Add max_response_length option to http module

This option limits the maximum body length that will be read from the
HTTP server. It's meant to prevent misconfigured servers from causing
the probe to use too many resources, even if temporarily. It's not an
additional check on the response, for that, use the resulting metrics
(probe_http_content_length, probe_http_uncompressed_body_length, etc).

Signed-off-by: Marcelo E. Magallon <[email protected]>

CONFIGURATION.md

@@ -49,6 +49,11 @@ The other placeholders are specified separately.
   headers:
     [ <string>: <string> ... ]
 
+  # The maximum body length in bytes that will be read from the server. A value of 0 means no limit.
+  # If the response includes a Content-Length header, it is NOT validated against this value. This
+  # setting is only meant to limit the amount of data that you are willing to read from the server.
+  [ max_response_length: <int> | default = 0 ]
+
   # The compression algorithm to use to decompress the response (gzip, br, deflate, identity).
   #
   # If an "Accept-Encoding" header is specified, it MUST be such that the compression algorithm

config/config.go

@@ -207,6 +207,7 @@ type HTTPProbe struct {
 	Body                         string                  `yaml:"body,omitempty"`
 	HTTPClientConfig             config.HTTPClientConfig `yaml:"http_client_config,inline"`
 	Compression                  string                  `yaml:"compression,omitempty"`
+	MaxResponseLength            int64                   `yaml:"max_response_length,omitempty"`
 }
 
 type HeaderMatch struct {
@@ -287,6 +288,11 @@ func (s *HTTPProbe) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	if err := unmarshal((*plain)(s)); err != nil {
 		return err
 	}
+
+	if s.MaxResponseLength < 0 {
+		return fmt.Errorf(`invalid max_response_length value: %d`, s.MaxResponseLength)
+	}
+
 	if err := s.HTTPClientConfig.Validate(); err != nil {
 		return err
 	}

prober/http.go

@@ -474,6 +474,14 @@ func ProbeHTTP(ctx context.Context, target string, module config.Module, registr
 			}
 		}
 
+		// If there's a configured max_response_length, wrap the body in the response in a http.MaxBytesReader.
+		// This will read up to MaxResponseLength bytes from the body, and return an error if the response is
+		// larger. It forwards the Close call to the original resp.Body to make sure the TCP connection is
+		// correctly shut down. The limit is applied _before_ decompression_.
+		if httpConfig.MaxResponseLength > 0 {
+			resp.Body = http.MaxBytesReader(nil, resp.Body, httpConfig.MaxResponseLength)
+		}
+
 		// Since the configuration specifies a compression algorithm, blindly treat the response body as a
 		// compressed payload; if we cannot decompress it it's a failure because the configuration says we
 		// should expect the response to be compressed in that way.

prober/http_test.go

@@ -513,6 +513,108 @@ func TestHandlingOfCompressionSetting(t *testing.T) {
 	}
 }
 
+func TestMaxResponseLength(t *testing.T) {
+	const max = 128
+
+	var gzippedPayload bytes.Buffer
+	enc := gzip.NewWriter(&gzippedPayload)
+	enc.Write(bytes.Repeat([]byte{'A'}, 2*max))
+	enc.Close()
+
+	testcases := map[string]struct {
+		target          string
+		compression     string
+		expectedMetrics map[string]float64
+		expectFailure   bool
+	}{
+		"short": {
+			target: "/short",
+			expectedMetrics: map[string]float64{
+				"probe_http_uncompressed_body_length": float64(max - 1),
+				"probe_http_content_length":           float64(max - 1),
+			},
+		},
+		"long": {
+			target:        "/long",
+			expectFailure: true,
+			expectedMetrics: map[string]float64{
+				"probe_http_content_length": float64(max + 1),
+			},
+		},
+		"compressed": {
+			target:      "/compressed",
+			compression: "gzip",
+			expectedMetrics: map[string]float64{
+				"probe_http_content_length":           float64(gzippedPayload.Len()),
+				"probe_http_uncompressed_body_length": float64(2 * max),
+			},
+		},
+	}
+
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		var resp []byte
+
+		switch r.URL.Path {
+		case "/compressed":
+			resp = gzippedPayload.Bytes()
+			w.Header().Add("Content-Encoding", "gzip")
+
+		case "/long":
+			resp = bytes.Repeat([]byte{'A'}, max+1)
+
+		case "/short":
+			resp = bytes.Repeat([]byte{'A'}, max-1)
+
+		default:
+			w.WriteHeader(http.StatusBadRequest)
+			return
+		}
+
+		w.Header().Set("Content-Length", strconv.Itoa(len(resp)))
+		w.WriteHeader(http.StatusOK)
+		w.Write(resp)
+	}))
+	defer ts.Close()
+
+	for name, tc := range testcases {
+		t.Run(name, func(t *testing.T) {
+			registry := prometheus.NewRegistry()
+			testCTX, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+			defer cancel()
+
+			result := ProbeHTTP(
+				testCTX,
+				ts.URL+tc.target,
+				config.Module{
+					Timeout: time.Second,
+					HTTP: config.HTTPProbe{
+						IPProtocolFallback: true,
+						MaxResponseLength:  max,
+						HTTPClientConfig:   pconfig.DefaultHTTPClientConfig,
+						Compression:        tc.compression,
+					},
+				},
+				registry,
+				log.NewNopLogger(),
+			)
+
+			switch {
+			case tc.expectFailure && result:
+				t.Fatalf("test passed unexpectedly")
+			case !tc.expectFailure && !result:
+				t.Fatalf("test failed unexpectedly")
+			}
+
+			mfs, err := registry.Gather()
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			checkRegistryResults(tc.expectedMetrics, mfs, t)
+		})
+	}
+}
+
 func TestRedirectFollowed(t *testing.T) {
 	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		if r.URL.Path == "/" {