KEYS: Perform RCU synchronisation on keys prior to key destruction

Make the keys garbage collector invoke synchronize_rcu() prior to destroying
keys with a zero usage count.  This means that a key can be examined under the
RCU read lock in the safe knowledge that it won't get deallocated until after
the lock is released - even if its usage count becomes zero whilst we're
looking at it.

This is useful in keyring search vs key link.  Consider a keyring containing a
link to a key.  That link can be replaced in-place in the keyring without
requiring an RCU copy-and-replace on the keyring contents without breaking a
search underway on that keyring when the displaced key is released, provided
the key is actually destroyed only after the RCU read lock held by the search
algorithm is released.

This permits __key_link() to replace a key without having to reallocate the key
payload.  A key gets replaced if a new key being linked into a keyring has the
same type and description.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Jeff Layton <jlayton@redhat.com>

diff --git a/include/linux/key.h b/include/linux/key.h
index 96933b1e5d24eeee3dde80b5336b0053a0c1d06e..c505f83c969104e64dd4c656c0704c97f810fb3f 100644 (file)
--- a/include/linux/key.h
+++ b/include/linux/key.h
@@ -124,7 +124,10 @@ static inline unsigned long is_key_possessed(const key_ref_t key_ref)
 struct key {
        atomic_t                usage;          /* number of references */
        key_serial_t            serial;         /* key serial number */
-       struct rb_node          serial_node;
+       union {
+               struct list_head graveyard_link;
+               struct rb_node  serial_node;
+       };
        struct key_type         *type;          /* type of key */
        struct rw_semaphore     sem;            /* change vs change sem */
        struct key_user         *user;          /* owner of this key */

diff --git a/security/keys/gc.c b/security/keys/gc.c
index a42b45531aac173a337498cfc26fd09ded18baee..27610bf7219501aa0e6dffce793efb756ced01b5 100644 (file)
--- a/security/keys/gc.c
+++ b/security/keys/gc.c
@@ -168,38 +168,45 @@ do_gc:
 }
 
 /*
- * Garbage collect an unreferenced, detached key
+ * Garbage collect a list of unreferenced, detached keys
  */
-static noinline void key_gc_unused_key(struct key *key)
+static noinline void key_gc_unused_keys(struct list_head *keys)
 {
-       key_check(key);
-
-       security_key_free(key);
-
-       /* deal with the user's key tracking and quota */
-       if (test_bit(KEY_FLAG_IN_QUOTA, &key->flags)) {
-               spin_lock(&key->user->lock);
-               key->user->qnkeys--;
-               key->user->qnbytes -= key->quotalen;
-               spin_unlock(&key->user->lock);
-       }
+       while (!list_empty(keys)) {
+               struct key *key =
+                       list_entry(keys->next, struct key, graveyard_link);
+               list_del(&key->graveyard_link);
+
+               kdebug("- %u", key->serial);
+               key_check(key);
+
+               security_key_free(key);
+
+               /* deal with the user's key tracking and quota */
+               if (test_bit(KEY_FLAG_IN_QUOTA, &key->flags)) {
+                       spin_lock(&key->user->lock);
+                       key->user->qnkeys--;
+                       key->user->qnbytes -= key->quotalen;
+                       spin_unlock(&key->user->lock);
+               }
 
-       atomic_dec(&key->user->nkeys);
-       if (test_bit(KEY_FLAG_INSTANTIATED, &key->flags))
-               atomic_dec(&key->user->nikeys);
+               atomic_dec(&key->user->nkeys);
+               if (test_bit(KEY_FLAG_INSTANTIATED, &key->flags))
+                       atomic_dec(&key->user->nikeys);
 
-       key_user_put(key->user);
+               key_user_put(key->user);
 
-       /* now throw away the key memory */
-       if (key->type->destroy)
-               key->type->destroy(key);
+               /* now throw away the key memory */
+               if (key->type->destroy)
+                       key->type->destroy(key);
 
-       kfree(key->description);
+               kfree(key->description);
 
 #ifdef KEY_DEBUGGING
-       key->magic = KEY_DEBUG_MAGIC_X;
+               key->magic = KEY_DEBUG_MAGIC_X;
 #endif
-       kmem_cache_free(key_jar, key);
+               kmem_cache_free(key_jar, key);
+       }
 }
 
 /*
@@ -211,6 +218,7 @@ static noinline void key_gc_unused_key(struct key *key)
  */
 static void key_garbage_collector(struct work_struct *work)
 {
+       static LIST_HEAD(graveyard);
        static u8 gc_state;             /* Internal persistent state */
 #define KEY_GC_REAP_AGAIN      0x01    /* - Need another cycle */
 #define KEY_GC_REAPING_LINKS   0x02    /* - We need to reap links */
@@ -316,15 +324,22 @@ maybe_resched:
                key_schedule_gc(new_timer);
        }
 
-       if (unlikely(gc_state & KEY_GC_REAPING_DEAD_2)) {
-               /* Make sure everyone revalidates their keys if we marked a
-                * bunch as being dead and make sure all keyring ex-payloads
-                * are destroyed.
+       if (unlikely(gc_state & KEY_GC_REAPING_DEAD_2) ||
+           !list_empty(&graveyard)) {
+               /* Make sure that all pending keyring payload destructions are
+                * fulfilled and that people aren't now looking at dead or
+                * dying keys that they don't have a reference upon or a link
+                * to.
                 */
-               kdebug("dead sync");
+               kdebug("gc sync");
                synchronize_rcu();
        }
 
+       if (!list_empty(&graveyard)) {
+               kdebug("gc keys");
+               key_gc_unused_keys(&graveyard);
+       }
+
        if (unlikely(gc_state & (KEY_GC_REAPING_DEAD_1 |
                                 KEY_GC_REAPING_DEAD_2))) {
                if (!(gc_state & KEY_GC_FOUND_DEAD_KEY)) {
@@ -359,7 +374,7 @@ found_unreferenced_key:
        rb_erase(&key->serial_node, &key_serial_tree);
        spin_unlock(&key_serial_lock);
 
-       key_gc_unused_key(key);
+       list_add_tail(&key->graveyard_link, &graveyard);
        gc_state |= KEY_GC_REAP_AGAIN;
        goto maybe_resched;