Add support for session policies in STS APIs (#7747)

This PR adds support for adding session policies
for further restrictions on STS credentials, useful
in situations when applications want to generate
creds for multiple interested parties with different
set of policy restrictions.

This session policy is not mandatory, but optional.

Fixes #7732

cmd/auth-handler.go

@@ -200,6 +200,39 @@ func getClaimsFromToken(r *http.Request) (map[string]interface{}, error) {
 	if _, ok = v.(string); !ok {
 		return nil, errInvalidAccessKeyID
 	}
+
+	if globalPolicyOPA == nil {
+		// If OPA is not set, session token should
+		// have a policy and its mandatory, reject
+		// requests without policy claim.
+		p, pok := claims[iampolicy.PolicyName]
+		if !pok {
+			return nil, errAuthentication
+		}
+		if _, pok = p.(string); !pok {
+			return nil, errAuthentication
+		}
+		sp, spok := claims[iampolicy.SessionPolicyName]
+		// Sub policy is optional, if not set return success.
+		if !spok {
+			return claims, nil
+		}
+		// Sub policy is set but its not a string, reject such requests
+		spStr, spok := sp.(string)
+		if !spok {
+			return nil, errAuthentication
+		}
+		// Looks like subpolicy is set and is a string, if set then its
+		// base64 encoded, decode it. Decoding fails reject such requests.
+		spBytes, err := base64.StdEncoding.DecodeString(spStr)
+		if err != nil {
+			// Base64 decoding fails, we should log to indicate
+			// something is malforming the request sent by client.
+			logger.LogIf(context.Background(), err)
+			return nil, errAuthentication
+		}
+		claims[iampolicy.SessionPolicyName] = string(spBytes)
+	}
 	return claims, nil
 }