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 /*
  29  * exception list locking rules:
  30  * hold devcgroup_mutex for update/read.
  31  * hold rcu_read_lock() for read.
  32  */
  33
  34 struct dev_exception_item {
  35         u32 major, minor;
  36         short type;
  37         short access;
  38         struct list_head list;
  39         struct rcu_head rcu;
  40 };
  41
  42 struct dev_cgroup {
  43         struct cgroup_subsys_state css;
  44         struct list_head exceptions;
  45         enum {
  46                 DEVCG_DEFAULT_ALLOW,
  47                 DEVCG_DEFAULT_DENY,
  48         } behavior;
  49 };
  50
  51 static inline struct dev_cgroup *css_to_devcgroup(struct cgroup_subsys_state *s)
  52 {
  53         return container_of(s, struct dev_cgroup, css);
  54 }
  55
  56 static inline struct dev_cgroup *cgroup_to_devcgroup(struct cgroup *cgroup)
  57 {
  58         return css_to_devcgroup(cgroup_subsys_state(cgroup, devices_subsys_id));
  59 }
  60
  61 static inline struct dev_cgroup *task_devcgroup(struct task_struct *task)
  62 {
  63         return css_to_devcgroup(task_subsys_state(task, devices_subsys_id));
  64 }
  65
  66 struct cgroup_subsys devices_subsys;
  67
  68 static int devcgroup_can_attach(struct cgroup *new_cgrp,
  69                                 struct cgroup_taskset *set)
  70 {
  71         struct task_struct *task = cgroup_taskset_first(set);
  72
  73         if (current != task && !capable(CAP_SYS_ADMIN))
  74                 return -EPERM;
  75         return 0;
  76 }
  77
  78 /*
  79  * called under devcgroup_mutex
  80  */
  81 static int dev_exceptions_copy(struct list_head *dest, struct list_head *orig)
  82 {
  83         struct dev_exception_item *ex, *tmp, *new;
  84
  85         lockdep_assert_held(&devcgroup_mutex);
  86
  87         list_for_each_entry(ex, orig, list) {
  88                 new = kmemdup(ex, sizeof(*ex), GFP_KERNEL);
  89                 if (!new)
  90                         goto free_and_exit;
  91                 list_add_tail(&new->list, dest);
  92         }
  93
  94         return 0;
  95
  96 free_and_exit:
  97         list_for_each_entry_safe(ex, tmp, dest, list) {
  98                 list_del(&ex->list);
  99                 kfree(ex);
 100         }
 101         return -ENOMEM;
 102 }
 103
 104 /*
 105  * called under devcgroup_mutex
 106  */
 107 static int dev_exception_add(struct dev_cgroup *dev_cgroup,
 108                              struct dev_exception_item *ex)
 109 {
 110         struct dev_exception_item *excopy, *walk;
 111
 112         lockdep_assert_held(&devcgroup_mutex);
 113
 114         excopy = kmemdup(ex, sizeof(*ex), GFP_KERNEL);
 115         if (!excopy)
 116                 return -ENOMEM;
 117
 118         list_for_each_entry(walk, &dev_cgroup->exceptions, list) {
 119                 if (walk->type != ex->type)
 120                         continue;
 121                 if (walk->major != ex->major)
 122                         continue;
 123                 if (walk->minor != ex->minor)
 124                         continue;
 125
 126                 walk->access |= ex->access;
 127                 kfree(excopy);
 128                 excopy = NULL;
 129         }
 130
 131         if (excopy != NULL)
 132                 list_add_tail_rcu(&excopy->list, &dev_cgroup->exceptions);
 133         return 0;
 134 }
 135
 136 /*
 137  * called under devcgroup_mutex
 138  */
 139 static void dev_exception_rm(struct dev_cgroup *dev_cgroup,
 140                              struct dev_exception_item *ex)
 141 {
 142         struct dev_exception_item *walk, *tmp;
 143
 144         lockdep_assert_held(&devcgroup_mutex);
 145
 146         list_for_each_entry_safe(walk, tmp, &dev_cgroup->exceptions, list) {
 147                 if (walk->type != ex->type)
 148                         continue;
 149                 if (walk->major != ex->major)
 150                         continue;
 151                 if (walk->minor != ex->minor)
 152                         continue;
 153
 154                 walk->access &= ~ex->access;
 155                 if (!walk->access) {
 156                         list_del_rcu(&walk->list);
 157                         kfree_rcu(walk, rcu);
 158                 }
 159         }
 160 }
 161
 162 static void __dev_exception_clean(struct dev_cgroup *dev_cgroup)
 163 {
 164         struct dev_exception_item *ex, *tmp;
 165
 166         list_for_each_entry_safe(ex, tmp, &dev_cgroup->exceptions, list) {
 167                 list_del_rcu(&ex->list);
 168                 kfree_rcu(ex, rcu);
 169         }
 170 }
 171
 172 /**
 173  * dev_exception_clean - frees all entries of the exception list
 174  * @dev_cgroup: dev_cgroup with the exception list to be cleaned
 175  *
 176  * called under devcgroup_mutex
 177  */
 178 static void dev_exception_clean(struct dev_cgroup *dev_cgroup)
 179 {
 180         lockdep_assert_held(&devcgroup_mutex);
 181
 182         __dev_exception_clean(dev_cgroup);
 183 }
 184
 185 /*
 186  * called from kernel/cgroup.c with cgroup_lock() held.
