From 1ebdb19ff194120109b08bbf888bdcc502f83211 Mon Sep 17 00:00:00 2001
From: Ronan Pigott <ronan@rjp.ie>
Date: Sat, 24 Feb 2024 18:21:24 -0700
Subject: [PATCH] resolved: limit the number of signature validations in a
transaction
It has been demonstrated that tolerating an unbounded number of dnssec
signature validations is a bad idea. It is easy for a maliciously
crafted DNS reply to contain as many keytag collisions as desired,
causing us to iterate every dnskey and signature combination in vain.
The solution is to impose a maximum number of validations we will
tolerate. While collisions are not hard to craft, I still expect they
are unlikely in the wild so it should be safe to pick fairly small
values.
Here two limits are imposed: one on the maximum number of invalid
signatures encountered per rrset, and another on the total number of
validations performed per transaction.
(cherry picked from commit 67d0ce8843d612a2245d0966197d4f528b911b66)
Conflict:NA
Reference:https://github.com/systemd/systemd-stable/commit/1ebdb19ff194120109b08bbf888bdcc502f83211
---
src/resolve/resolved-dns-dnssec.c | 16 ++++++++++++++--
src/resolve/resolved-dns-dnssec.h | 9 ++++++++-
src/resolve/resolved-dns-transaction.c | 19 ++++++++++++++++---
3 files changed, 38 insertions(+), 6 deletions(-)
diff --git a/src/resolve/resolved-dns-dnssec.c b/src/resolve/resolved-dns-dnssec.c
index 2580c2333c..aa87820dca 100644
--- a/src/resolve/resolved-dns-dnssec.c
+++ b/src/resolve/resolved-dns-dnssec.c
@@ -1169,6 +1169,7 @@ int dnssec_verify_rrset_search(
DnsResourceRecord **ret_rrsig) {
bool found_rrsig = false, found_invalid = false, found_expired_rrsig = false, found_unsupported_algorithm = false;
+ unsigned nvalidations = 0;
DnsResourceRecord *rrsig;
int r;
@@ -1214,6 +1215,14 @@ int dnssec_verify_rrset_search(
if (realtime == USEC_INFINITY)
realtime = now(CLOCK_REALTIME);
+ /* Have we seen an unreasonable number of invalid signaures? */
+ if (nvalidations > DNSSEC_INVALID_MAX) {
+ if (ret_rrsig)
+ *ret_rrsig = NULL;
+ *result = DNSSEC_TOO_MANY_VALIDATIONS;
+ return (int) nvalidations;
+ }
+
/* Yay, we found a matching RRSIG with a matching
* DNSKEY, awesome. Now let's verify all entries of
* the RRSet against the RRSIG and DNSKEY
@@ -1223,6 +1232,8 @@ int dnssec_verify_rrset_search(
if (r < 0)
return r;
+ nvalidations++;
+
switch (one_result) {
case DNSSEC_VALIDATED:
@@ -1233,7 +1244,7 @@ int dnssec_verify_rrset_search(
*ret_rrsig = rrsig;
*result = one_result;
- return 0;
+ return (int) nvalidations;
case DNSSEC_INVALID:
/* If the signature is invalid, let's try another
@@ -1280,7 +1291,7 @@ int dnssec_verify_rrset_search(
if (ret_rrsig)
*ret_rrsig = NULL;
- return 0;
+ return (int) nvalidations;
}
int dnssec_has_rrsig(DnsAnswer *a, const DnsResourceKey *key) {
@@ -2564,6 +2575,7 @@ static const char* const dnssec_result_table[_DNSSEC_RESULT_MAX] = {
[DNSSEC_FAILED_AUXILIARY] = "failed-auxiliary",
[DNSSEC_NSEC_MISMATCH] = "nsec-mismatch",
[DNSSEC_INCOMPATIBLE_SERVER] = "incompatible-server",
+ [DNSSEC_TOO_MANY_VALIDATIONS] = "too-many-validations",
};
DEFINE_STRING_TABLE_LOOKUP(dnssec_result, DnssecResult);
diff --git a/src/resolve/resolved-dns-dnssec.h b/src/resolve/resolved-dns-dnssec.h
