security/keys/gc.c

   1 // SPDX-License-Identifier: GPL-2.0-or-later
   2 /* Key garbage collector
   3  *
   4  * Copyright (C) 2009-2011 Red Hat, Inc. All Rights Reserved.
   5  * Written by David Howells (dhowells@redhat.com)
   6  */
   7
   8 #include <linux/slab.h>
   9 #include <linux/security.h>
  10 #include <keys/keyring-type.h>
  11 #include "internal.h"
  12
  13 /*
  14  * Delay between key revocation/expiry in seconds
  15  */
  16 unsigned key_gc_delay = 5 * 60;
  17
  18 /*
  19  * Reaper for unused keys.
  20  */
  21 static void key_garbage_collector(struct work_struct *work);
  22 DECLARE_WORK(key_gc_work, key_garbage_collector);
  23
  24 /*
  25  * Reaper for links from keyrings to dead keys.
  26  */
  27 static void key_gc_timer_func(struct timer_list *);
  28 static DEFINE_TIMER(key_gc_timer, key_gc_timer_func);
  29
  30 static time64_t key_gc_next_run = TIME64_MAX;
  31 static struct key_type *key_gc_dead_keytype;
  32
  33 static unsigned long key_gc_flags;
  34 #define KEY_GC_KEY_EXPIRED      0       /* A key expired and needs unlinking */
  35 #define KEY_GC_REAP_KEYTYPE     1       /* A keytype is being unregistered */
  36 #define KEY_GC_REAPING_KEYTYPE  2       /* Cleared when keytype reaped */
  37
  38
  39 /*
  40  * Any key whose type gets unregistered will be re-typed to this if it can't be
  41  * immediately unlinked.
  42  */
  43 struct key_type key_type_dead = {
  44         .name = ".dead",
  45 };
  46
  47 /*
  48  * Schedule a garbage collection run.
  49  * - time precision isn't particularly important
  50  */
  51 void key_schedule_gc(time64_t gc_at)
  52 {
  53         unsigned long expires;
  54         time64_t now = ktime_get_real_seconds();
  55
  56         kenter("%lld", gc_at - now);
  57
  58         if (gc_at <= now || test_bit(KEY_GC_REAP_KEYTYPE, &key_gc_flags)) {
  59                 kdebug("IMMEDIATE");
  60                 schedule_work(&key_gc_work);
  61         } else if (gc_at < key_gc_next_run) {
  62                 kdebug("DEFERRED");
  63                 key_gc_next_run = gc_at;
  64                 expires = jiffies + (gc_at - now) * HZ;
  65                 mod_timer(&key_gc_timer, expires);
  66         }
  67 }
  68
  69 /*
  70  * Set the expiration time on a key.
  71  */
  72 void key_set_expiry(struct key *key, time64_t expiry)
  73 {
  74         key->expiry = expiry;
  75         if (expiry != TIME64_MAX) {
  76                 if (!(key->type->flags & KEY_TYPE_INSTANT_REAP))
  77                         expiry += key_gc_delay;
  78                 key_schedule_gc(expiry);
  79         }
  80 }
  81
  82 /*
  83  * Schedule a dead links collection run.
  84  */
  85 void key_schedule_gc_links(void)
  86 {
  87         set_bit(KEY_GC_KEY_EXPIRED, &key_gc_flags);
  88         schedule_work(&key_gc_work);
  89 }
  90
  91 /*
  92  * Some key's cleanup time was met after it expired, so we need to get the
  93  * reaper to go through a cycle finding expired keys.
  94  */
  95 static void key_gc_timer_func(struct timer_list *unused)
  96 {
  97         kenter("");
  98         key_gc_next_run = TIME64_MAX;
  99         key_schedule_gc_links();
 100 }
 101
 102 /*
 103  * Reap keys of dead type.
 104  *
 105  * We use three flags to make sure we see three complete cycles of the garbage
 106  * collector: the first to mark keys of that type as being dead, the second to
 107  * collect dead links and the third to clean up the dead keys.  We have to be
 108  * careful as there may already be a cycle in progress.
 109  *
 110  * The caller must be holding key_types_sem.
