http: add MaxBytesReader to limit request body size

This adds http.MaxBytesReader, similar to io.LimitReader,
but specific to http, and for preventing a class of DoS
attacks.

This also makes the 10MB ParseForm limit optional (if
not already set by a MaxBytesReader), documents it,
and also adds "PUT" as a valid verb for parsing forms
in the request body.

Improves issue 2093 (DoS protection)
Fixes #2165 (PUT form parsing)

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/4921049

src/pkg/http/request.go

@@ -608,19 +608,63 @@ func ReadRequest(b *bufio.Reader) (req *Request, err os.Error) {
 	return req, nil
 }
 
-// ParseForm parses the raw query.
-// For POST requests, it also parses the request body as a form.
+// MaxBytesReader is similar to io.LimitReader, but is intended for
+// limiting the size of incoming request bodies. In contrast to
+// io.LimitReader, MaxBytesReader is a ReadCloser, returns a non-EOF
+// error if the body is too large, and also takes care of closing the
+// underlying io.ReadCloser connection (if applicable, usually a TCP
+// connection) when the limit is hit.  This prevents clients from
+// accidentally or maliciously sending a large request and wasting
+// server resources.
+func MaxBytesReader(w ResponseWriter, r io.ReadCloser, n int64) io.ReadCloser {
+	return &maxBytesReader{w: w, r: r, n: n}
+}
+
+type maxBytesReader struct {
+	w       ResponseWriter
+	r       io.ReadCloser // underlying reader
+	n       int64         // max bytes remaining
+	stopped bool
+}
+
+func (l *maxBytesReader) Read(p []byte) (n int, err os.Error) {
+	if l.n <= 0 {
+		if !l.stopped {
+			l.stopped = true
+			if res, ok := l.w.(*response); ok {
+				res.requestTooLarge()
+			}
+		}
+		return 0, os.NewError("http: request body too large")
+	}
+	if int64(len(p)) > l.n {
+		p = p[:l.n]
+	}
+	n, err = l.r.Read(p)
+	l.n -= int64(n)
+	return
+}
+
+func (l *maxBytesReader) Close() os.Error {
+	return l.r.Close()
+}
+
+// ParseForm parses the raw query from the URL.
+//
+// For POST or PUT requests, it also parses the request body as a form.
+// If the request Body's size has not already been limited by MaxBytesReader,
+// the size is capped at 10MB.
+//
 // ParseMultipartForm calls ParseForm automatically.
 // It is idempotent.
 func (r *Request) ParseForm() (err os.Error) {
 	if r.Form != nil {
 		return
 	}
-
 	if r.URL != nil {
 		r.Form, err = url.ParseQuery(r.URL.RawQuery)
 	}
-	if r.Method == "POST" {
+	if r.Method == "POST" || r.Method == "PUT" {
 		if r.Body == nil {
 			return os.NewError("missing form body")
 		}
@@ -628,8 +672,13 @@ func (r *Request) ParseForm() (err os.Error) {
 		ct, _, err := mime.ParseMediaType(ct)
 		switch {
 		case ct == "text/plain" || ct == "application/x-www-form-urlencoded" || ct == "":
-			const maxFormSize = int64(10 << 20) // 10 MB is a lot of text.
-			b, e := ioutil.ReadAll(io.LimitReader(r.Body, maxFormSize+1))
+			var reader io.Reader = r.Body
+			maxFormSize := int64((1 << 63) - 1)
+			if _, ok := r.Body.(*maxBytesReader); !ok {
+				maxFormSize = int64(10 << 20) // 10 MB is a lot of text.
+				reader = io.LimitReader(r.Body, maxFormSize+1)
+			}
+			b, e := ioutil.ReadAll(reader)
 			if e != nil {
 				if err == nil {
 					err = e

