v1-0002-libpq-Disallow-cancel-keys-longer-than-256-bytes.patch

From 591ed4beea42e0982d5fbb0aaae1abf27ad8b099 Mon Sep 17 00:00:00 2001
From: Jelte Fennema-Nio <github-tech@jeltef.nl>
Date: Wed, 25 Jun 2025 08:54:15 +0200
Subject: [PATCH v1 2/2] libpq: Disallow cancel keys longer than 256 bytes

The protocol documentation states that the maximum length of a cancel key
is 256 bytes. This starts checking for that limit in libpq. Otherwise
third party backend implementations will probably start using more bytes
anyway.
---
 src/interfaces/libpq/fe-protocol3.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/interfaces/libpq/fe-protocol3.c b/src/interfaces/libpq/fe-protocol3.c
index beb1c889aad..fc34fb52766 100644
--- a/src/interfaces/libpq/fe-protocol3.c
+++ b/src/interfaces/libpq/fe-protocol3.c
@@ -1546,6 +1546,12 @@ getBackendKeyData(PGconn *conn, int msgLength)
 
 	cancel_key_len = 5 + msgLength - (conn->inCursor - conn->inStart);
 
+	if (cancel_key_len > 256)
+	{
+		libpq_append_conn_error(conn, "received invalid BackendKeyData message: cancel key length %d exceeds maximum of 256 bytes", cancel_key_len);
+		return EOF;
+	}
+
 	conn->be_cancel_key = malloc(cancel_key_len);
 	if (conn->be_cancel_key == NULL)
 	{
-- 
2.43.0