 111  */
 112 void key_gc_keytype(struct key_type *ktype)
 113 {
 114         kenter("%s", ktype->name);
 115
 116         key_gc_dead_keytype = ktype;
 117         set_bit(KEY_GC_REAPING_KEYTYPE, &key_gc_flags);
 118         smp_mb();
 119         set_bit(KEY_GC_REAP_KEYTYPE, &key_gc_flags);
 120
 121         kdebug("schedule");
 122         schedule_work(&key_gc_work);
 123
 124         kdebug("sleep");
 125         wait_on_bit(&key_gc_flags, KEY_GC_REAPING_KEYTYPE,
 126                     TASK_UNINTERRUPTIBLE);
 127
 128         key_gc_dead_keytype = NULL;
 129         kleave("");
 130 }
 131
 132 /*
 133  * Garbage collect a list of unreferenced, detached keys
 134  */
 135 static noinline void key_gc_unused_keys(struct list_head *keys)
 136 {
 137         while (!list_empty(keys)) {
 138                 struct key *key =
 139                         list_entry(keys->next, struct key, graveyard_link);
 140                 short state = key->state;
 141
 142                 list_del(&key->graveyard_link);
 143
 144                 kdebug("- %u", key->serial);
 145                 key_check(key);
 146
 147 #ifdef CONFIG_KEY_NOTIFICATIONS
 148                 remove_watch_list(key->watchers, key->serial);
 149                 key->watchers = NULL;
 150 #endif
 151
 152                 /* Throw away the key data if the key is instantiated */
 153                 if (state == KEY_IS_POSITIVE && key->type->destroy)
 154                         key->type->destroy(key);
 155
 156                 security_key_free(key);
 157
 158                 atomic_dec(&key->user->nkeys);
 159                 if (state != KEY_IS_UNINSTANTIATED)
 160                         atomic_dec(&key->user->nikeys);
 161
 162                 key_user_put(key->user);
 163                 key_put_tag(key->domain_tag);
 164                 kfree(key->description);
 165
 166                 memzero_explicit(key, sizeof(*key));
 167                 kmem_cache_free(key_jar, key);
 168         }
 169 }
 170
 171 /*
 172  * Garbage collector for unused keys.
 173  *
 174  * This is done in process context so that we don't have to disable interrupts
 175  * all over the place.  key_put() schedules this rather than trying to do the
 176  * cleanup itself, which means key_put() doesn't have to sleep.
 177  */
 178 static void key_garbage_collector(struct work_struct *work)
 179 {
 180         static LIST_HEAD(graveyard);
 181         static u8 gc_state;             /* Internal persistent state */
 182 #define KEY_GC_REAP_AGAIN       0x01    /* - Need another cycle */
 183 #define KEY_GC_REAPING_LINKS    0x02    /* - We need to reap links */
 184 #define KEY_GC_REAPING_DEAD_1   0x10    /* - We need to mark dead keys */
 185 #define KEY_GC_REAPING_DEAD_2   0x20    /* - We need to reap dead key links */
 186 #define KEY_GC_REAPING_DEAD_3   0x40    /* - We need to reap dead keys */
 187 #define KEY_GC_FOUND_DEAD_KEY   0x80    /* - We found at least one dead key */
 188
 189         struct rb_node *cursor;
 190         struct key *key;
 191         time64_t new_timer, limit, expiry;
 192
 193         kenter("[%lx,%x]", key_gc_flags, gc_state);
 194
 195         limit = ktime_get_real_seconds();
 196
 197         /* Work out what we're going to be doing in this pass */
 198         gc_state &= KEY_GC_REAPING_DEAD_1 | KEY_GC_REAPING_DEAD_2;
 199         gc_state <<= 1;
 200         if (test_and_clear_bit(KEY_GC_KEY_EXPIRED, &key_gc_flags))
 201                 gc_state |= KEY_GC_REAPING_LINKS;
 202
 203         if (test_and_clear_bit(KEY_GC_REAP_KEYTYPE, &key_gc_flags))
 204                 gc_state |= KEY_GC_REAPING_DEAD_1;
 205         kdebug("new pass %x", gc_state);
 206
 207         new_timer = TIME64_MAX;
 208
 209         /* As only this function is permitted to remove things from the key
 210          * serial tree, if cursor is non-NULL then it will always point to a
 211          * valid node in the tree - even if lock got dropped.
 212          */
 213         spin_lock(&key_serial_lock);
 214         cursor = rb_first(&key_serial_tree);
 215
 216 continue_scanning:
 217         while (cursor) {
 218                 key = rb_entry(cursor, struct key, serial_node);
 219                 cursor = rb_next(cursor);
 220
 221                 if (refcount_read(&key->usage) == 0)
 222                         goto found_unreferenced_key;
 223
 224                 if (unlikely(gc_state & KEY_GC_REAPING_DEAD_1)) {
 225                         if (key->type == key_gc_dead_keytype) {
 226                                 gc_state |= KEY_GC_FOUND_DEAD_KEY;
 227                                 set_bit(KEY_FLAG_DEAD, &key->flags);
 228                                 key->perm = 0;
 229                                 goto skip_dead_key;
 230                         } else if (key->type == &key_type_keyring &&
 231                                    key->restrict_link) {
 232                                 goto found_restricted_keyring;
 233                         }
 234                 }
 235
 236                 expiry = key->expiry;
 237                 if (expiry != TIME64_MAX) {
 238                         if (!(key->type->flags & KEY_TYPE_INSTANT_REAP))
 239                                 expiry += key_gc_delay;
 240                         if (expiry > limit && expiry < new_timer) {
 241                                 kdebug("will expire %x in %lld",
 242                                        key_serial(key), key->expiry - limit);
 243                                 new_timer = key->expiry;
 244                         }
 245                 }
 246
 247                 if (unlikely(gc_state & KEY_GC_REAPING_DEAD_2))
 248                         if (key->type == key_gc_dead_keytype)
 249                                 gc_state |= KEY_GC_FOUND_DEAD_KEY;
 250
 251                 if ((gc_state & KEY_GC_REAPING_LINKS) ||
 252                     unlikely(gc_state & KEY_GC_REAPING_DEAD_2)) {
 253                         if (key->type == &key_type_keyring)
 254                                 goto found_keyring;
 255                 }
 256
 257                 if (unlikely(gc_state & KEY_GC_REAPING_DEAD_3))
 258                         if (key->type == key_gc_dead_keytype)
 259                                 goto destroy_dead_key;
 260
 261         skip_dead_key:
 262                 if (spin_is_contended(&key_serial_lock) || need_resched())
 263                         goto contended;
 264         }
 265
 266 contended:
 267         spin_unlock(&key_serial_lock);
 268
 269 maybe_resched:
 270         if (cursor) {
 271                 cond_resched();
 272                 spin_lock(&key_serial_lock);
 273                 goto continue_scanning;
 274         }
 275
 276         /* We've completed the pass.  Set the timer if we need to and queue a
 277          * new cycle if necessary.  We keep executing cycles until we find one
 278          * where we didn't reap any keys.
