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A83-xds-gcp-authn-filter.md

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A83: xDS GCP Authentication Filter

Abstract

In service mesh environments, there are cases where intermediate proxies make it impossible to rely on mTLS for end-to-end authentication. These cases can be addressed instead by the use of service account identity JWT tokens. The xDS GCP Authentication filter provides a mechanism for attaching such JWT tokens as gRPC call credentials on GCP. We will add support for this filter in gRPC.

Background

gRPC already supports a framework for xDS HTTP filters, as described in gRFC A39. We will support the GCP Authentication filter under this framework.

Related Proposals:

Proposal

We will support the GCP Authentication xDS HTTP filter in the gRPC client.

Call Credentials

Note: This section is intended for gRPC implementations that need to implement a new call credential type for GCP service account identity tokens. Implementations that already support this functionality (e.g., by depending on an external Google Auth library) may continue to use their existing functionality, even if the behavior differs in small ways from what is described in this section.

gRPC should support a GcpServiceAccountIdentityCallCredentials call credentials type, which is not xDS-specific. This credential type will be instantiated with one parameter, which is the audience to be encoded into the JWT token. The credential object will handle fetching the token on-demand and caching it based on the token's expiration time.

To handle potential clock skew issues and to account for processing time on the server, the credential will set the cache expiration time to be 30 seconds before the expiration time encoded in the token. All logic in the call credential code will use this modified expiration time instead of the expiration time encoded in the token.

When the credential is asked for a token for a data plane RPC, if the token is not yet cached or the cached token will expire within some fixed refresh interval (typically 1 minute), the credential will start an HTTP request (if there is not already one pending) to http://metadata.google.internal/computeMetadata/v1/instance/service-accounts/default/identity?audience=[AUDIENCE], where [AUDIENCE] is replaced with the audience specified when the credential object was instantiated. The HTTP request will include the header Metadata-Flavor: Google.

When a data plane RPC starts, if the token is cached and is not expired, the token will immediately be added to the RPC, and the RPC will continue. Otherwise (i.e., before the token is initially obtained or after the cached token has expired), the data plane RPC will be queued until the HTTP request completes. When the HTTP request completes, the result (either success or failure, as described below) will be applied to all queued data plane RPCs.

Note that when the token's expiration time is less than the refresh interval in the future, a new data plane RPC being started will trigger a new HTTP request, but the cached token value will still be used for that data plane RPC. This pre-emptive re-fetching is intended to avoid periodic latency spikes when refreshing the token.

If the HTTP request fails, all queued data plane RPCs will be failed with a gRPC status determined based on the returned HTTP status. If the returned HTTP status maps to UNAVAILABLE in HTTP to gRPC Status Code Mapping, then the data plane RPCs will be failed with status UNAVAILABLE; otherwise, they will be failed with status UNAUTHENTICATED. If the request fails without an HTTP status (e.g., an I/O error), all queued data plane RPCs will be failed with UNAVAILABLE status.

If the HTTP request succeeds, the body of the response will contain the JWT token. which the client will cache. The client does not need to do full RFC-7519 validation of the token (that is the responsibility of the server side), but it does need to extract the exp field for caching purposes. If the exp field cannot be extracted (i.e., the JWT token is invalid), all queued data plane RPCs will be failed with status UNAUTHENTICATED. Otherwise, the cache is updated, and the returned token is added to all queued data plane RPCs, which may then continue.

If the HTTP request does not result in the cache being updated (i.e., if the HTTP request fails or if it returns an invalid JWT token), backoff must be applied before the next attempt may be started. If a data plane RPC is started when there is no cached token available and while in backoff delay, it will be failed with the status from the last HTTP request attempt. When the backoff delay expires, the next data plane RPC will trigger a new attempt. Note that no attempt should be started until and unless a data plane RPC is started, since we do not want to unnecessarily retry if the channel is idle. The backoff state will be reset once there is a successful HTTP request.