 187  */
 188 static struct cgroup_subsys_state *devcgroup_css_alloc(struct cgroup *cgroup)
 189 {
 190         struct dev_cgroup *dev_cgroup, *parent_dev_cgroup;
 191         struct cgroup *parent_cgroup;
 192         int ret;
 193
 194         dev_cgroup = kzalloc(sizeof(*dev_cgroup), GFP_KERNEL);
 195         if (!dev_cgroup)
 196                 return ERR_PTR(-ENOMEM);
 197         INIT_LIST_HEAD(&dev_cgroup->exceptions);
 198         parent_cgroup = cgroup->parent;
 199
 200         if (parent_cgroup == NULL)
 201                 dev_cgroup->behavior = DEVCG_DEFAULT_ALLOW;
 202         else {
 203                 parent_dev_cgroup = cgroup_to_devcgroup(parent_cgroup);
 204                 mutex_lock(&devcgroup_mutex);
 205                 ret = dev_exceptions_copy(&dev_cgroup->exceptions,
 206                                           &parent_dev_cgroup->exceptions);
 207                 dev_cgroup->behavior = parent_dev_cgroup->behavior;
 208                 mutex_unlock(&devcgroup_mutex);
 209                 if (ret) {
 210                         kfree(dev_cgroup);
 211                         return ERR_PTR(ret);
 212                 }
 213         }
 214
 215         return &dev_cgroup->css;
 216 }
 217
 218 static void devcgroup_css_free(struct cgroup *cgroup)
 219 {
 220         struct dev_cgroup *dev_cgroup;
 221
 222         dev_cgroup = cgroup_to_devcgroup(cgroup);
 223         __dev_exception_clean(dev_cgroup);
 224         kfree(dev_cgroup);
 225 }
 226
 227 #define DEVCG_ALLOW 1
 228 #define DEVCG_DENY 2
 229 #define DEVCG_LIST 3
 230
 231 #define MAJMINLEN 13
 232 #define ACCLEN 4
 233
 234 static void set_access(char *acc, short access)
 235 {
 236         int idx = 0;
 237         memset(acc, 0, ACCLEN);
 238         if (access & ACC_READ)
 239                 acc[idx++] = 'r';
 240         if (access & ACC_WRITE)
 241                 acc[idx++] = 'w';
 242         if (access & ACC_MKNOD)
 243                 acc[idx++] = 'm';
 244 }
 245
 246 static char type_to_char(short type)
 247 {
 248         if (type == DEV_ALL)
 249                 return 'a';
 250         if (type == DEV_CHAR)
 251                 return 'c';
 252         if (type == DEV_BLOCK)
 253                 return 'b';
 254         return 'X';
 255 }
 256
 257 static void set_majmin(char *str, unsigned m)
 258 {
 259         if (m == ~0)
 260                 strcpy(str, "*");
 261         else
 262                 sprintf(str, "%u", m);
 263 }
 264
 265 static int devcgroup_seq_read(struct cgroup *cgroup, struct cftype *cft,
 266                                 struct seq_file *m)
 267 {
 268         struct dev_cgroup *devcgroup = cgroup_to_devcgroup(cgroup);
 269         struct dev_exception_item *ex;
 270         char maj[MAJMINLEN], min[MAJMINLEN], acc[ACCLEN];
 271
 272         rcu_read_lock();
 273         /*
 274          * To preserve the compatibility:
 275          * - Only show the "all devices" when the default policy is to allow
 276          * - List the exceptions in case the default policy is to deny
 277          * This way, the file remains as a "whitelist of devices"
 278          */
 279         if (devcgroup->behavior == DEVCG_DEFAULT_ALLOW) {
 280                 set_access(acc, ACC_MASK);
 281                 set_majmin(maj, ~0);
 282                 set_majmin(min, ~0);
 283                 seq_printf(m, "%c %s:%s %s\n", type_to_char(DEV_ALL),
 284                            maj, min, acc);
 285         } else {
 286                 list_for_each_entry_rcu(ex, &devcgroup->exceptions, list) {
 287                         set_access(acc, ex->access);
 288                         set_majmin(maj, ex->major);
 289                         set_majmin(min, ex->minor);
 290                         seq_printf(m, "%c %s:%s %s\n", type_to_char(ex->type),
 291                                    maj, min, acc);
 292                 }
 293         }
 294         rcu_read_unlock();
 295
 296         return 0;
 297 }
 298
 299 /**
 300  * may_access - verifies if a new exception is part of what is allowed
 301  *              by a dev cgroup based on the default policy +
 302  *              exceptions. This is used to make sure a child cgroup
 303  *              won't have more privileges than its parent or to
 304  *              verify if a certain access is allowed.
