The Design and Implementation of the FreeBSD Operating System, Second Edition
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FreeBSD/Linux Kernel Cross Reference
sys/security/device_cgroup.c

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

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