To add the token to a data plane RPC, the call credential will add a header named authorization. The header value will be the string Bearer (note trailing space) followed by the token value.

xDS HTTP Filter Configuration

The xDS HTTP filter will be configured via the extensions.filters.http.gcp_authn.v3.GcpAuthnFilterConfig message. The fields will be interpretted as follows:

  • cache_config: Optional. Within this message:
    • cache_size: Optional. If set, must be greater than 0. Defaults to 10. Implementations that cannot support caches as large as UINT64_MAX may cap this value at their maximum supported size.
  • http_uri: Ignored by gRPC.
  • token_header: Ignored by gRPC.
  • retry_policy: Ignored by gRPC.
  • cluster: Ignored by gRPC.
  • timeout: Ignored by gRPC.

Note that this filter does not support having its config overridden in a typed_per_filter_config field on a per-route, per-virtualhost, or per-clusterweight basis. If the filter's config message appears in a typed_per_filter_config field, it will be validated as part of the normal resource validation, but the configuration will not actually be used.

xDS Cluster Metadata

The GCP Authentication filter uses cluster metadata from the Cluster.metadata field to configure the audience. We will process this field when validating the CDS resource and convert it into a map, which will be added to the parsed cluster resource that is passed to the XdsClient watcher.

The metadata field is a message that actually contains two maps:

  • filter_metadata: This map contains google.protobuf.Struct values, which we will convert to parsed JSON form.
  • typed_filter_metadata: This map contains google.protobuf.Any fields. To support this, we will use a registry-like approach for metadata types (may be an actual registry or just a block of code that supports the known protobuf message types) that handles parsing the google.protobuf.Any field and converting it some internal form appropriate for the implementation (e.g., JSON or a native struct).

The value for a given metadata key will come from only one of the two maps; the value from filter_metadata will be used only if the key is not present or is of an unknown protobuf message type in typed_filter_metadata. In the resulting map in the parsed cluster resource, the map value will contain the type of the original message (google.protobuf.Struct if it came from the filter_metadata map) and a parsed representation of the content. The parsed representation may be either JSON or the appropriate internal form, depending on which of the two maps the entry came from.

The logic to validate cluster metadata will look something like this (pseudo-code):

parsed_metadata = {}  # Value is either JSON or parsed object
# First process typed_filter_metadata.
for key, any_field in cluster_metadata.typed_filter_metadata.items():
  parser = metadata_registry.FindParser(any_field.type_url)
  if parser is not None:
    value = parser.Parse(any_field.value)
    if value is None:
      return NACK  # Parsing failed, reject resource
    parsed_metadata[key] = value
# Now process filter_metadata.  We look only at keys that were not
# already added from typed_filter_metadata.
for key, struct_field in cluster_metadata.filter_metadata.items():
  if key not in parsed_metadata:
    parsed_metadata[key] = ConvertToJson(struct_field)

For now, the only registered metadata type we support is extensions.filters.http.gcp_authn.v3.Audience. In this message, the url field must be non-empty; if empty, the resource will be NACKed. The parsed representation of this message can be a simple string.

xDS ConfigSelector Behavior

As per gRFC A60, we currently pass the selected cluster name via a call attribute for access in filters. However, the filters will now also need access to the CDS resource for the selected cluster, so that the GCP Authentication filter can access the cluster metadata for the selected cluster. This data is available via the XdsConfig attribute introduced in A74. If the xDS ConfigSelector is not already passing that attribute to the filters, it will need to be changed to do so.

Filter Call Credentials Cache

The filter will maintain a cache of GcpServiceAccountIdentityCallCredentials instances, one for each audience, along with a last-used list that tracks how recently the entries have been used. As an entry is used, it is moved to the front of the last-used list. The maximum number of entries in the cache is bounded by the config field cache_config.cache_size; if the cache exceeds that size, then entries will be removed starting from the end of the last-used list.

Note that the cache_config.cache_size parameter in the filter config is a channel-level parameter, not settable per-route, and we want the cache itself to be shared across all routes. Implementations that create separate filter/interceptor instances for each route should share the cache between those instances.

It is desirable to avoid losing this cache when we get an xDS Listener or RouteConfiguration update, so that we don't wind up needlessly refetching tokens after the update. Implementations should provide a mechanism for new instances of the filter to retain the cache from previous instances.

