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