype, char *buffer)
 581 {
 582         const char *b;
 583         char temp[12];          /* 11 + 1 characters needed for a u32 */
 584         int count, rc = 0;
 585         struct dev_exception_item ex;
 586         struct dev_cgroup *parent = css_to_devcgroup(css_parent(&devcgroup->css));
 587
 588         if (!capable(CAP_SYS_ADMIN))
 589                 return -EPERM;
 590
 591         memset(&ex, 0, sizeof(ex));
 592         b = buffer;
 593
 594         switch (*b) {
 595         case 'a':
 596                 switch (filetype) {
 597                 case DEVCG_ALLOW:
 598                         if (has_children(devcgroup))
 599                                 return -EINVAL;
 600
 601                         if (!may_allow_all(parent))
 602                                 return -EPERM;
 603                         dev_exception_clean(devcgroup);
 604                         devcgroup->behavior = DEVCG_DEFAULT_ALLOW;
 605                         if (!parent)
 606                                 break;
 607
 608                         rc = dev_exceptions_copy(&devcgroup->exceptions,
 609                                                  &parent->exceptions);
 610                         if (rc)
 611                                 return rc;
 612                         break;
 613                 case DEVCG_DENY:
 614                         if (has_children(devcgroup))
 615                                 return -EINVAL;
 616
 617                         dev_exception_clean(devcgroup);
 618                         devcgroup->behavior = DEVCG_DEFAULT_DENY;
 619                         break;
 620                 default:
 621                         return -EINVAL;
 622                 }
 623                 return 0;
 624         case 'b':
 625                 ex.type = DEV_BLOCK;
 626                 break;
 627         case 'c':
 628                 ex.type = DEV_CHAR;
 629                 break;
 630         default:
 631                 return -EINVAL;
 632         }
 633         b++;
 634         if (!isspace(*b))
 635                 return -EINVAL;
 636         b++;
 637         if (*b == '*') {
 638                 ex.major = ~0;
 639                 b++;
 640         } else if (isdigit(*b)) {
 641                 memset(temp, 0, sizeof(temp));
 642                 for (count = 0; count < sizeof(temp) - 1; count++) {
 643                         temp[count] = *b;
 644                         b++;
 645                         if (!isdigit(*b))
 646                                 break;
 647                 }
 648                 rc = kstrtou32(temp, 10, &ex.major);
 649                 if (rc)
 650                         return -EINVAL;
 651         } else {
 652                 return -EINVAL;
 653         }
 654         if (*b != ':')
 655                 return -EINVAL;
 656         b++;
 657
 658         /* read minor */
 659         if (*b == '*') {
 660                 ex.minor = ~0;
 661                 b++;
 662         } else if (isdigit(*b)) {
 663                 memset(temp, 0, sizeof(temp));
 664                 for (count = 0; count < sizeof(temp) - 1; count++) {
 665                         temp[count] = *b;
 666                         b++;
 667                         if (!isdigit(*b))
 668                                 break;
 669                 }
 670                 rc = kstrtou32(temp, 10, &ex.minor);
 671                 if (rc)
 672                         return -EINVAL;
 673         } else {
 674                 return -EINVAL;
 675         }
 676         if (!isspace(*b))
 677                 return -EINVAL;
 678         for (b++, count = 0; count < 3; count++, b++) {
 679                 switch (*b) {
 680                 case 'r':
 681                         ex.access |= ACC_READ;
 682                         break;
 683                 case 'w':
 684                         ex.access |= ACC_WRITE;
 685                         break;
 686                 case 'm':
 687                         ex.access |= ACC_MKNOD;
 688                         break;
 689                 case '\n':
 690                 case '\0':
 691                         count = 3;
 692                         break;
 693                 default:
 694                         return -EINVAL;
 695                 }
 696         }
 697
 698         switch (filetype) {
 699         case DEVCG_ALLOW:
 700                 if (!parent_has_perm(devcgroup, &ex))
 701                         return -EPERM;
 702                 /*
 703                  * If the default policy is to allow by default, try to remove
 704                  * an matching exception instead. And be silent about it: we
 705                  * don't want to break compatibility
 706                  */
 707                 if (devcgroup->behavior == DEVCG_DEFAULT_ALLOW) {