 305  * @dev_cgroup: dev cgroup to be tested against
 306  * @refex: new exception
 307  */
 308 static bool may_access(struct dev_cgroup *dev_cgroup,
 309                        struct dev_exception_item *refex)
 310 {
 311         struct dev_exception_item *ex;
 312         bool match = false;
 313
 314         rcu_lockdep_assert(rcu_read_lock_held() ||
 315                            lockdep_is_held(&devcgroup_mutex),
 316                            "device_cgroup::may_access() called without proper synchronization");
 317
 318         list_for_each_entry_rcu(ex, &dev_cgroup->exceptions, list) {
 319                 if ((refex->type & DEV_BLOCK) && !(ex->type & DEV_BLOCK))
 320                         continue;
 321                 if ((refex->type & DEV_CHAR) && !(ex->type & DEV_CHAR))
 322                         continue;
 323                 if (ex->major != ~0 && ex->major != refex->major)
 324                         continue;
 325                 if (ex->minor != ~0 && ex->minor != refex->minor)
 326                         continue;
 327                 if (refex->access & (~ex->access))
 328                         continue;
 329                 match = true;
 330                 break;
 331         }
 332
 333         /*
 334          * In two cases we'll consider this new exception valid:
 335          * - the dev cgroup has its default policy to deny + exception list:
 336          *   the new exception *should* match the exceptions
 337          * - the dev cgroup has its default policy to allow + exception list:
 338          *   the new exception should *not* match any of the exceptions
 339          */
 340         if (dev_cgroup->behavior == DEVCG_DEFAULT_DENY) {
 341                 if (match)
 342                         return true;
 343         } else {
 344                 if (!match)
 345                         return true;
 346         }
 347         return false;
 348 }
 349
 350 /*
 351  * parent_has_perm:
 352  * when adding a new allow rule to a device exception list, the rule
 353  * must be allowed in the parent device
 354  */
 355 static int parent_has_perm(struct dev_cgroup *childcg,
 356                                   struct dev_exception_item *ex)
 357 {
 358         struct cgroup *pcg = childcg->css.cgroup->parent;
 359         struct dev_cgroup *parent;
 360
 361         if (!pcg)
 362                 return 1;
 363         parent = cgroup_to_devcgroup(pcg);
 364         return may_access(parent, ex);
 365 }
 366
 367 /**
 368  * may_allow_all - checks if it's possible to change the behavior to
 369  *                 allow based on parent's rules.
 370  * @parent: device cgroup's parent
 371  * returns: != 0 in case it's allowed, 0 otherwise
 372  */
 373 static inline int may_allow_all(struct dev_cgroup *parent)
 374 {
 375         if (!parent)
 376                 return 1;
 377         return parent->behavior == DEVCG_DEFAULT_ALLOW;
 378 }
 379
 380 /*
 381  * Modify the exception list using allow/deny rules.
 382  * CAP_SYS_ADMIN is needed for this.  It's at least separate from CAP_MKNOD
 383  * so we can give a container CAP_MKNOD to let it create devices but not
 384  * modify the exception list.
 385  * It seems likely we'll want to add a CAP_CONTAINER capability to allow
 386  * us to also grant CAP_SYS_ADMIN to containers without giving away the
 387  * device exception list controls, but for now we'll stick with CAP_SYS_ADMIN
 388  *
 389  * Taking rules away is always allowed (given CAP_SYS_ADMIN).  Granting
 390  * new access is only allowed if you're in the top-level cgroup, or your
 391  * parent cgroup has the access you're asking for.
