[PATCH] keys: Discard key spinlock and use RCU for key payload

The attached patch changes the key implementation in a number of ways:

 (1) It removes the spinlock from the key structure.

 (2) The key flags are now accessed using atomic bitops instead of
     write-locking the key spinlock and using C bitwise operators.

     The three instantiation flags are dealt with with the construction
     semaphore held during the request_key/instantiate/negate sequence, thus
     rendering the spinlock superfluous.

     The key flags are also now bit numbers not bit masks.

 (3) The key payload is now accessed using RCU. This permits the recursive
     keyring search algorithm to be simplified greatly since no locks need be
     taken other than the usual RCU preemption disablement. Searching now does
     not require any locks or semaphores to be held; merely that the starting
     keyring be pinned.

 (4) The keyring payload now includes an RCU head so that it can be disposed
     of by call_rcu(). This requires that the payload be copied on unlink to
     prevent introducing races in copy-down vs search-up.

 (5) The user key payload is now a structure with the data following it. It
     includes an RCU head like the keyring payload and for the same reason. It
     also contains a data length because the data length in the key may be
     changed on another CPU whilst an RCU protected read is in progress on the
     payload. This would then see the supposed RCU payload and the on-key data
     length getting out of sync.

     I'm tempted to drop the key's datalen entirely, except that it's used in
     conjunction with quota management and so is a little tricky to get rid
     of.

 (6) Update the keys documentation.
Signed-Off-By: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/key-ui.h

@@ -31,8 +31,10 @@ extern spinlock_t key_serial_lock;
  * subscribed
  */
 struct keyring_list {
-	unsigned	maxkeys;	/* max keys this list can hold */
-	unsigned	nkeys;		/* number of keys currently held */
+	struct rcu_head	rcu;		/* RCU deletion hook */
+	unsigned short	maxkeys;	/* max keys this list can hold */
+	unsigned short	nkeys;		/* number of keys currently held */
+	unsigned short	delkey;		/* key to be unlinked by RCU */
 	struct key	*keys[0];
 };
 

include/linux/key.h

@@ -18,7 +18,7 @@
 #include <linux/types.h>
 #include <linux/list.h>
 #include <linux/rbtree.h>
-#include <linux/spinlock.h>
+#include <linux/rcupdate.h>
 #include <asm/atomic.h>
 
 #ifdef __KERNEL__
@@ -78,7 +78,6 @@ struct key {
 	key_serial_t		serial;		/* key serial number */
 	struct rb_node		serial_node;
 	struct key_type		*type;		/* type of key */
-	rwlock_t		lock;		/* examination vs change lock */
 	struct rw_semaphore	sem;		/* change vs change sem */
 	struct key_user		*user;		/* owner of this key */
 	time_t			expiry;		/* time at which key expires (or 0) */
@@ -86,14 +85,10 @@ struct key {
 	gid_t			gid;
 	key_perm_t		perm;		/* access permissions */
 	unsigned short		quotalen;	/* length added to quota */
-	unsigned short		datalen;	/* payload data length */
-	unsigned short		flags;		/* status flags (change with lock writelocked) */
-#define KEY_FLAG_INSTANTIATED	0x00000001	/* set if key has been instantiated */
-#define KEY_FLAG_DEAD		0x00000002	/* set if key type has been deleted */
-#define KEY_FLAG_REVOKED	0x00000004	/* set if key had been revoked */
-#define KEY_FLAG_IN_QUOTA	0x00000008	/* set if key consumes quota */
-#define KEY_FLAG_USER_CONSTRUCT	0x00000010	/* set if key is being constructed in userspace */
-#define KEY_FLAG_NEGATIVE	0x00000020	/* set if key is negative */
+	unsigned short		datalen;	/* payload data length
+						 * - may not match RCU dereferenced payload
+						 * - payload should contain own length
+						 */
 
 #ifdef KEY_DEBUGGING
 	unsigned		magic;
@@ -101,6 +96,14 @@ struct key {
 #define KEY_DEBUG_MAGIC_X	0xf8e9dacbu
 #endif
 