 279          */
 280         kdebug("pass complete");
 281
 282         if (new_timer != TIME64_MAX) {
 283                 new_timer += key_gc_delay;
 284                 key_schedule_gc(new_timer);
 285         }
 286
 287         if (unlikely(gc_state & KEY_GC_REAPING_DEAD_2) ||
 288             !list_empty(&graveyard)) {
 289                 /* Make sure that all pending keyring payload destructions are
 290                  * fulfilled and that people aren't now looking at dead or
 291                  * dying keys that they don't have a reference upon or a link
 292                  * to.
 293                  */
 294                 kdebug("gc sync");
 295                 synchronize_rcu();
 296         }
 297
 298         if (!list_empty(&graveyard)) {
 299                 kdebug("gc keys");
 300                 key_gc_unused_keys(&graveyard);
 301         }
 302
 303         if (unlikely(gc_state & (KEY_GC_REAPING_DEAD_1 |
 304                                  KEY_GC_REAPING_DEAD_2))) {
 305                 if (!(gc_state & KEY_GC_FOUND_DEAD_KEY)) {
 306                         /* No remaining dead keys: short circuit the remaining
 307                          * keytype reap cycles.
 308                          */
 309                         kdebug("dead short");
 310                         gc_state &= ~(KEY_GC_REAPING_DEAD_1 | KEY_GC_REAPING_DEAD_2);
 311                         gc_state |= KEY_GC_REAPING_DEAD_3;
 312                 } else {
 313                         gc_state |= KEY_GC_REAP_AGAIN;
 314                 }
 315         }
 316
 317         if (unlikely(gc_state & KEY_GC_REAPING_DEAD_3)) {
 318                 kdebug("dead wake");
 319                 smp_mb();
 320                 clear_bit(KEY_GC_REAPING_KEYTYPE, &key_gc_flags);
 321                 wake_up_bit(&key_gc_flags, KEY_GC_REAPING_KEYTYPE);
 322         }
 323
 324         if (gc_state & KEY_GC_REAP_AGAIN)
 325                 schedule_work(&key_gc_work);
 326         kleave(" [end %x]", gc_state);
 327         return;
 328
 329         /* We found an unreferenced key - once we've removed it from the tree,
 330          * we can safely drop the lock.
 331          */
 332 found_unreferenced_key:
 333         kdebug("unrefd key %d", key->serial);
 334         rb_erase(&key->serial_node, &key_serial_tree);
 335         spin_unlock(&key_serial_lock);
 336
 337         list_add_tail(&key->graveyard_link, &graveyard);
 338         gc_state |= KEY_GC_REAP_AGAIN;
 339         goto maybe_resched;
 340
 341         /* We found a restricted keyring and need to update the restriction if
 342          * it is associated with the dead key type.
 343          */
 344 found_restricted_keyring:
 345         spin_unlock(&key_serial_lock);
 346         keyring_restriction_gc(key, key_gc_dead_keytype);
 347         goto maybe_resched;
 348
 349         /* We found a keyring and we need to check the payload for links to
 350          * dead or expired keys.  We don't flag another reap immediately as we
 351          * have to wait for the old payload to be destroyed by RCU before we
 352          * can reap the keys to which it refers.
 353          */
 354 found_keyring:
 355         spin_unlock(&key_serial_lock);
 356         keyring_gc(key, limit);
 357         goto maybe_resched;
 358
 359         /* We found a dead key that is still referenced.  Reset its type and
 360          * destroy its payload with its semaphore held.
 361          */
 362 destroy_dead_key:
 363         spin_unlock(&key_serial_lock);
 364         kdebug("destroy key %d", key->serial);
 365         down_write(&key->sem);
 366         key->type = &key_type_dead;
 367         if (key_gc_dead_keytype->destroy)
 368                 key_gc_dead_keytype->destroy(key);
 369         memset(&key->payload, KEY_DESTROY, sizeof(key->payload));
 370         up_write(&key->sem);
 371         goto maybe_resched;
 372 }