keys: Move the user and user-session keyrings to the user_namespace

Move the user and user-session keyrings to the user_namespace struct rather
than pinning them from the user_struct struct.  This prevents these
keyrings from propagating across user-namespaces boundaries with regard to
the KEY_SPEC_* flags, thereby making them more useful in a containerised
environment.

The issue is that a single user_struct may be represent UIDs in several
different namespaces.

The way the patch does this is by attaching a 'register keyring' in each
user_namespace and then sticking the user and user-session keyrings into
that.  It can then be searched to retrieve them.
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jann Horn <jannh@google.com>

include/linux/sched/user.h

@@ -7,8 +7,6 @@
 #include <linux/refcount.h>
 #include <linux/ratelimit.h>
 
-struct key;
-
 /*
  * Some day this will be a full-fledged user tracking system..
  */
@@ -30,18 +28,6 @@ struct user_struct {
 	unsigned long unix_inflight;	/* How many files in flight in unix sockets */
 	atomic_long_t pipe_bufs;  /* how many pages are allocated in pipe buffers */
 
-#ifdef CONFIG_KEYS
-	/*
-	 * These pointers can only change from NULL to a non-NULL value once.
-	 * Writes are protected by key_user_keyring_mutex.
-	 * Unlocked readers should use READ_ONCE() unless they know that
-	 * install_user_keyrings() has been called successfully (which sets
-	 * these members to non-NULL values, preventing further modifications).
-	 */
-	struct key *uid_keyring;	/* UID specific keyring */
-	struct key *session_keyring;	/* UID's default session keyring */
-#endif
-
 	/* Hash table maintenance information */
 	struct hlist_node uidhash_node;
 	kuid_t uid;

include/linux/user_namespace.h

@@ -65,14 +65,19 @@ struct user_namespace {
 	unsigned long		flags;
 
 #ifdef CONFIG_KEYS
-	/* List of joinable keyrings in this namespace */
+	/* List of joinable keyrings in this namespace.  Modification access of
+	 * these pointers is controlled by keyring_sem.  Once
+	 * user_keyring_register is set, it won't be changed, so it can be
+	 * accessed directly with READ_ONCE().
+	 */
 	struct list_head	keyring_name_list;
+	struct key		*user_keyring_register;
+	struct rw_semaphore	keyring_sem;
 #endif
 
 	/* Register of per-UID persistent keyrings for this namespace */
 #ifdef CONFIG_PERSISTENT_KEYRINGS
 	struct key		*persistent_keyring_register;
-	struct rw_semaphore	persistent_keyring_register_sem;
 #endif
 	struct work_struct	work;
 #ifdef CONFIG_SYSCTL

kernel/user.c

@@ -64,10 +64,7 @@ struct user_namespace init_user_ns = {
 	.flags = USERNS_INIT_FLAGS,
 #ifdef CONFIG_KEYS
 	.keyring_name_list = LIST_HEAD_INIT(init_user_ns.keyring_name_list),
-#endif
-#ifdef CONFIG_PERSISTENT_KEYRINGS
-	.persistent_keyring_register_sem =
-	__RWSEM_INITIALIZER(init_user_ns.persistent_keyring_register_sem),
+	.keyring_sem = __RWSEM_INITIALIZER(init_user_ns.keyring_sem),
 #endif
 };
 EXPORT_SYMBOL_GPL(init_user_ns);
@@ -143,8 +140,6 @@ static void free_user(struct user_struct *up, unsigned long flags)
 {
 	uid_hash_remove(up);
 	spin_unlock_irqrestore(&uidhash_lock, flags);
-	key_put(up->uid_keyring);
-	key_put(up->session_keyring);
 	kmem_cache_free(uid_cachep, up);
 }
 

kernel/user_namespace.c

@@ -135,9 +135,7 @@ int create_user_ns(struct cred *new)
 
 #ifdef CONFIG_KEYS
 	INIT_LIST_HEAD(&ns->keyring_name_list);
-#endif
-#ifdef CONFIG_PERSISTENT_KEYRINGS
-	init_rwsem(&ns->persistent_keyring_register_sem);
+	init_rwsem(&ns->keyring_sem);
 #endif
 	ret = -ENOMEM;
 	if (!setup_userns_sysctls(ns))

security/keys/internal.h

@@ -148,7 +148,8 @@ extern key_ref_t search_process_keyrings_rcu(struct keyring_search_context *ctx)
 
 extern struct key *find_keyring_by_name(const char *name, bool uid_keyring);
 