 392  */
 393 static int devcgroup_update_access(struct dev_cgroup *devcgroup,
 394                                    int filetype, const char *buffer)
 395 {
 396         const char *b;
 397         char temp[12];          /* 11 + 1 characters needed for a u32 */
 398         int count, rc;
 399         struct dev_exception_item ex;
 400         struct cgroup *p = devcgroup->css.cgroup;
 401         struct dev_cgroup *parent = NULL;
 402
 403         if (!capable(CAP_SYS_ADMIN))
 404                 return -EPERM;
 405
 406         if (p->parent)
 407                 parent = cgroup_to_devcgroup(p->parent);
 408
 409         memset(&ex, 0, sizeof(ex));
 410         b = buffer;
 411
 412         switch (*b) {
 413         case 'a':
 414                 switch (filetype) {
 415                 case DEVCG_ALLOW:
 416                         if (!may_allow_all(parent))
 417                                 return -EPERM;
 418                         dev_exception_clean(devcgroup);
 419                         devcgroup->behavior = DEVCG_DEFAULT_ALLOW;
 420                         if (!parent)
 421                                 break;
 422
 423                         rc = dev_exceptions_copy(&devcgroup->exceptions,
 424                                                  &parent->exceptions);
 425                         if (rc)
 426                                 return rc;
 427                         break;
 428                 case DEVCG_DENY:
 429                         dev_exception_clean(devcgroup);
 430                         devcgroup->behavior = DEVCG_DEFAULT_DENY;
 431                         break;
 432                 default:
 433                         return -EINVAL;
 434                 }
 435                 return 0;
 436         case 'b':
 437                 ex.type = DEV_BLOCK;
 438                 break;
 439         case 'c':
 440                 ex.type = DEV_CHAR;
 441                 break;
 442         default:
 443                 return -EINVAL;
 444         }
 445         b++;
 446         if (!isspace(*b))
 447                 return -EINVAL;
 448         b++;
 449         if (*b == '*') {
 450                 ex.major = ~0;
 451                 b++;
 452         } else if (isdigit(*b)) {
 453                 memset(temp, 0, sizeof(temp));
 454                 for (count = 0; count < sizeof(temp) - 1; count++) {
 455                         temp[count] = *b;
 456                         b++;
 457                         if (!isdigit(*b))
 458                                 break;
 459                 }
 460                 rc = kstrtou32(temp, 10, &ex.major);
 461                 if (rc)
 462                         return -EINVAL;
 463         } else {
 464                 return -EINVAL;
 465         }
 466         if (*b != ':')
 467                 return -EINVAL;
 468         b++;
 469
 470         /* read minor */
 471         if (*b == '*') {
 472                 ex.minor = ~0;
 473                 b++;
 474         } else if (isdigit(*b)) {
 475                 memset(temp, 0, sizeof(temp));
 476                 for (count = 0; count < sizeof(temp) - 1; count++) {
 477                         temp[count] = *b;
 478                         b++;
 479                         if (!isdigit(*b))
 480                                 break;
 481                 }
 482                 rc = kstrtou32(temp, 10, &ex.minor);
 483                 if (rc)
 484                         return -EINVAL;
 485         } else {
 486                 return -EINVAL;
 487         }
 488         if (!isspace(*b))
 489                 return -EINVAL;
 490         for (b++, count = 0; count < 3; count++, b++) {
 491                 switch (*b) {
 492                 case 'r':
 493                         ex.access |= ACC_READ;
 494                         break;
 495                 case 'w':
 496                         ex.access |= ACC_WRITE;
 497                         break;
 498                 case 'm':
 499                         ex.access |= ACC_MKNOD;
 500                         break;
 501                 case '\n':
 502                 case '\0':
 503                         count = 3;
 504                         break;
 505                 default:
 506                         return -EINVAL;
 507                 }
 508         }
 509
 510         switch (filetype) {
 511         case DEVCG_ALLOW:
 512                 if (!parent_has_perm(devcgroup, &ex))
 513                         return -EPERM;
 514                 /*
 515                  * If the default policy is to allow by default, try to remove
 516                  * an matching exception instead. And be silent about it: we
 517                  * don't want to break compatibility
 518                  */
 519                 if (devcgroup->behavior == DEVCG_DEFAULT_ALLOW) {