+	unsigned long		flags;		/* status flags (change with bitops) */
+#define KEY_FLAG_INSTANTIATED	0	/* set if key has been instantiated */
+#define KEY_FLAG_DEAD		1	/* set if key type has been deleted */
+#define KEY_FLAG_REVOKED	2	/* set if key had been revoked */
+#define KEY_FLAG_IN_QUOTA	3	/* set if key consumes quota */
+#define KEY_FLAG_USER_CONSTRUCT	4	/* set if key is being constructed in userspace */
+#define KEY_FLAG_NEGATIVE	5	/* set if key is negative */
+
 	/* the description string
 	 * - this is used to match a key against search criteria
 	 * - this should be a printable string
@@ -250,6 +253,8 @@ extern int keyring_add_key(struct key *keyring,
 
 extern struct key *key_lookup(key_serial_t id);
 
+extern void keyring_replace_payload(struct key *key, void *replacement);
+
 #define key_serial(key) ((key) ? (key)->serial : 0)
 
 /*

security/keys/key.c

@@ -294,7 +294,6 @@ struct key *key_alloc(struct key_type *type, const char *desc,
 	}
 
 	atomic_set(&key->usage, 1);
-	rwlock_init(&key->lock);
 	init_rwsem(&key->sem);
 	key->type = type;
 	key->user = user;
@@ -308,7 +307,7 @@ struct key *key_alloc(struct key_type *type, const char *desc,
 	key->payload.data = NULL;
 
 	if (!not_in_quota)
-		key->flags |= KEY_FLAG_IN_QUOTA;
+		key->flags |= 1 << KEY_FLAG_IN_QUOTA;
 
 	memset(&key->type_data, 0, sizeof(key->type_data));
 
@@ -359,7 +358,7 @@ int key_payload_reserve(struct key *key, size_t datalen)
 	key_check(key);
 
 	/* contemplate the quota adjustment */
-	if (delta != 0 && key->flags & KEY_FLAG_IN_QUOTA) {
+	if (delta != 0 && test_bit(KEY_FLAG_IN_QUOTA, &key->flags)) {
 		spin_lock(&key->user->lock);
 
 		if (delta > 0 &&
@@ -405,23 +404,17 @@ static int __key_instantiate_and_link(struct key *key,
 	down_write(&key_construction_sem);
 
 	/* can't instantiate twice */
-	if (!(key->flags & KEY_FLAG_INSTANTIATED)) {
+	if (!test_bit(KEY_FLAG_INSTANTIATED, &key->flags)) {
 		/* instantiate the key */
 		ret = key->type->instantiate(key, data, datalen);
 
 		if (ret == 0) {
 			/* mark the key as being instantiated */
-			write_lock(&key->lock);
-
 			atomic_inc(&key->user->nikeys);
-			key->flags |= KEY_FLAG_INSTANTIATED;
+			set_bit(KEY_FLAG_INSTANTIATED, &key->flags);
 
-			if (key->flags & KEY_FLAG_USER_CONSTRUCT) {
-				key->flags &= ~KEY_FLAG_USER_CONSTRUCT;
+			if (test_and_clear_bit(KEY_FLAG_USER_CONSTRUCT, &key->flags))
 				awaken = 1;
-			}
-
-			write_unlock(&key->lock);
 
 			/* and link it into the destination keyring */
 			if (keyring)
@@ -486,21 +479,17 @@ int key_negate_and_link(struct key *key,
 	down_write(&key_construction_sem);
 
 	/* can't instantiate twice */
-	if (!(key->flags & KEY_FLAG_INSTANTIATED)) {
+	if (!test_bit(KEY_FLAG_INSTANTIATED, &key->flags)) {
 		/* mark the key as being negatively instantiated */
-		write_lock(&key->lock);
-
 		atomic_inc(&key->user->nikeys);
-		key->flags |= KEY_FLAG_INSTANTIATED | KEY_FLAG_NEGATIVE;
+		set_bit(KEY_FLAG_NEGATIVE, &key->flags);
+		set_bit(KEY_FLAG_INSTANTIATED, &key->flags);
 		now = current_kernel_time();
 		key->expiry = now.tv_sec + timeout;
 
-		if (key->flags & KEY_FLAG_USER_CONSTRUCT) {
-			key->flags &= ~KEY_FLAG_USER_CONSTRUCT;
+		if (test_and_clear_bit(KEY_FLAG_USER_CONSTRUCT, &key->flags))
 			awaken = 1;
-		}
 
-		write_unlock(&key->lock);
 		ret = 0;
 
 		/* and link it into the destination keyring */
@@ -553,8 +542,10 @@ static void key_cleanup(void *data)
 	rb_erase(&key->serial_node, &key_serial_tree);
 	spin_unlock(&key_serial_lock);
 