index 954bb3ef9d..29b90130a3 100644
--- a/src/resolve/resolved-dns-dnssec.h
+++ b/src/resolve/resolved-dns-dnssec.h
@@ -9,12 +9,13 @@ typedef enum DnssecVerdict DnssecVerdict;
#include "resolved-dns-rr.h"
enum DnssecResult {
- /* These five are returned by dnssec_verify_rrset() */
+ /* These six are returned by dnssec_verify_rrset() */
DNSSEC_VALIDATED,
DNSSEC_VALIDATED_WILDCARD, /* Validated via a wildcard RRSIG, further NSEC/NSEC3 checks necessary */
DNSSEC_INVALID,
DNSSEC_SIGNATURE_EXPIRED,
DNSSEC_UNSUPPORTED_ALGORITHM,
+ DNSSEC_TOO_MANY_VALIDATIONS,
/* These two are added by dnssec_verify_rrset_search() */
DNSSEC_NO_SIGNATURE,
@@ -45,6 +46,12 @@ enum DnssecVerdict {
/* The longest digest we'll ever generate, of all digest algorithms we support */
#define DNSSEC_HASH_SIZE_MAX (MAX(20, 32))
+/* The most invalid signatures we will tolerate for a single rrset */
+#define DNSSEC_INVALID_MAX 5
+
+/* The total number of signature validations we will tolerate for a single transaction */
+#define DNSSEC_VALIDATION_MAX 64
+
int dnssec_rrsig_match_dnskey(DnsResourceRecord *rrsig, DnsResourceRecord *dnskey, bool revoked_ok);
int dnssec_key_match_rrsig(const DnsResourceKey *key, DnsResourceRecord *rrsig);
diff --git a/src/resolve/resolved-dns-transaction.c b/src/resolve/resolved-dns-transaction.c
index 6c931d71dc..8ff5653dff 100644
--- a/src/resolve/resolved-dns-transaction.c
+++ b/src/resolve/resolved-dns-transaction.c
@@ -3163,11 +3163,14 @@ static int dnssec_validate_records(
DnsTransaction *t,
Phase phase,
bool *have_nsec,
+ unsigned *nvalidations,
DnsAnswer **validated) {
DnsResourceRecord *rr;
int r;
+ assert(nvalidations);
+
/* Returns negative on error, 0 if validation failed, 1 to restart validation, 2 when finished. */
DNS_ANSWER_FOREACH(rr, t->answer) {
@@ -3209,6 +3212,7 @@ static int dnssec_validate_records(
&rrsig);
if (r < 0)
return r;
+ *nvalidations += r;
log_debug("Looking at %s: %s", strna(dns_resource_record_to_string(rr)), dnssec_result_to_string(result));
@@ -3406,7 +3410,8 @@ static int dnssec_validate_records(
DNSSEC_SIGNATURE_EXPIRED,
DNSSEC_NO_SIGNATURE))
manager_dnssec_verdict(t->scope->manager, DNSSEC_BOGUS, rr->key);
- else /* DNSSEC_MISSING_KEY or DNSSEC_UNSUPPORTED_ALGORITHM */
+ else /* DNSSEC_MISSING_KEY, DNSSEC_UNSUPPORTED_ALGORITHM,
+ or DNSSEC_TOO_MANY_VALIDATIONS */
manager_dnssec_verdict(t->scope->manager, DNSSEC_INDETERMINATE, rr->key);
/* This is a primary response to our question, and it failed validation.
@@ -3499,13 +3504,21 @@ int dns_transaction_validate_dnssec(DnsTransaction *t) {
return r;
phase = DNSSEC_PHASE_DNSKEY;
- for (;;) {
+ for (unsigned nvalidations = 0;;) {
bool have_nsec = false;
- r = dnssec_validate_records(t, phase, &have_nsec, &validated);
+ r = dnssec_validate_records(t, phase, &have_nsec, &nvalidations, &validated);
if (r <= 0)
return r;
+ if (nvalidations > DNSSEC_VALIDATION_MAX) {
+ /* This reply requires an onerous number of signature validations to verify. Let's
+ * not waste our time trying, as this shouldn't happen for well-behaved domains
+ * anyway. */
+ t->answer_dnssec_result = DNSSEC_TOO_MANY_VALIDATIONS;
+ return 0;
+ }
+
/* Try again as long as we managed to achieve something */
if (r == 1)
continue;
--
2.33.0