src/pkg/http/serve_test.go

@@ -896,6 +896,60 @@ func TestRequestLimit(t *testing.T) {
 	}
 }
 
+type neverEnding byte
+
+func (b neverEnding) Read(p []byte) (n int, err os.Error) {
+	for i := range p {
+		p[i] = byte(b)
+	}
+	return len(p), nil
+}
+
+type countReader struct {
+	r io.Reader
+	n *int64
+}
+
+func (cr countReader) Read(p []byte) (n int, err os.Error) {
+	n, err = cr.r.Read(p)
+	*cr.n += int64(n)
+	return
+}
+
+func TestRequestBodyLimit(t *testing.T) {
+	const limit = 1 << 20
+	ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {
+		r.Body = MaxBytesReader(w, r.Body, limit)
+		n, err := io.Copy(ioutil.Discard, r.Body)
+		if err == nil {
+			t.Errorf("expected error from io.Copy")
+		}
+		if n != limit {
+			t.Errorf("io.Copy = %d, want %d", n, limit)
+		}
+	}))
+	defer ts.Close()
+
+	nWritten := int64(0)
+	req, _ := NewRequest("POST", ts.URL, io.LimitReader(countReader{neverEnding('a'), &nWritten}, limit*200))
+
+	// Send the POST, but don't care it succeeds or not.  The
+	// remote side is going to reply and then close the TCP
+	// connection, and HTTP doesn't really define if that's
+	// allowed or not.  Some HTTP clients will get the response
+	// and some (like ours, currently) will complain that the
+	// request write failed, without reading the response.
+	//
+	// But that's okay, since what we're really testing is that
+	// the remote side hung up on us before we wrote too much.
+	_, _ = DefaultClient.Do(req)
+
+	if nWritten > limit*2 {
+		t.Errorf("handler restricted the request body to %d bytes, but client managed to write %d",
+			limit, nWritten)
+	}
+}
+
 type errorListener struct {
 	errs []os.Error
 }

src/pkg/http/server.go

@@ -122,6 +122,25 @@ type response struct {
 	// "Connection: keep-alive" response header and a
 	// Content-Length.
 	closeAfterReply bool
+
+	// requestBodyLimitHit is set by requestTooLarge when
+	// maxBytesReader hits its max size. It is checked in
+	// WriteHeader, to make sure we don't consume the the
+	// remaining request body to try to advance to the next HTTP
+	// request. Instead, when this is set, we stop doing
+	// subsequent requests on this connection and stop reading
+	// input from it.
+	requestBodyLimitHit bool
+}
+
+// requestTooLarge is called by maxBytesReader when too much input has
+// been read from the client.
+func (r *response) requestTooLarge() {
+	r.closeAfterReply = true
+	r.requestBodyLimitHit = true
+	if !r.wroteHeader {
+		r.Header().Set("Connection", "close")
+	}
 }
 
 type writerOnly struct {
@@ -257,7 +276,7 @@ func (w *response) WriteHeader(code int) {
 
 	// Per RFC 2616, we should consume the request body before
 	// replying, if the handler hasn't already done so.
-	if w.req.ContentLength != 0 {
+	if w.req.ContentLength != 0 && !w.requestBodyLimitHit {
 		ecr, isExpecter := w.req.Body.(*expectContinueReader)
 		if !isExpecter || ecr.resp.wroteContinue {
 			w.req.Body.Close()
@@ -543,7 +562,11 @@ func (w *response) finishRequest() {
 		io.WriteString(w.conn.buf, "\r\n")
 	}
 	w.conn.buf.Flush()
+	// Close the body, unless we're about to close the whole TCP connection
+	// anyway.
+	if !w.closeAfterReply {
 		w.req.Body.Close()
+	}
 	if w.req.MultipartForm != nil {
 		w.req.MultipartForm.RemoveAll()
 	}

src/pkg/http/transfer.go

@@ -478,6 +478,8 @@ type body struct {
 	r       *bufio.Reader // underlying wire-format reader for the trailer
 	closing bool          // is the connection to be closed after reading body?
 	closed  bool
+
+	res *response // response writer for server requests, else nil
 }
 
 // ErrBodyReadAfterClose is returned when reading a Request Body after
@@ -506,6 +508,15 @@ func (b *body) Close() os.Error {
 		return nil
 	}
 
+	// In a server request, don't continue reading from the client
+	// if we've already hit the maximum body size set by the
+	// handler. If this is set, that also means the TCP connection
+	// is about to be closed, so getting to the next HTTP request
+	// in the stream is not necessary.
+	if b.res != nil && b.res.requestBodyLimitHit {
+		return nil
+	}
+
 	if _, err := io.Copy(ioutil.Discard, b); err != nil {
 		return err
 	}