+	key_check(key);
+
 	/* deal with the user's key tracking and quota */
-	if (key->flags & KEY_FLAG_IN_QUOTA) {
+	if (test_bit(KEY_FLAG_IN_QUOTA, &key->flags)) {
 		spin_lock(&key->user->lock);
 		key->user->qnkeys--;
 		key->user->qnbytes -= key->quotalen;
@@ -562,7 +553,7 @@ static void key_cleanup(void *data)
 	}
 
 	atomic_dec(&key->user->nkeys);
-	if (key->flags & KEY_FLAG_INSTANTIATED)
+	if (test_bit(KEY_FLAG_INSTANTIATED, &key->flags))
 		atomic_dec(&key->user->nikeys);
 
 	key_user_put(key->user);
@@ -631,9 +622,9 @@ struct key *key_lookup(key_serial_t id)
 	goto error;
 
  found:
-	/* pretent doesn't exist if it's dead */
+	/* pretend it doesn't exist if it's dead */
 	if (atomic_read(&key->usage) == 0 ||
-	    (key->flags & KEY_FLAG_DEAD) ||
+	    test_bit(KEY_FLAG_DEAD, &key->flags) ||
 	    key->type == &key_type_dead)
 		goto not_found;
 
@@ -708,12 +699,9 @@ static inline struct key *__key_update(struct key *key, const void *payload,
 
 	ret = key->type->update(key, payload, plen);
 
-	if (ret == 0) {
+	if (ret == 0)
 		/* updating a negative key instantiates it */
-		write_lock(&key->lock);
-		key->flags &= ~KEY_FLAG_NEGATIVE;
-		write_unlock(&key->lock);
-	}
+		clear_bit(KEY_FLAG_NEGATIVE, &key->flags);
 
 	up_write(&key->sem);
 
@@ -841,12 +829,9 @@ int key_update(struct key *key, const void *payload, size_t plen)
 		down_write(&key->sem);
 		ret = key->type->update(key, payload, plen);
 
-		if (ret == 0) {
+		if (ret == 0)
 			/* updating a negative key instantiates it */
-			write_lock(&key->lock);
-			key->flags &= ~KEY_FLAG_NEGATIVE;
-			write_unlock(&key->lock);
-		}
+			clear_bit(KEY_FLAG_NEGATIVE, &key->flags);
 
 		up_write(&key->sem);
 	}
@@ -892,10 +877,7 @@ struct key *key_duplicate(struct key *source, const char *desc)
 		goto error2;
 
 	atomic_inc(&key->user->nikeys);
-
-	write_lock(&key->lock);
-	key->flags |= KEY_FLAG_INSTANTIATED;
-	write_unlock(&key->lock);
+	set_bit(KEY_FLAG_INSTANTIATED, &key->flags);
 
  error_k:
 	up_read(&key_types_sem);
@@ -922,9 +904,7 @@ void key_revoke(struct key *key)
 	/* make sure no one's trying to change or use the key when we mark
 	 * it */
 	down_write(&key->sem);
-	write_lock(&key->lock);
-	key->flags |= KEY_FLAG_REVOKED;
-	write_unlock(&key->lock);
+	set_bit(KEY_FLAG_REVOKED, &key->flags);
 	up_write(&key->sem);
 
 } /* end key_revoke() */
@@ -975,24 +955,33 @@ void unregister_key_type(struct key_type *ktype)
 	/* withdraw the key type */
 	list_del_init(&ktype->link);
 
-	/* need to withdraw all keys of this type */
+	/* mark all the keys of this type dead */
 	spin_lock(&key_serial_lock);
 
 	for (_n = rb_first(&key_serial_tree); _n; _n = rb_next(_n)) {
 		key = rb_entry(_n, struct key, serial_node);
 
-		if (key->type != ktype)
-			continue;
+		if (key->type == ktype)
+			key->type = &key_type_dead;
+	}
+
+	spin_unlock(&key_serial_lock);
+
+	/* make sure everyone revalidates their keys */
+	synchronize_kernel();
+
+	/* we should now be able to destroy the payloads of all the keys of
+	 * this type with impunity */
+	spin_lock(&key_serial_lock);
 