-extern int install_user_keyrings(void);
+extern int look_up_user_keyrings(struct key **, struct key **);
+extern struct key *get_user_session_keyring_rcu(const struct cred *);
 extern int install_thread_keyring_to_cred(struct cred *);
 extern int install_process_keyring_to_cred(struct cred *);
 extern int install_session_keyring_to_cred(struct cred *, struct key *);

security/keys/keyring.c

@@ -62,6 +62,7 @@ void key_free_user_ns(struct user_namespace *ns)
 	list_del_init(&ns->keyring_name_list);
 	write_unlock(&keyring_name_lock);
 
+	key_put(ns->user_keyring_register);
 #ifdef CONFIG_PERSISTENT_KEYRINGS
 	key_put(ns->persistent_keyring_register);
 #endif

security/keys/persistent.c

@@ -91,9 +91,9 @@ static long key_get_persistent(struct user_namespace *ns, kuid_t uid,
 
 	if (ns->persistent_keyring_register) {
 		reg_ref = make_key_ref(ns->persistent_keyring_register, true);
-		down_read(&ns->persistent_keyring_register_sem);
+		down_read(&ns->keyring_sem);
 		persistent_ref = find_key_to_update(reg_ref, &index_key);
-		up_read(&ns->persistent_keyring_register_sem);
+		up_read(&ns->keyring_sem);
 
 		if (persistent_ref)
 			goto found;
@@ -102,9 +102,9 @@ static long key_get_persistent(struct user_namespace *ns, kuid_t uid,
 	/* It wasn't in the register, so we'll need to create it.  We might
 	 * also need to create the register.
 	 */
-	down_write(&ns->persistent_keyring_register_sem);
+	down_write(&ns->keyring_sem);
 	persistent_ref = key_create_persistent(ns, uid, &index_key);
-	up_write(&ns->persistent_keyring_register_sem);
+	up_write(&ns->keyring_sem);
 	if (!IS_ERR(persistent_ref))
 		goto found;
 

security/keys/request_key.c

@@ -121,7 +121,7 @@ static int call_sbin_request_key(struct key *authkey, void *aux)
 	struct request_key_auth *rka = get_request_key_auth(authkey);
 	const struct cred *cred = current_cred();
 	key_serial_t prkey, sskey;
-	struct key *key = rka->target_key, *keyring, *session;
+	struct key *key = rka->target_key, *keyring, *session, *user_session;
 	char *argv[9], *envp[3], uid_str[12], gid_str[12];
 	char key_str[12], keyring_str[3][12];
 	char desc[20];
@@ -129,9 +129,9 @@ static int call_sbin_request_key(struct key *authkey, void *aux)
 
 	kenter("{%d},{%d},%s", key->serial, authkey->serial, rka->op);
 
-	ret = install_user_keyrings();
+	ret = look_up_user_keyrings(NULL, &user_session);
 	if (ret < 0)
-		goto error_alloc;
+		goto error_us;
 
 	/* allocate a new session keyring */
 	sprintf(desc, "_req.%u", key->serial);
@@ -169,7 +169,7 @@ static int call_sbin_request_key(struct key *authkey, void *aux)
 
 	session = cred->session_keyring;
 	if (!session)
-		session = cred->user->session_keyring;
+		session = user_session;
 	sskey = session->serial;
 
 	sprintf(keyring_str[2], "%d", sskey);
@@ -211,6 +211,8 @@ static int call_sbin_request_key(struct key *authkey, void *aux)
 	key_put(keyring);
 
 error_alloc:
+	key_put(user_session);
+error_us:
 	complete_request_key(authkey, ret);
 	kleave(" = %d", ret);
 	return ret;
@@ -317,13 +319,15 @@ static int construct_get_dest_keyring(struct key **_dest_keyring)
 
 			/* fall through */
 		case KEY_REQKEY_DEFL_USER_SESSION_KEYRING:
-			dest_keyring =
-				key_get(READ_ONCE(cred->user->session_keyring));
+			ret = look_up_user_keyrings(NULL, &dest_keyring);
+			if (ret < 0)
+				return ret;
 			break;
 
 		case KEY_REQKEY_DEFL_USER_KEYRING:
-			dest_keyring =
-				key_get(READ_ONCE(cred->user->uid_keyring));
+			ret = look_up_user_keyrings(&dest_keyring, NULL);
+			if (ret < 0)
+				return ret;
 			break;
 
 		case KEY_REQKEY_DEFL_GROUP_KEYRING: