feat: add flag to disable IMDSv1 fallback

.changelog/a1050420a5f7409fbc4d1d82c882d168.json

@@ -0,0 +1,8 @@
+{
+    "id": "a1050420-a5f7-409f-bc4d-1d82c882d168",
+    "type": "feature",
+    "description": "Add flag to disable IMDSv1 fallback",
+    "modules": [
+        "feature/ec2/imds"
+    ]
+}

feature/ec2/imds/api_client.go

@@ -174,6 +174,16 @@ type Options struct {
 	// The logger writer interface to write logging messages to.
 	Logger logging.Logger
 
+	// Configure IMDSv1 fallback behavior. By default, the client will attempt
+	// to fall back to IMDSv1 as needed for backwards compatibility. When set to [aws.FalseTernary]
+	// the client will return any errors encountered from attempting to fetch a token
+	// instead of silently using the insecure data flow of IMDSv1.
+	//
+	// See [configuring IMDS] for more information.
+	//
+	// [configuring IMDS]: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/configuring-instance-metadata-service.html
+	EnableFallback aws.Ternary
+
 	// provides the caching of API tokens used for operation calls. If unset,
 	// the API token will not be retrieved for the operation.
 	tokenProvider *tokenProvider

feature/ec2/imds/request_middleware_test.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"encoding/hex"
 	"fmt"
+	"github.com/aws/aws-sdk-go-v2/aws"
 	"io"
 	"io/ioutil"
 	"net/http"
@@ -330,11 +331,12 @@ func (h *successAPIResponseHandler) ServeHTTP(w http.ResponseWriter, r *http.Req
 
 func TestRequestGetToken(t *testing.T) {
 	cases := map[string]struct {
-		GetHandler    func(*testing.T) http.Handler
-		APICallCount  int
-		ExpectTrace   []string
-		ExpectContent []byte
-		ExpectErr     string
+		GetHandler     func(*testing.T) http.Handler
+		APICallCount   int
+		ExpectTrace    []string
+		ExpectContent  []byte
+		ExpectErr      string
+		EnableFallback aws.Ternary
 	}{
 		"secure": {
 			ExpectTrace: []string{
@@ -496,8 +498,69 @@ func TestRequestGetToken(t *testing.T) {
 						}),
 					))
 			},
+			ExpectErr: "failed to get API token",
+		},
+
+		// retryable token error with fallback enabled (default)
+		"token failure fallback enabled": {
+			ExpectTrace: []string{
+				getTokenPath,
+				getTokenPath,
+				getTokenPath,
+				"/latest/foo",
+			},
+			APICallCount: 1,
+			GetHandler: func(t *testing.T) http.Handler {
+				return newTestServeMux(t,
+					newInsecureAPIHandler(t,
+						500,
+						&successAPIResponseHandler{t: t,
+							path:   "/latest/foo",
+							method: "GET",
+							body:   []byte("hello"),
+						},
+					))
+			},
 			ExpectContent: []byte("hello"),
-			ExpectErr:     "EC2 IMDS failed",
+		},
+		// retryable token error with fallback disabled
+		"token failure fallback disabled": {
+			ExpectTrace: []string{
+				getTokenPath,
+				getTokenPath,
+				getTokenPath,
+			},
+			APICallCount: 1,
+			GetHandler: func(t *testing.T) http.Handler {
+				return newTestServeMux(t,
+					newInsecureAPIHandler(t,
+						500,
+						http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+							t.Errorf("expected no call to API handler")
+							http.Error(w, "", 400)
+						}),
+					))
+			},
+			ExpectErr:      "failed to get API token",
+			EnableFallback: aws.BoolTernary(false),
+		},
+		"insecure 403 fallback disabled": {
+			ExpectTrace: []string{
+				getTokenPath,
+			},
+			APICallCount: 1,
+			GetHandler: func(t *testing.T) http.Handler {
+				return newTestServeMux(t,
+					newInsecureAPIHandler(t,
+						403,
+						http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+							t.Errorf("expected no call to API handler")
+							http.Error(w, "", 400)
+						}),
+					))
+			},
+			ExpectErr:      "failed to get API token",
+			EnableFallback: aws.BoolTernary(false),
 		},
 	}
 
@@ -515,7 +578,8 @@ func TestRequestGetToken(t *testing.T) {
 			defer server.Close()
 
 			client := New(Options{
-				Endpoint: server.URL,
+				Endpoint:       server.URL,
+				EnableFallback: c.EnableFallback,
 			})
 
 			ctx := context.Background()

feature/ec2/imds/token_provider.go

@@ -4,12 +4,14 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"github.com/aws/aws-sdk-go-v2/aws"
+	"github.com/aws/smithy-go"
+	"github.com/aws/smithy-go/logging"
 	"net/http"
 	"sync"
 	"sync/atomic"
 	"time"
 