-		write_lock(&key->lock);
-		key->type = &key_type_dead;
-		write_unlock(&key->lock);
+	for (_n = rb_first(&key_serial_tree); _n; _n = rb_next(_n)) {
+		key = rb_entry(_n, struct key, serial_node);
 
-		/* there shouldn't be anyone looking at the description or
-		 * payload now */
-		if (ktype->destroy)
-			ktype->destroy(key);
-		memset(&key->payload, 0xbd, sizeof(key->payload));
+		if (key->type == ktype) {
+			if (ktype->destroy)
+				ktype->destroy(key);
+			memset(&key->payload, 0xbd, sizeof(key->payload));
+		}
 	}
 
 	spin_unlock(&key_serial_lock);
@@ -1037,4 +1026,5 @@ void __init key_init(void)
 
 	/* link the two root keyrings together */
 	key_link(&root_session_keyring, &root_user_keyring);
+
 } /* end key_init() */

security/keys/keyctl.c

@@ -728,7 +728,6 @@ long keyctl_chown_key(key_serial_t id, uid_t uid, gid_t gid)
 	/* make the changes with the locks held to prevent chown/chown races */
 	ret = -EACCES;
 	down_write(&key->sem);
-	write_lock(&key->lock);
 
 	if (!capable(CAP_SYS_ADMIN)) {
 		/* only the sysadmin can chown a key to some other UID */
@@ -755,7 +754,6 @@ long keyctl_chown_key(key_serial_t id, uid_t uid, gid_t gid)
 	ret = 0;
 
  no_access:
-	write_unlock(&key->lock);
 	up_write(&key->sem);
 	key_put(key);
  error:
@@ -784,26 +782,19 @@ long keyctl_setperm_key(key_serial_t id, key_perm_t perm)
 		goto error;
 	}
 
-	/* make the changes with the locks held to prevent chown/chmod
-	 * races */
+	/* make the changes with the locks held to prevent chown/chmod races */
 	ret = -EACCES;
 	down_write(&key->sem);
-	write_lock(&key->lock);
 
-	/* if we're not the sysadmin, we can only chmod a key that we
-	 * own */
-	if (!capable(CAP_SYS_ADMIN) && key->uid != current->fsuid)
-		goto no_access;
-
-	/* changing the permissions mask */
-	key->perm = perm;
-	ret = 0;
+	/* if we're not the sysadmin, we can only change a key that we own */
+	if (capable(CAP_SYS_ADMIN) || key->uid == current->fsuid) {
+		key->perm = perm;
+		ret = 0;
+	}
 
- no_access:
-	write_unlock(&key->lock);
 	up_write(&key->sem);
 	key_put(key);
- error:
+error:
 	return ret;
 
 } /* end keyctl_setperm_key() */

security/keys/proc.c

@@ -140,7 +140,7 @@ static int proc_keys_show(struct seq_file *m, void *v)
 
 	now = current_kernel_time();
 
-	read_lock(&key->lock);
+	rcu_read_lock();
 
 	/* come up with a suitable timeout value */
 	if (key->expiry == 0) {
@@ -164,14 +164,17 @@ static int proc_keys_show(struct seq_file *m, void *v)
 			sprintf(xbuf, "%luw", timo / (60*60*24*7));
 	}
 
+#define showflag(KEY, LETTER, FLAG) \
+	(test_bit(FLAG,	&(KEY)->flags) ? LETTER : '-')
+
 	seq_printf(m, "%08x %c%c%c%c%c%c %5d %4s %06x %5d %5d %-9.9s ",
 		   key->serial,
-		   key->flags & KEY_FLAG_INSTANTIATED	? 'I' : '-',
-		   key->flags & KEY_FLAG_REVOKED	? 'R' : '-',
-		   key->flags & KEY_FLAG_DEAD		? 'D' : '-',
-		   key->flags & KEY_FLAG_IN_QUOTA	? 'Q' : '-',
-		   key->flags & KEY_FLAG_USER_CONSTRUCT	? 'U' : '-',
-		   key->flags & KEY_FLAG_NEGATIVE	? 'N' : '-',
+		   showflag(key, 'I', KEY_FLAG_INSTANTIATED),
+		   showflag(key, 'R', KEY_FLAG_REVOKED),
+		   showflag(key, 'D', KEY_FLAG_DEAD),
+		   showflag(key, 'Q', KEY_FLAG_IN_QUOTA),
+		   showflag(key, 'U', KEY_FLAG_USER_CONSTRUCT),
+		   showflag(key, 'N', KEY_FLAG_NEGATIVE),
 		   atomic_read(&key->usage),
 		   xbuf,
 		   key->perm,
@@ -179,11 +182,13 @@ static int proc_keys_show(struct seq_file *m, void *v)
 		   key->gid,
 		   key->type->name);
 
