HTTP inbound: Fix a potential panic in readResponseAndHandle100Continue() (#6545)

proxy/http/server.go

@@ -332,7 +332,7 @@ func readResponseAndHandle100Continue(r *bufio.Reader, req *http.Request, writer
 					return nil, errors.New("failed to read http 1xx response").Base(err)
 				}
 				ResponseHeader1xx = append(ResponseHeader1xx, data...)
-				if bytes.Equal(ResponseHeader1xx[len(ResponseHeader1xx)-4:], []byte{'\r', '\n', '\r', '\n'}) {
+				if len(ResponseHeader1xx) >= 4 && bytes.Equal(ResponseHeader1xx[len(ResponseHeader1xx)-4:], []byte{'\r', '\n', '\r', '\n'}) {
 					break
 				}
 				if len(ResponseHeader1xx) > 1024 {

proxy/http/server_test.go

@@ -0,0 +1,49 @@
+package http
+
+import (
+	"bufio"
+	"bytes"
+	"io"
+	"net/http"
+	"strings"
+	"testing"
+)
+
+// A malformed upstream response containing a bare '\n' before the real
+// status line used to crash readResponseAndHandle100Continue: the first
+// ReadSlice('\n') returns fewer than 4 bytes, and slicing
+// ResponseHeader1xx[len(ResponseHeader1xx)-4:] panicked with a negative
+// index instead of returning an error.
+func TestReadResponseAndHandle100ContinueDoesNotPanicOnEarlyNewline(t *testing.T) {
+	payload := "X\nHTTP/1.1 100 Continue\r\n\r\n" + strings.Repeat("A", 40)
+	r := bufio.NewReader(bytes.NewReader([]byte(payload)))
+	req, err := http.NewRequest("GET", "http://example.com/", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Must not panic; a parse error for the garbage trailing bytes is fine.
+	_, _ = readResponseAndHandle100Continue(r, req, io.Discard)
+}
+
+func TestReadResponseAndHandle100ContinueForwardsAndParsesFinalResponse(t *testing.T) {
+	payload := "HTTP/1.1 100 Continue\r\n\r\n" +
+		"HTTP/1.1 200 OK\r\nContent-Length: 5\r\n\r\nhello"
+	r := bufio.NewReader(bytes.NewReader([]byte(payload)))
+	req, err := http.NewRequest("GET", "http://example.com/", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var forwarded bytes.Buffer
+	resp, err := readResponseAndHandle100Continue(r, req, &forwarded)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if resp.StatusCode != 200 {
+		t.Fatalf("expected status 200, got %d", resp.StatusCode)
+	}
+	if !strings.Contains(forwarded.String(), "100 Continue") {
+		t.Fatalf("expected 1xx response to be forwarded, got %q", forwarded.String())
+	}
+}