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Linux/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 }
651 

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