http2: stop reading from socket if writes are in progress

If a write to the underlying socket finishes asynchronously, that
means that we cannot write any more data at that point without waiting
for it to finish. If this happens, we should also not be producing any
more input.

This is part of mitigating CVE-2019-9511/CVE-2019-9517.

src/node_http2.cc

@@ -1542,9 +1542,18 @@ void Http2Session::HandleSettingsFrame(const nghttp2_frame* frame) {
 void Http2Session::OnStreamAfterWrite(WriteWrap* w, int status) {
   Debug(this, "write finished with status %d", status);
 
+  CHECK_NE(flags_ & SESSION_STATE_WRITE_IN_PROGRESS, 0);
+  flags_ &= ~SESSION_STATE_WRITE_IN_PROGRESS;
+
   // Inform all pending writes about their completion.
   ClearOutgoing(status);
 
+  if ((flags_ & SESSION_STATE_READING_STOPPED) &&
+      nghttp2_session_want_read(session_)) {
+    flags_ &= ~SESSION_STATE_READING_STOPPED;
+    stream_->ReadStart();
+  }
+
   if (!(flags_ & SESSION_STATE_WRITE_SCHEDULED)) {
     // Schedule a new write if nghttp2 wants to send data.
     MaybeScheduleWrite();
@@ -1582,10 +1591,13 @@ void Http2Session::MaybeScheduleWrite() {
 }
 
 void Http2Session::MaybeStopReading() {
+  if (flags_ & SESSION_STATE_READING_STOPPED) return;
   int want_read = nghttp2_session_want_read(session_);
   Debug(this, "wants read? %d", want_read);
-  if (want_read == 0)
+  if (want_read == 0 || (flags_ & SESSION_STATE_WRITE_IN_PROGRESS)) {
+    flags_ |= SESSION_STATE_READING_STOPPED;
     stream_->ReadStop();
+  }
 }
 
 // Unset the sending state, finish up all current writes, and reset
@@ -1716,8 +1728,11 @@ uint8_t Http2Session::SendPendingData() {
 
   chunks_sent_since_last_write_++;
 
+  CHECK_EQ(flags_ & SESSION_STATE_WRITE_IN_PROGRESS, 0);
+  flags_ |= SESSION_STATE_WRITE_IN_PROGRESS;
   StreamWriteResult res = underlying_stream()->Write(*bufs, count);
   if (!res.async) {
+    flags_ &= ~SESSION_STATE_WRITE_IN_PROGRESS;
     ClearOutgoing(res.err);
   }
 

src/node_http2.h

@@ -335,6 +335,8 @@ enum session_state_flags {
   SESSION_STATE_CLOSED = 0x4,
   SESSION_STATE_CLOSING = 0x8,
   SESSION_STATE_SENDING = 0x10,
+  SESSION_STATE_WRITE_IN_PROGRESS = 0x20,
+  SESSION_STATE_READING_STOPPED = 0x40,
 };
 
 typedef uint32_t(*get_setting)(nghttp2_session* session,