Add credentials.WithPolicy method to narrow down policy (#2007)

The `NewSTSWebIdentityWithPolicy` allows fetching credentials that are 
narrowed down to the specified policy. This is useful when a client requires 
fewer rights than it has. The returned credentials will have the intersection 
of the requested policy and the assigned policies.

* add `credentials.NewKubernetesIdentity`

pkg/credentials/sts_web_identity.go

@@ -25,6 +25,7 @@ import (
 	"io"
 	"net/http"
 	"net/url"
+	"os"
 	"strconv"
 	"strings"
 	"time"
@@ -85,29 +86,59 @@ type STSWebIdentity struct {
 	// assuming.
 	RoleARN string
 
+	// Policy is the policy where the credentials should be limited too.
+	Policy string
+
 	// roleSessionName is the identifier for the assumed role session.
 	roleSessionName string
 }
 
 // NewSTSWebIdentity returns a pointer to a new
 // Credentials object wrapping the STSWebIdentity.
-func NewSTSWebIdentity(stsEndpoint string, getWebIDTokenExpiry func() (*WebIdentityToken, error)) (*Credentials, error) {
+func NewSTSWebIdentity(stsEndpoint string, getWebIDTokenExpiry func() (*WebIdentityToken, error), opts ...func(*STSWebIdentity)) (*Credentials, error) {
 	if stsEndpoint == "" {
 		return nil, errors.New("STS endpoint cannot be empty")
 	}
 	if getWebIDTokenExpiry == nil {
 		return nil, errors.New("Web ID token and expiry retrieval function should be defined")
 	}
-	return New(&STSWebIdentity{
+	i := &STSWebIdentity{
 		Client: &http.Client{
 			Transport: http.DefaultTransport,
 		},
 		STSEndpoint:         stsEndpoint,
 		GetWebIDTokenExpiry: getWebIDTokenExpiry,
-	}), nil
+	}
+	for _, o := range opts {
+		o(i)
+	}
+	return New(i), nil
+}
+
+// NewKubernetesIdentity returns a pointer to a new
+// Credentials object using the Kubernetes service account
+func NewKubernetesIdentity(stsEndpoint string, opts ...func(*STSWebIdentity)) (*Credentials, error) {
+	return NewSTSWebIdentity(stsEndpoint, func() (*WebIdentityToken, error) {
+		token, err := os.ReadFile("/var/run/secrets/kubernetes.io/serviceaccount/token")
+		if err != nil {
+			return nil, err
+		}
+
+		return &WebIdentityToken{
+			Token: string(token),
+		}, nil
+	}, opts...)
+}
+
+// WithPolicy option will enforce that the returned credentials
+// will be scoped down to the specified policy
+func WithPolicy(policy string) func(*STSWebIdentity) {
+	return func(i *STSWebIdentity) {
+		i.Policy = policy
+	}
 }
 
-func getWebIdentityCredentials(clnt *http.Client, endpoint, roleARN, roleSessionName string,
+func getWebIdentityCredentials(clnt *http.Client, endpoint, roleARN, roleSessionName string, policy string,
 	getWebIDTokenExpiry func() (*WebIdentityToken, error),
 ) (AssumeRoleWithWebIdentityResponse, error) {
 	idToken, err := getWebIDTokenExpiry()
@@ -133,6 +164,9 @@ func getWebIdentityCredentials(clnt *http.Client, endpoint, roleARN, roleSession
 	if idToken.Expiry > 0 {
 		v.Set("DurationSeconds", fmt.Sprintf("%d", idToken.Expiry))
 	}
+	if policy != "" {
+		v.Set("Policy", policy)
+	}
 	v.Set("Version", STSVersion)
 
 	u, err := url.Parse(endpoint)
@@ -183,7 +217,7 @@ func getWebIdentityCredentials(clnt *http.Client, endpoint, roleARN, roleSession
 // Retrieve retrieves credentials from the MinIO service.
 // Error will be returned if the request fails.
 func (m *STSWebIdentity) Retrieve() (Value, error) {
-	a, err := getWebIdentityCredentials(m.Client, m.STSEndpoint, m.RoleARN, m.roleSessionName, m.GetWebIDTokenExpiry)
+	a, err := getWebIdentityCredentials(m.Client, m.STSEndpoint, m.RoleARN, m.roleSessionName, m.Policy, m.GetWebIDTokenExpiry)
 	if err != nil {
 		return Value{}, err
 	}