From 65d87fe68abf2fc226a9e96be61160f65d6b4680 Mon Sep 17 00:00:00 2001 From: David Howells Date: Fri, 11 May 2012 10:56:56 +0100 Subject: [PATCH] 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 Acked-by: Jeff Layton --- include/linux/key.h | 5 +++- security/keys/gc.c | 73 +++++++++++++++++++++++++++------------------ 2 files changed, 48 insertions(+), 30 deletions(-) diff --git a/include/linux/key.h b/include/linux/key.h index 96933b1e5d24..c505f83c9691 100644 --- 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 a42b45531aac..27610bf72195 100644 --- 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; -- 2.20.1