-	smithy "github.com/aws/smithy-go"
 	"github.com/aws/smithy-go/middleware"
 	smithyhttp "github.com/aws/smithy-go/transport/http"
 )
@@ -68,7 +70,7 @@ func (t *tokenProvider) HandleFinalize(
 ) (
 	out middleware.FinalizeOutput, metadata middleware.Metadata, err error,
 ) {
-	if !t.enabled() {
+	if t.fallbackEnabled() && !t.enabled() {
 		// short-circuits to insecure data flow if token provider is disabled.
 		return next.HandleFinalize(ctx, input)
 	}
@@ -115,23 +117,15 @@ func (t *tokenProvider) HandleDeserialize(
 	}
 
 	if resp.StatusCode == http.StatusUnauthorized { // unauthorized
-		err = &retryableError{Err: err}
 		t.enable()
+		err = &retryableError{Err: err, isRetryable: true}
 	}
 
 	return out, metadata, err
 }
 
-type retryableError struct {
-	Err error
-}
-
-func (*retryableError) RetryableError() bool { return true }
-
-func (e *retryableError) Error() string { return e.Err.Error() }
-
 func (t *tokenProvider) getToken(ctx context.Context) (tok *apiToken, err error) {
-	if !t.enabled() {
+	if t.fallbackEnabled() && !t.enabled() {
 		return nil, &bypassTokenRetrievalError{
 			Err: fmt.Errorf("cannot get API token, provider disabled"),
 		}
@@ -147,7 +141,7 @@ func (t *tokenProvider) getToken(ctx context.Context) (tok *apiToken, err error)
 
 	tok, err = t.updateToken(ctx)
 	if err != nil {
-		return nil, fmt.Errorf("cannot get API token, %w", err)
+		return nil, err
 	}
 
 	return tok, nil
@@ -167,17 +161,19 @@ func (t *tokenProvider) updateToken(ctx context.Context) (*apiToken, error) {
 		TokenTTL: t.tokenTTL,
 	})
 	if err != nil {
-		// change the disabled flag on token provider to true, when error is request timeout error.
 		var statusErr interface{ HTTPStatusCode() int }
 		if errors.As(err, &statusErr) {
 			switch statusErr.HTTPStatusCode() {
-
-			// Disable get token if failed because of 403, 404, or 405
+			// Disable future get token if failed because of 403, 404, or 405
 			case http.StatusForbidden,
 				http.StatusNotFound,
 				http.StatusMethodNotAllowed:
 
-				t.disable()
+				if t.fallbackEnabled() {
+					logger := middleware.GetLogger(ctx)
+					logger.Logf(logging.Warn, "falling back to IMDSv1: %v", err)
+					t.disable()
+				}
 
 			// 400 errors are terminal, and need to be upstreamed
 			case http.StatusBadRequest:
@@ -192,8 +188,17 @@ func (t *tokenProvider) updateToken(ctx context.Context) (*apiToken, error) {
 			atomic.StoreUint32(&t.disabled, 1)
 		}
 
-		// Token couldn't be retrieved, but bypass this, and allow the
-		// request to continue.
+		if !t.fallbackEnabled() {
+			// NOTE: getToken() is an implementation detail of some outer operation
+			// (e.g. GetMetadata). It has its own retries that have already been exhausted.
+			// Mark the underlying error as a terminal error.
+			err = &retryableError{Err: err, isRetryable: false}
+			return nil, err
+		}
+
+		// Token couldn't be retrieved, fallback to IMDSv1 insecure flow for this request
+		// and allow the request to proceed. Future requests _may_ re-attempt fetching a
+		// token if not disabled.
 		return nil, &bypassTokenRetrievalError{Err: err}
 	}
 
@@ -206,21 +211,21 @@ func (t *tokenProvider) updateToken(ctx context.Context) (*apiToken, error) {
 	return tok, nil
 }
 
-type bypassTokenRetrievalError struct {
-	Err error
-}
-
-func (e *bypassTokenRetrievalError) Error() string {
-	return fmt.Sprintf("bypass token retrieval, %v", e.Err)
-}
-
-func (e *bypassTokenRetrievalError) Unwrap() error { return e.Err }
-
 // enabled returns if the token provider is current enabled or not.
 func (t *tokenProvider) enabled() bool {
 	return atomic.LoadUint32(&t.disabled) == 0
 }
 
+// fallbackEnabled returns false if EnableFallback is [aws.FalseTernary], true otherwise
+func (t *tokenProvider) fallbackEnabled() bool {
+	switch t.client.options.EnableFallback {
+	case aws.FalseTernary:
+		return false
+	default:
+		return true
+	}
+}
+
 // disable disables the token provider and it will no longer attempt to inject
 // the token, nor request updates.
 func (t *tokenProvider) disable() {
@@ -235,3 +240,22 @@ func (t *tokenProvider) enable() {
 	t.tokenMux.Unlock()
 	atomic.StoreUint32(&t.disabled, 0)
 }
+
+type bypassTokenRetrievalError struct {
+	Err error
+}
+
+func (e *bypassTokenRetrievalError) Error() string {
+	return fmt.Sprintf("bypass token retrieval, %v", e.Err)
+}
+
+func (e *bypassTokenRetrievalError) Unwrap() error { return e.Err }
+
+type retryableError struct {
+	Err         error
+	isRetryable bool
+}
+
+func (e *retryableError) RetryableError() bool { return e.isRetryable }
+
+func (e *retryableError) Error() string { return e.Err.Error() }