+#undef showflag
+
 	if (key->type->describe)
 		key->type->describe(key, m);
 	seq_putc(m, '\n');
 
-	read_unlock(&key->lock);
+	rcu_read_unlock();
 
 	return 0;
 

security/keys/process_keys.c

@@ -38,10 +38,9 @@ struct key root_user_keyring = {
 	.serial		= 2,
 	.type		= &key_type_keyring,
 	.user		= &root_key_user,
-	.lock		= RW_LOCK_UNLOCKED,
 	.sem		= __RWSEM_INITIALIZER(root_user_keyring.sem),
 	.perm		= KEY_USR_ALL,
-	.flags		= KEY_FLAG_INSTANTIATED,
+	.flags		= 1 << KEY_FLAG_INSTANTIATED,
 	.description	= "_uid.0",
 #ifdef KEY_DEBUGGING
 	.magic		= KEY_DEBUG_MAGIC,
@@ -54,10 +53,9 @@ struct key root_session_keyring = {
 	.serial		= 1,
 	.type		= &key_type_keyring,
 	.user		= &root_key_user,
-	.lock		= RW_LOCK_UNLOCKED,
 	.sem		= __RWSEM_INITIALIZER(root_session_keyring.sem),
 	.perm		= KEY_USR_ALL,
-	.flags		= KEY_FLAG_INSTANTIATED,
+	.flags		= 1 << KEY_FLAG_INSTANTIATED,
 	.description	= "_uid_ses.0",
 #ifdef KEY_DEBUGGING
 	.magic		= KEY_DEBUG_MAGIC,
@@ -349,9 +347,7 @@ void key_fsuid_changed(struct task_struct *tsk)
 	/* update the ownership of the thread keyring */
 	if (tsk->thread_keyring) {
 		down_write(&tsk->thread_keyring->sem);
-		write_lock(&tsk->thread_keyring->lock);
 		tsk->thread_keyring->uid = tsk->fsuid;
-		write_unlock(&tsk->thread_keyring->lock);
 		up_write(&tsk->thread_keyring->sem);
 	}
 
@@ -366,9 +362,7 @@ void key_fsgid_changed(struct task_struct *tsk)
 	/* update the ownership of the thread keyring */
 	if (tsk->thread_keyring) {
 		down_write(&tsk->thread_keyring->sem);
-		write_lock(&tsk->thread_keyring->lock);
 		tsk->thread_keyring->gid = tsk->fsgid;
-		write_unlock(&tsk->thread_keyring->lock);
 		up_write(&tsk->thread_keyring->sem);
 	}
 
@@ -588,7 +582,7 @@ struct key *lookup_user_key(key_serial_t id, int create, int partial,
 	}
 
 	ret = -EIO;
-	if (!partial && !(key->flags & KEY_FLAG_INSTANTIATED))
+	if (!partial && !test_bit(KEY_FLAG_INSTANTIATED, &key->flags))
 		goto invalid_key;
 
 	ret = -EACCES;

security/keys/request_key.c

@@ -105,7 +105,7 @@ static struct key *__request_key_construction(struct key_type *type,
 	struct key_construction cons;
 	struct timespec now;
 	struct key *key;
-	int ret, negative;
+	int ret, negated;
 
 	/* create a key and add it to the queue */
 	key = key_alloc(type, description,
@@ -113,9 +113,7 @@ static struct key *__request_key_construction(struct key_type *type,
 	if (IS_ERR(key))
 		goto alloc_failed;
 
-	write_lock(&key->lock);
-	key->flags |= KEY_FLAG_USER_CONSTRUCT;
-	write_unlock(&key->lock);
+	set_bit(KEY_FLAG_USER_CONSTRUCT, &key->flags);
 
 	cons.key = key;
 	list_add_tail(&cons.link, &key->user->consq);
@@ -130,7 +128,7 @@ static struct key *__request_key_construction(struct key_type *type,
 