 520                         dev_exception_rm(devcgroup, &ex);
 521                         return 0;
 522                 }
 523                 return dev_exception_add(devcgroup, &ex);
 524         case DEVCG_DENY:
 525                 /*
 526                  * If the default policy is to deny by default, try to remove
 527                  * an matching exception instead. And be silent about it: we
 528                  * don't want to break compatibility
 529                  */
 530                 if (devcgroup->behavior == DEVCG_DEFAULT_DENY) {
 531                         dev_exception_rm(devcgroup, &ex);
 532                         return 0;
 533                 }
 534                 return dev_exception_add(devcgroup, &ex);
 535         default:
 536                 return -EINVAL;
 537         }
 538         return 0;
 539 }
 540
 541 static int devcgroup_access_write(struct cgroup *cgrp, struct cftype *cft,
 542                                   const char *buffer)
 543 {
 544         int retval;
 545
 546         mutex_lock(&devcgroup_mutex);
 547         retval = devcgroup_update_access(cgroup_to_devcgroup(cgrp),
 548                                          cft->private, buffer);
 549         mutex_unlock(&devcgroup_mutex);
 550         return retval;
 551 }
 552
 553 static struct cftype dev_cgroup_files[] = {
 554         {
 555                 .name = "allow",
 556                 .write_string  = devcgroup_access_write,
 557                 .private = DEVCG_ALLOW,
 558         },
 559         {
 560                 .name = "deny",
 561                 .write_string = devcgroup_access_write,
 562                 .private = DEVCG_DENY,
 563         },
 564         {
 565                 .name = "list",
 566                 .read_seq_string = devcgroup_seq_read,
 567                 .private = DEVCG_LIST,
 568         },
 569         { }     /* terminate */
 570 };
 571
 572 struct cgroup_subsys devices_subsys = {
 573         .name = "devices",
 574         .can_attach = devcgroup_can_attach,
 575         .css_alloc = devcgroup_css_alloc,
 576         .css_free = devcgroup_css_free,
 577         .subsys_id = devices_subsys_id,
 578         .base_cftypes = dev_cgroup_files,
 579
 580         /*
 581          * While devices cgroup has the rudimentary hierarchy support which
 582          * checks the parent's restriction, it doesn't properly propagates
 583          * config changes in ancestors to their descendents.  A child
 584          * should only be allowed to add more restrictions to the parent's
 585          * configuration.  Fix it and remove the following.
 586          */
 587         .broken_hierarchy = true,
 588 };
 589
 590 /**
 591  * __devcgroup_check_permission - checks if an inode operation is permitted
 592  * @dev_cgroup: the dev cgroup to be tested against
 593  * @type: device type
 594  * @major: device major number
 595  * @minor: device minor number
 596  * @access: combination of ACC_WRITE, ACC_READ and ACC_MKNOD
 597  *
 598  * returns 0 on success, -EPERM case the operation is not permitted
 599  */
 600 static int __devcgroup_check_permission(short type, u32 major, u32 minor,
 601                                         short access)
 602 {
 603         struct dev_cgroup *dev_cgroup;
 604         struct dev_exception_item ex;
 605         int rc;
 606
 607         memset(&ex, 0, sizeof(ex));
 608         ex.type = type;
 609         ex.major = major;
 610         ex.minor = minor;
 611         ex.access = access;
 612
 613         rcu_read_lock();
 614         dev_cgroup = task_devcgroup(current);
 615         rc = may_access(dev_cgroup, &ex);
 616         rcu_read_unlock();
 617
 618         if (!rc)
 619                 return -EPERM;
 620
 621         return 0;
 622 }
 623
 624 int __devcgroup_inode_permission(struct inode *inode, int mask)
 625 {
 626         short type, access = 0;
 627
 628         if (S_ISBLK(inode->i_mode))
 629                 type = DEV_BLOCK;
 630         if (S_ISCHR(inode->i_mode))
 631                 type = DEV_CHAR;
 632         if (mask & MAY_WRITE)
 633                 access |= ACC_WRITE;
 634         if (mask & MAY_READ)
 635                 access |= ACC_READ;
 636
 637         return __devcgroup_check_permission(type, imajor(inode), iminor(inode),
 638                         access);
 639 }
 640
 641 int devcgroup_inode_mknod(int mode, dev_t dev)
 642 {
 643         short type;
 644
 645         if (!S_ISBLK(mode) && !S_ISCHR(mode))
 646                 return 0;
 647
 648         if (S_ISBLK(mode))
 649                 type = DEV_BLOCK;
 650         else
 651                 type = DEV_CHAR;
 652
 653         return __devcgroup_check_permission(type, MAJOR(dev), MINOR(dev),
 654                         ACC_MKNOD);
 655
 656 }