keys: Provide request_key_rcu()

Provide a request_key_rcu() function that can be used to request a key
under RCU conditions.  It can only search and check permissions; it cannot
allocate a new key, upcall or wait for an upcall to complete.  It may
return a partially constructed key.
Signed-off-by: David Howells <dhowells@redhat.com>

Documentation/security/keys/core.rst

@@ -1147,6 +1147,16 @@ payload contents" for more information.
     case error ERESTARTSYS will be returned.
 
 
+ *  To search for a key under RCU conditions, call::
+
+	struct key *request_key_rcu(const struct key_type *type,
+				    const char *description);
+
+    which is similar to request_key() except that it does not check for keys
+    that are under construction and it will not call out to userspace to
+    construct a key if it can't find a match.
+
+
  *  When it is no longer required, the key should be released using::
 
 	void key_put(struct key *key);

Documentation/security/keys/request-key.rst

@@ -36,6 +36,11 @@ or::
 					     	   size_t callout_len,
 						   void *aux);
 
+or::
+
+	struct key *request_key_rcu(const struct key_type *type,
+				    const char *description);
+
 Or by userspace invoking the request_key system call::
 
 	key_serial_t request_key(const char *type,
@@ -57,6 +62,10 @@ The two async in-kernel calls may return keys that are still in the process of
 being constructed.  The two non-async ones will wait for construction to
 complete first.
 
+The request_key_rcu() call is like the in-kernel request_key() call, except
+that it doesn't check for keys that are under construction and doesn't attempt
+to construct missing keys.
+
 The userspace interface links the key to a keyring associated with the process
 to prevent the key from going away, and returns the serial number of the key to
 the caller.

include/linux/key.h

@@ -274,6 +274,9 @@ extern struct key *request_key(struct key_type *type,
 			       const char *description,
 			       const char *callout_info);
 
+extern struct key *request_key_rcu(struct key_type *type,
+				   const char *description);
+
 extern struct key *request_key_with_auxdata(struct key_type *type,
 					    const char *description,
 					    const void *callout_info,

security/keys/request_key.c

@@ -756,3 +756,47 @@ struct key *request_key_async_with_auxdata(struct key_type *type,
 				    callout_len, aux, NULL, KEY_ALLOC_IN_QUOTA);
 }
 EXPORT_SYMBOL(request_key_async_with_auxdata);
+
+/**
+ * request_key_rcu - Request key from RCU-read-locked context
+ * @type: The type of key we want.
+ * @description: The name of the key we want.
+ *
+ * Request a key from a context that we may not sleep in (such as RCU-mode
+ * pathwalk).  Keys under construction are ignored.
+ *
+ * Return a pointer to the found key if successful, -ENOKEY if we couldn't find
+ * a key or some other error if the key found was unsuitable or inaccessible.
+ */
+struct key *request_key_rcu(struct key_type *type, const char *description)
+{
+	struct keyring_search_context ctx = {
+		.index_key.type		= type,
+		.index_key.description	= description,
+		.index_key.desc_len	= strlen(description),
+		.cred			= current_cred(),
+		.match_data.cmp		= key_default_cmp,
+		.match_data.raw_data	= description,
+		.match_data.lookup_type	= KEYRING_SEARCH_LOOKUP_DIRECT,
+		.flags			= (KEYRING_SEARCH_DO_STATE_CHECK |
+					   KEYRING_SEARCH_SKIP_EXPIRED),
+	};
+	struct key *key;
+	key_ref_t key_ref;
+
+	kenter("%s,%s", type->name, description);
+
+	/* search all the process keyrings for a key */
+	key_ref = search_process_keyrings_rcu(&ctx);
+	if (IS_ERR(key_ref)) {
+		key = ERR_CAST(key_ref);
+		if (PTR_ERR(key_ref) == -EAGAIN)
+			key = ERR_PTR(-ENOKEY);
+	} else {
+		key = key_ref_to_ptr(key_ref);
+	}
+
+	kleave(" = %p", key);
+	return key;
+}
+EXPORT_SYMBOL(request_key_rcu);