Limit the kex deferred packet queue length

src/common-session.c

@@ -117,6 +117,7 @@ void common_session_init(int sock_in, int sock_out) {
 	ses.lastpacket = 0;
 	ses.reply_queue_head = NULL;
 	ses.reply_queue_tail = NULL;
+	ses.reply_queue_len = 0;
 
 	/* set all the algos to none */
 	ses.keys = (struct key_context*)m_malloc(sizeof(struct key_context));

src/packet.c

@@ -477,6 +477,12 @@ static int packet_is_okay_kex(unsigned char type) {
 
 static void enqueue_reply_packet(void) {
 	struct packetlist * new_item = NULL;
+
+	if (ses.reply_queue_len > MAX_DEFER_REPLY_QUEUE) {
+		/* This limit should never be reached by a normal peer. */
+		dropbear_exit("Full reply queue");
+	}
+
 	new_item = m_malloc(sizeof(struct packetlist));
 	new_item->next = NULL;
 
@@ -490,6 +496,7 @@ static void enqueue_reply_packet(void) {
 		ses.reply_queue_head = new_item;
 	}
 	ses.reply_queue_tail = new_item;
+	ses.reply_queue_len++;
 }
 
 void maybe_flush_reply_queue() {
@@ -512,6 +519,7 @@ void maybe_flush_reply_queue() {
 		encrypt_packet();
 	}
 	ses.reply_queue_head = ses.reply_queue_tail = NULL;
+	ses.reply_queue_len = 0;
 }
 
 /* encrypt the writepayload, putting into writebuf, ready for write_packet()

src/session.h

@@ -206,6 +206,7 @@ struct sshsession {
 	/* a list of queued replies that should be sent after a KEX has
 	   concluded (ie, while dataallowed was unset)*/
 	struct packetlist *reply_queue_head, *reply_queue_tail;
+	size_t reply_queue_len;
 
 	void(*remoteclosed)(void); /* A callback to handle closure of the
 									  remote connection */

src/sysoptions.h

@@ -259,6 +259,10 @@
 #define MAX_STRING_LEN (MAX(MAX_CMD_LEN, 2400)) /* Sun SSH needs 2400 for algos,
                                                    MAX_CMD_LEN is usually longer */
 
+/* Count of packets ot enqueue deferring while a KEX is in progress.
+ * We wouldn't expect to be anywhere near one-per-channel in normal
+ * operation, but all replies are small so this is a reasonable upper bound */
+#define MAX_DEFER_REPLY_QUEUE MAX_CHANNELS
 
 /* Key type sizes are ordered large to small, all are
  determined empirically, and rounded up */