fix: usr: Fix rndc-confgen aborting on HMAC-SHA-384/512 keys above 512 bits

`rndc-confgen -A hmac-sha384` and `-A hmac-sha512` documented a `-b`
range of 1..1024, but any value above 512 aborted on hardened builds
instead of producing a key. The full advertised range now works.

Closes #5903

Merge branch '5903-hmac-generate-stack-overflow' into 'main'

See merge request isc-projects/bind9!11903

bin/confgen/keygen.c

@@ -21,6 +21,7 @@
 #include <isc/base64.h>
 #include <isc/buffer.h>
 #include <isc/file.h>
+#include <isc/md.h>
 #include <isc/mem.h>
 #include <isc/result.h>
 #include <isc/string.h>
@@ -117,7 +118,7 @@ generate_key(isc_mem_t *mctx, dns_secalg_t alg, int keysize,
 	isc_result_t result = ISC_R_SUCCESS;
 	isc_buffer_t key_rawbuffer;
 	isc_region_t key_rawregion;
-	char key_rawsecret[64];
+	char key_rawsecret[ISC_MAX_BLOCK_SIZE];
 	dst_key_t *key = NULL;
 
 	switch (alg) {

bin/tests/system/rndc_confgen/tests_rndc_confgen.py

@@ -9,7 +9,9 @@
 # See the COPYRIGHT file distributed with this work for additional
 # information regarding copyright ownership.
 
+import base64
 import os
+import re
 import subprocess
 
 import pytest
@@ -67,3 +69,33 @@ def test_ddns_confgen_default():
 def test_ddns_confgen_rejects_injection(args):
     with pytest.raises(subprocess.CalledProcessError):
         isctest.run.cmd([DDNSCONFGEN, "-q", *args])
+
+
+def _extract_secret(stdout: bytes) -> bytes:
+    match = re.search(rb'secret\s+"([^"]+)"', stdout)
+    assert match is not None, f"no secret in output: {stdout!r}"
+    return base64.b64decode(match.group(1))
+
+
+@pytest.mark.parametrize(
+    "algorithm,bits",
+    [
+        ("hmac-sha256", 1),
+        ("hmac-sha256", 256),
+        ("hmac-sha256", 512),
+        ("hmac-sha384", 1),
+        ("hmac-sha384", 384),
+        ("hmac-sha384", 513),
+        ("hmac-sha384", 768),
+        ("hmac-sha384", 1024),
+        ("hmac-sha512", 1),
+        ("hmac-sha512", 512),
+        ("hmac-sha512", 513),
+        ("hmac-sha512", 1024),
+    ],
+)
+def test_rndc_confgen_hmac_keysize(algorithm, bits):
+    cmd = isctest.run.cmd([os.environ["RNDCCONFGEN"], "-A", algorithm, "-b", str(bits)])
+    secret = _extract_secret(cmd.proc.stdout)
+    assert len(secret) == (bits + 7) // 8
+    assert f"algorithm {algorithm};".encode() in cmd.proc.stdout

lib/dns/hmac_link.c

@@ -249,7 +249,7 @@ hmac_generate(isc_md_type_t type, dst_key_t *key) {
 	isc_buffer_t b;
 	isc_result_t result;
 	unsigned int bytes, len;
-	unsigned char data[ISC_MAX_MD_SIZE] = { 0 };
+	unsigned char data[ISC_MAX_BLOCK_SIZE] = { 0 };
 
 	len = isc_md_type_get_block_size(type);