 708                         dev_exception_rm(devcgroup, &ex);
 709                         return 0;
 710                 }
 711                 rc = dev_exception_add(devcgroup, &ex);
 712                 break;
 713         case DEVCG_DENY:
 714                 /*
 715                  * If the default policy is to deny by default, try to remove
 716                  * an matching exception instead. And be silent about it: we
 717                  * don't want to break compatibility
 718                  */
 719                 if (devcgroup->behavior == DEVCG_DEFAULT_DENY)
 720                         dev_exception_rm(devcgroup, &ex);
 721                 else
 722                         rc = dev_exception_add(devcgroup, &ex);
 723
 724                 if (rc)
 725                         break;
 726                 /* we only propagate new restrictions */
 727                 rc = propagate_exception(devcgroup, &ex);
 728                 break;
 729         default:
 730                 rc = -EINVAL;
 731         }
 732         return rc;
 733 }
 734
 735 static int devcgroup_access_write(struct cgroup_subsys_state *css,
 736                                   struct cftype *cft, char *buffer)
 737 {
 738         int retval;
 739
 740         mutex_lock(&devcgroup_mutex);
 741         retval = devcgroup_update_access(css_to_devcgroup(css),
 742                                          cft->private, buffer);
 743         mutex_unlock(&devcgroup_mutex);
 744         return retval;
 745 }
 746
 747 static struct cftype dev_cgroup_files[] = {
 748         {
 749                 .name = "allow",
 750                 .write_string  = devcgroup_access_write,
 751                 .private = DEVCG_ALLOW,
 752         },
 753         {
 754                 .name = "deny",
 755                 .write_string = devcgroup_access_write,
 756                 .private = DEVCG_DENY,
 757         },
 758         {
 759                 .name = "list",
 760                 .seq_show = devcgroup_seq_show,
 761                 .private = DEVCG_LIST,
 762         },
 763         { }     /* terminate */
 764 };
 765
 766 struct cgroup_subsys devices_cgrp_subsys = {
 767         .css_alloc = devcgroup_css_alloc,
 768         .css_free = devcgroup_css_free,
 769         .css_online = devcgroup_online,
 770         .css_offline = devcgroup_offline,
 771         .base_cftypes = dev_cgroup_files,
 772 };
 773
 774 /**
 775  * __devcgroup_check_permission - checks if an inode operation is permitted
 776  * @dev_cgroup: the dev cgroup to be tested against
 777  * @type: device type
 778  * @major: device major number
 779  * @minor: device minor number
 780  * @access: combination of ACC_WRITE, ACC_READ and ACC_MKNOD
 781  *
 782  * returns 0 on success, -EPERM case the operation is not permitted
 783  */
 784 static int __devcgroup_check_permission(short type, u32 major, u32 minor,
 785                                         short access)
 786 {
 787         struct dev_cgroup *dev_cgroup;
 788         bool rc;
 789
 790         rcu_read_lock();
 791         dev_cgroup = task_devcgroup(current);
 792         if (dev_cgroup->behavior == DEVCG_DEFAULT_ALLOW)
 793                 /* Can't match any of the exceptions, even partially */
 794                 rc = !match_exception_partial(&dev_cgroup->exceptions,
 795                                               type, major, minor, access);
 796         else
 797                 /* Need to match completely one exception to be allowed */
 798                 rc = match_exception(&dev_cgroup->exceptions, type, major,
 799                                      minor, access);
 800         rcu_read_unlock();
 801
 802         if (!rc)
 803                 return -EPERM;
 804
 805         return 0;
 806 }
 807
 808 int __devcgroup_inode_permission(struct inode *inode, int mask)
 809 {
 810         short type, access = 0;
 811
 812         if (S_ISBLK(inode->i_mode))
 813                 type = DEV_BLOCK;
 814         if (S_ISCHR(inode->i_mode))
 815                 type = DEV_CHAR;
 816         if (mask & MAY_WRITE)
 817                 access |= ACC_WRITE;
 818         if (mask & MAY_READ)
 819                 access |= ACC_READ;
 820
 821         return __devcgroup_check_permission(type, imajor(inode), iminor(inode),
 822                         access);
 823 }
 824
 825 int devcgroup_inode_mknod(int mode, dev_t dev)
 826 {
 827         short type;
 828
 829         if (!S_ISBLK(mode) && !S_ISCHR(mode))
 830                 return 0;
 831
 832         if (S_ISBLK(mode))
 833                 type = DEV_BLOCK;
 834         else
 835                 type = DEV_CHAR;
 836
 837         return __devcgroup_check_permission(type, MAJOR(dev), MINOR(dev),
 838                         ACC_MKNOD);
 839
 840 }