If an LDS update changes the cache size, the filter must apply that change to the cache. If the cache currently has more entries in it than the new cache size, then the least recently used entries will be removed to make the cache adhere to the new size limit.

Filter Behavior

When the filter processes the RPC's initial metadata, it will first check to see what cluster the RPC is being sent to. If the RPC is being sent to a route that uses a cluster specifier plugin instead of a fixed cluster, then the filter is a no-op. Otherwise, the filter will attempt to determine the audience by looking at the CDS resource for the cluster that the RPC is being sent to.

If the CDS resource is not available (e.g., because the client received an error without having previously received a valid resource, or because the server indicated that the resource has been deleted), then the filter will fail the RPC with status UNAVAILABLE. Note that this does yield sub-optimal behavior for wait_for_ready RPCs, since we will fail them instead of queuing them, but we don't currently have a good alternative: the filter cannot queue the call until the client gets a valid CDS resource, because once that happens, a new instance of the filter will be swapped in for subsequent calls, but the queued call would already be tied to the original filter instance, which will never see the update.

Otherwise, the filter will look in the CDS resource's metadata for a key corresponding to the filter's instance name. Note that in Envoy, the cluster metadata keys must exactly match the legacy filter name (e.g., "envoy.filters.http.gcp_authn"). However, as per envoyproxy/envoy#34251, it is desirable to instead use the HTTP filter instance name from the HttpFilter.name field. We will implement that behavior in gRPC.

If the cluster metadata does not contain a key matching the filter's instance name, then the filter is a no-op. If a cluster metadata entry exists for the filter's instance name, but the entry is of a type other than extensions.filters.http.gcp_authn.v3.Audience, then the filter will fail data plane RPCs with status UNAVAILABLE. Otherwise, the audience is the value of the url field in the Audience proto.

The filter will then check to see if it already has a cached GcpServiceAccountIdentityCallCredentials instance for the specified audience. If it does not, it will create a new instance, adding it to its cache, removing the least recently used entry from the cache if the cache is already at its max size. It will then attach that GcpServiceAccountIdentityCallCredentials instance to the RPC.

Note that implementations must ensure that the token is not added to RPCs sent on insecure connections. However, the GCP Authentication filter will run before load balancing has chosen a connection, so the filter cannot directly add the token to the RPC. Instead, it must add the call credential to the RPC, and the call credential will do the work of adding the token to the RPC later, after load balancing has chosen a connection.

Temporary environment variable protection

Support for the GCP Authentication filter in the xDS HTTP filter registry and the extensions.filters.http.gcp_authn.v3.Audience entry in the metadata registry will be guarded by the GRPC_EXPERIMENTAL_XDS_GCP_AUTHENTICATION_FILTER env var. The env var guard will be removed once the feature passes interop tests.

Rationale

It is not our intention to support this mechanism for GCP only; in principle, it should be possible to support JWT identity tokens for any cloud provider. However, at present, the existing xDS HTTP filter supports only GCP, so that's what we're initially focusing on, for compatibility with Envoy. We would be open to future contributions from the OSS community to provide similar functionality for other cloud providers, in both gRPC and Envoy.

Note that the cache structure in this design is a bit different from Envoy's implementation. In Envoy, the GCP Authentication filter directly maintains a single cache containing the tokens for each audience, with expiration based on the tokens' expiration times. In contrast, gRPC will essentially have a two-level cache here: the filter will maintain a cache of GcpServiceAccountIdentityCallCredentials instances for each audience with expiration based on their respective last-used times, and each of those GcpServiceAccountIdentityCallCredentials instances will internally cache the token for its audience based on the token's expiration time. In the majority of cases, this is expected to result in the same behavior, although it is conceivably possible for there to be edge cases where a given GcpServiceAccountIdentityCallCredentials instance is retained in the cache due to being used more recently even though it has actually been failing to obtain a token. However, this approach allows for cleaner code and better reuse of existing call credentials implementations in some languages.

Implementation

C-core implementation:

Will be implemented in all other languages, timelines TBD.