[9.20] fix: usr: Fix zone verification of NSEC3 signed zones

Previously, when computing the compressed bitmap during verification of an NSEC3-signed zone, an undersized buffer was used that resulted in an out-of-bounds write if there were too many active windows in the bitmap. This impacted mirror zones which are NSEC3-signed, `dnssec-signzone` and `dnssec-verifyzone`. This has been fixed.

Closes #5834

Backport of MR !11804

Merge branch 'backport-5834-fix-cbm-size-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11833

bin/tests/system/dnssec/tests.sh

@@ -3492,6 +3492,35 @@ n=$((n + 1))
 test "$ret" -eq 0 || echo_i "failed"
 status=$((status + ret))
 
+echo_i "checking maximal sized compresses bit map works ($n)"
+ret=0
+(
+  cd signer || exit 0
+  key1=$(${KEYGEN} -a "${DEFAULT_ALGORITHM}" -f KSK maxcbm.example)
+  key2=$(${KEYGEN} -a "${DEFAULT_ALGORITHM}" maxcbm.example)
+  cat >>maxcbm.example.db <<EOF
+\$TTL 3600
+@ SOA . . 0 0 0 0 3600
+@ NS .
+\$INCLUDE "${key1}.key"
+\$INCLUDE "${key2}.key"
+; the last data type in the first window
+data TYPE127 \# 0
+EOF
+  # add a record at the end of each cbm window less 1
+  type=$((256 + 254))
+  while test $type -lt 65536; do
+    echo "data TYPE$type \\# 0" >>maxcbm.example.db
+    type=$((type + 256))
+  done
+  "${SIGNER}" -3 - -o maxcbm.example maxcbm.example.db >signer.out.$n
+  "${CHECKZONE}" -q -D maxcbm.example maxcbm.example.db.signed \
+    | grep '^M7L6E3AJUD7LRVUMMQS595OGHBMT4DFT.*NSEC3.*TYPE65534$' >/dev/null || ret=1
+) || ret=1
+n=$((n + 1))
+if [ "$ret" -ne 0 ]; then echo_i "failed"; fi
+status=$((status + ret))
+
 echo_i "check that 'dnssec-keygen -S' works for all supported algorithms ($n)"
 ret=0
 alg=1

bin/tests/system/dnssec/tests_sh_dnssec.py

@@ -172,6 +172,7 @@ pytestmark = pytest.mark.extra_artifacts(
         "signer/general/signed.expect",
         "signer/general/signed.zone",
         "signer/general/signer.out.*",
+        "signer/maxcbm.example.db",
         "signer/nsec3param.out",
         "signer/prepub.db",
         "signer/revoke.example.db",

lib/dns/include/dns/nsec.h

@@ -23,7 +23,12 @@
 #include <dns/name.h>
 #include <dns/types.h>
 
-#define DNS_NSEC_BUFFERSIZE (DNS_NAME_MAXWIRE + 8192 + 512)
+/*
+ * max compressed bitmap size:
+ * 256 windows * (window number + window length + bitmap (max 256 bits))
+ */
+#define DNS_NSEC_MAXCBMSIZE (256 * ((256 / 8) + 2))
+#define DNS_NSEC_BUFFERSIZE (DNS_NAME_MAXWIRE + DNS_NSEC_MAXCBMSIZE)
 
 ISC_LANG_BEGINDECLS
 

lib/dns/zoneverify.c

@@ -460,7 +460,7 @@ match_nsec3(const vctx_t *vctx, const dns_name_t *name,
 	    const unsigned char types[8192], unsigned int maxtype,
 	    const unsigned char *rawhash, size_t rhsize,
 	    isc_result_t *vresult) {
-	unsigned char cbm[8244];
+	unsigned char cbm[DNS_NSEC_MAXCBMSIZE];
 	char namebuf[DNS_NAME_FORMATSIZE];
 	dns_rdata_nsec3_t nsec3;
 	isc_result_t result;