 	/* if the key wasn't instantiated, then we want to give an error */
 	ret = -ENOKEY;
-	if (!(key->flags & KEY_FLAG_INSTANTIATED))
+	if (!test_bit(KEY_FLAG_INSTANTIATED, &key->flags))
 		goto request_failed;
 
 	down_write(&key_construction_sem);
@@ -139,7 +137,7 @@ static struct key *__request_key_construction(struct key_type *type,
 
 	/* also give an error if the key was negatively instantiated */
  check_not_negative:
-	if (key->flags & KEY_FLAG_NEGATIVE) {
+	if (test_bit(KEY_FLAG_NEGATIVE, &key->flags)) {
 		key_put(key);
 		key = ERR_PTR(-ENOKEY);
 	}
@@ -152,24 +150,23 @@ static struct key *__request_key_construction(struct key_type *type,
 	 * - remove from construction queue
 	 * - mark the key as dead
 	 */
-	negative = 0;
+	negated = 0;
 	down_write(&key_construction_sem);
 
 	list_del(&cons.link);
 
-	write_lock(&key->lock);
-	key->flags &= ~KEY_FLAG_USER_CONSTRUCT;
-
 	/* check it didn't get instantiated between the check and the down */
-	if (!(key->flags & KEY_FLAG_INSTANTIATED)) {
-		key->flags |= KEY_FLAG_INSTANTIATED | KEY_FLAG_NEGATIVE;
-		negative = 1;
+	if (!test_bit(KEY_FLAG_INSTANTIATED, &key->flags)) {
+		set_bit(KEY_FLAG_NEGATIVE, &key->flags);
+		set_bit(KEY_FLAG_INSTANTIATED, &key->flags);
+		negated = 1;
 	}
 
-	write_unlock(&key->lock);
+	clear_bit(KEY_FLAG_USER_CONSTRUCT, &key->flags);
+
 	up_write(&key_construction_sem);
 
-	if (!negative)
+	if (!negated)
 		goto check_not_negative; /* surprisingly, the key got
 					  * instantiated */
 
@@ -250,7 +247,7 @@ static struct key *request_key_construction(struct key_type *type,
 
 	for (;;) {
 		set_current_state(TASK_UNINTERRUPTIBLE);
-		if (!(ckey->flags & KEY_FLAG_USER_CONSTRUCT))
+		if (!test_bit(KEY_FLAG_USER_CONSTRUCT, &ckey->flags))
 			break;
 		schedule();
 	}
@@ -339,7 +336,8 @@ int key_validate(struct key *key)
 	if (key) {
 		/* check it's still accessible */
 		ret = -EKEYREVOKED;
-		if (key->flags & (KEY_FLAG_REVOKED | KEY_FLAG_DEAD))
+		if (test_bit(KEY_FLAG_REVOKED, &key->flags) ||
+		    test_bit(KEY_FLAG_DEAD, &key->flags))
 			goto error;
 
 		/* check it hasn't expired */

security/keys/user_defined.c

@@ -42,12 +42,19 @@ struct key_type key_type_user = {
 	.read		= user_read,
 };
 
+struct user_key_payload {
+	struct rcu_head	rcu;		/* RCU destructor */
+	unsigned short	datalen;	/* length of this data */
+	char		data[0];	/* actual data */
+};
+
 /*****************************************************************************/
 /*
  * instantiate a user defined key
  */
 static int user_instantiate(struct key *key, const void *data, size_t datalen)
 {
+	struct user_key_payload *upayload;
 	int ret;
 
 	ret = -EINVAL;
@@ -58,13 +65,15 @@ static int user_instantiate(struct key *key, const void *data, size_t datalen)
 	if (ret < 0)
 		goto error;
 
-	/* attach the data */
 	ret = -ENOMEM;
-	key->payload.data = kmalloc(datalen, GFP_KERNEL);
-	if (!key->payload.data)
+	upayload = kmalloc(sizeof(*upayload) + datalen, GFP_KERNEL);
+	if (!upayload)
 		goto error;
 
-	memcpy(key->payload.data, data, datalen);
+	/* attach the data */
+	upayload->datalen = datalen;
+	memcpy(upayload->data, data, datalen);
+	rcu_assign_pointer(key->payload.data, upayload);
 	ret = 0;
 
  error:
@@ -75,18 +84,25 @@ static int user_instantiate(struct key *key, const void *data, size_t datalen)
 /*****************************************************************************/
 /*
  * duplicate a user defined key
+ * - both keys' semaphores are locked against further modification
+ * - the new key cannot yet be accessed
  */
 static int user_duplicate(struct key *key, const struct key *source)
 {
+	struct user_key_payload *upayload, *spayload;
 	int ret;
 
