stateless-connect: send response end packet

Currently, remote-curl acts as a proxy and blindly forwards packets
between an HTTP server and fetch-pack. In the case of a stateless RPC
connection where the connection is terminated before the transaction is
complete, remote-curl will blindly forward the packets before waiting on
more input from fetch-pack. Meanwhile, fetch-pack will read the
transaction and continue reading, expecting more input to continue the
transaction. This results in a deadlock between the two processes.

This can be seen in the following command which does not terminate:

	$ git -c protocol.version=2 clone https://github.com/git/git.git --shallow-since=20151012
	Cloning into 'git'...

whereas the v1 version does terminate as expected:

	$ git -c protocol.version=1 clone https://github.com/git/git.git --shallow-since=20151012
	Cloning into 'git'...
	fatal: the remote end hung up unexpectedly

Instead of blindly forwarding packets, make remote-curl insert a
response end packet after proxying the responses from the remote server
when using stateless_connect(). On the RPC client side, ensure that each
response ends as described.

A separate control packet is chosen because we need to be able to
differentiate between what the remote server sends and remote-curl's
control packets. By ensuring in the remote-curl code that a server
cannot send response end packets, we prevent a malicious server from
being able to perform a denial of service attack in which they spoof a
response end packet and cause the described deadlock to happen.

Reported-by: Force Charlie <charlieio@outlook.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/remote-curl.c b/remote-curl.c
index d02cb547e9..75532a8bae 100644
--- a/remote-curl.c
+++ b/remote-curl.c
@@ -703,6 +703,8 @@ static void check_pktline(struct check_pktline_state *state, const char *ptr, si
 				state->remaining = packet_length(state->len_buf);
 				if (state->remaining < 0) {
 					die(_("remote-curl: bad line length character: %.4s"), state->len_buf);
+				} else if (state->remaining == 2) {
+					die(_("remote-curl: unexpected response end packet"));
 				} else if (state->remaining < 4) {
 					state->remaining = 0;
 				} else {
@@ -991,6 +993,9 @@ retry:
 	if (rpc_in_data.pktline_state.remaining)
 		err = error(_("%d bytes of body are still expected"), rpc_in_data.pktline_state.remaining);
 
+	if (stateless_connect)
+		packet_response_end(rpc->in);
+
 	curl_slist_free_all(headers);
 	free(gzip_body);
 	return err;