 	/* just copy the payload */
 	ret = -ENOMEM;
-	key->payload.data = kmalloc(source->datalen, GFP_KERNEL);
+	upayload = kmalloc(sizeof(*upayload) + source->datalen, GFP_KERNEL);
+	if (upayload) {
+		spayload = rcu_dereference(source->payload.data);
+		BUG_ON(source->datalen != spayload->datalen);
 
-	if (key->payload.data) {
-		key->datalen = source->datalen;
-		memcpy(key->payload.data, source->payload.data, source->datalen);
+		upayload->datalen = key->datalen = spayload->datalen;
+		memcpy(upayload->data, spayload->data, key->datalen);
+
+		key->payload.data = upayload;
 		ret = 0;
 	}
 
@@ -94,42 +110,56 @@ static int user_duplicate(struct key *key, const struct key *source)
 
 } /* end user_duplicate() */
 
+/*****************************************************************************/
+/*
+ * dispose of the old data from an updated user defined key
+ */
+static void user_update_rcu_disposal(struct rcu_head *rcu)
+{
+	struct user_key_payload *upayload;
+
+	upayload = container_of(rcu, struct user_key_payload, rcu);
+
+	kfree(upayload);
+
+} /* end user_update_rcu_disposal() */
+
 /*****************************************************************************/
 /*
  * update a user defined key
+ * - the key's semaphore is write-locked
  */
 static int user_update(struct key *key, const void *data, size_t datalen)
 {
-	void *new, *zap;
+	struct user_key_payload *upayload, *zap;
 	int ret;
 
 	ret = -EINVAL;
 	if (datalen <= 0 || datalen > 32767 || !data)
 		goto error;
 
-	/* copy the data */
+	/* construct a replacement payload */
 	ret = -ENOMEM;
-	new = kmalloc(datalen, GFP_KERNEL);
-	if (!new)
+	upayload = kmalloc(sizeof(*upayload) + datalen, GFP_KERNEL);
+	if (!upayload)
 		goto error;
 
-	memcpy(new, data, datalen);
+	upayload->datalen = datalen;
+	memcpy(upayload->data, data, datalen);
 
 	/* check the quota and attach the new data */
-	zap = new;
-	write_lock(&key->lock);
+	zap = upayload;
 
 	ret = key_payload_reserve(key, datalen);
 
 	if (ret == 0) {
 		/* attach the new data, displacing the old */
 		zap = key->payload.data;
-		key->payload.data = new;
+		rcu_assign_pointer(key->payload.data, upayload);
 		key->expiry = 0;
 	}
 
-	write_unlock(&key->lock);
-	kfree(zap);
+	call_rcu(&zap->rcu, user_update_rcu_disposal);
 
  error:
 	return ret;
@@ -152,13 +182,15 @@ static int user_match(const struct key *key, const void *description)
  */
 static void user_destroy(struct key *key)
 {
-	kfree(key->payload.data);
+	struct user_key_payload *upayload = key->payload.data;
+
+	kfree(upayload);
 
 } /* end user_destroy() */
 
 /*****************************************************************************/
 /*
- * describe the user
+ * describe the user key
  */
 static void user_describe(const struct key *key, struct seq_file *m)
 {
@@ -171,18 +203,23 @@ static void user_describe(const struct key *key, struct seq_file *m)
 /*****************************************************************************/
 /*
  * read the key data
+ * - the key's semaphore is read-locked
  */
 static long user_read(const struct key *key,
 		      char __user *buffer, size_t buflen)
 {
-	long ret = key->datalen;
+	struct user_key_payload *upayload;
+	long ret;
+
+	upayload = rcu_dereference(key->payload.data);
+	ret = upayload->datalen;
 
 	/* we can return the data as is */
 	if (buffer && buflen > 0) {
-		if (buflen > key->datalen)
-			buflen = key->datalen;
+		if (buflen > upayload->datalen)
+			buflen = upayload->datalen;
 
-		if (copy_to_user(buffer, key->payload.data, buflen) != 0)
+		if (copy_to_user(buffer, upayload->data, buflen) != 0)
 			ret = -EFAULT;
 	}