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185 lines
8.0 KiB
185 lines
8.0 KiB
8 months ago
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From bcb4e65f7ab49fd94b002ff9e1bba24237082726 Mon Sep 17 00:00:00 2001
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From: Jacek Migacz <jmigacz@redhat.com>
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Date: Mon, 26 Feb 2024 14:05:37 +0100
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Subject: [PATCH] resolved: limit the number of signature validations in a
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transaction
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It has been demonstrated that tolerating an unbounded number of dnssec
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signature validations is a bad idea. It is easy for a maliciously
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crafted DNS reply to contain as many keytag collisions as desired,
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causing us to iterate every dnskey and signature combination in vain.
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The solution is to impose a maximum number of validations we will
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tolerate. While collisions are not hard to craft, I still expect they
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are unlikely in the wild so it should be safe to pick fairly small
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values.
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Here two limits are imposed: one on the maximum number of invalid
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signatures encountered per rrset, and another on the total number of
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validations performed per transaction.
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(cherry picked from commit 67d0ce8843d612a2245d0966197d4f528b911b66)
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Resolves: RHEL-26643
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---
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src/resolve/resolved-dns-dnssec.c | 16 ++++++++++++++--
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src/resolve/resolved-dns-dnssec.h | 9 ++++++++-
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src/resolve/resolved-dns-transaction.c | 19 ++++++++++++++++---
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3 files changed, 38 insertions(+), 6 deletions(-)
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diff --git a/src/resolve/resolved-dns-dnssec.c b/src/resolve/resolved-dns-dnssec.c
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index 426ea945ca..de2660e317 100644
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--- a/src/resolve/resolved-dns-dnssec.c
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+++ b/src/resolve/resolved-dns-dnssec.c
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@@ -1176,6 +1176,7 @@ int dnssec_verify_rrset_search(
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DnsResourceRecord **ret_rrsig) {
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bool found_rrsig = false, found_invalid = false, found_expired_rrsig = false, found_unsupported_algorithm = false;
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+ unsigned nvalidations = 0;
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DnsResourceRecord *rrsig;
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int r;
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@@ -1221,6 +1222,14 @@ int dnssec_verify_rrset_search(
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if (realtime == USEC_INFINITY)
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realtime = now(CLOCK_REALTIME);
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+ /* Have we seen an unreasonable number of invalid signaures? */
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+ if (nvalidations > DNSSEC_INVALID_MAX) {
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+ if (ret_rrsig)
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+ *ret_rrsig = NULL;
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+ *result = DNSSEC_TOO_MANY_VALIDATIONS;
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+ return (int) nvalidations;
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+ }
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+
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/* Yay, we found a matching RRSIG with a matching
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* DNSKEY, awesome. Now let's verify all entries of
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* the RRSet against the RRSIG and DNSKEY
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@@ -1230,6 +1239,8 @@ int dnssec_verify_rrset_search(
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if (r < 0)
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return r;
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+ nvalidations++;
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+
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switch (one_result) {
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case DNSSEC_VALIDATED:
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@@ -1240,7 +1251,7 @@ int dnssec_verify_rrset_search(
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*ret_rrsig = rrsig;
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*result = one_result;
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- return 0;
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+ return (int) nvalidations;
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case DNSSEC_INVALID:
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/* If the signature is invalid, let's try another
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@@ -1287,7 +1298,7 @@ int dnssec_verify_rrset_search(
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if (ret_rrsig)
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*ret_rrsig = NULL;
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- return 0;
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+ return (int) nvalidations;
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}
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int dnssec_has_rrsig(DnsAnswer *a, const DnsResourceKey *key) {
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@@ -2571,6 +2582,7 @@ static const char* const dnssec_result_table[_DNSSEC_RESULT_MAX] = {
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[DNSSEC_FAILED_AUXILIARY] = "failed-auxiliary",
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[DNSSEC_NSEC_MISMATCH] = "nsec-mismatch",
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[DNSSEC_INCOMPATIBLE_SERVER] = "incompatible-server",
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+ [DNSSEC_TOO_MANY_VALIDATIONS] = "too-many-validations",
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};
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DEFINE_STRING_TABLE_LOOKUP(dnssec_result, DnssecResult);
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diff --git a/src/resolve/resolved-dns-dnssec.h b/src/resolve/resolved-dns-dnssec.h
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index 954bb3ef9d..29b90130a3 100644
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--- a/src/resolve/resolved-dns-dnssec.h
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+++ b/src/resolve/resolved-dns-dnssec.h
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@@ -9,12 +9,13 @@ typedef enum DnssecVerdict DnssecVerdict;
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#include "resolved-dns-rr.h"
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enum DnssecResult {
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- /* These five are returned by dnssec_verify_rrset() */
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+ /* These six are returned by dnssec_verify_rrset() */
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DNSSEC_VALIDATED,
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DNSSEC_VALIDATED_WILDCARD, /* Validated via a wildcard RRSIG, further NSEC/NSEC3 checks necessary */
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DNSSEC_INVALID,
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DNSSEC_SIGNATURE_EXPIRED,
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DNSSEC_UNSUPPORTED_ALGORITHM,
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+ DNSSEC_TOO_MANY_VALIDATIONS,
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/* These two are added by dnssec_verify_rrset_search() */
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DNSSEC_NO_SIGNATURE,
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@@ -45,6 +46,12 @@ enum DnssecVerdict {
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/* The longest digest we'll ever generate, of all digest algorithms we support */
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#define DNSSEC_HASH_SIZE_MAX (MAX(20, 32))
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+/* The most invalid signatures we will tolerate for a single rrset */
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+#define DNSSEC_INVALID_MAX 5
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+
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+/* The total number of signature validations we will tolerate for a single transaction */
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+#define DNSSEC_VALIDATION_MAX 64
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+
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int dnssec_rrsig_match_dnskey(DnsResourceRecord *rrsig, DnsResourceRecord *dnskey, bool revoked_ok);
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int dnssec_key_match_rrsig(const DnsResourceKey *key, DnsResourceRecord *rrsig);
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diff --git a/src/resolve/resolved-dns-transaction.c b/src/resolve/resolved-dns-transaction.c
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index 0306af84a2..ccca49d399 100644
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--- a/src/resolve/resolved-dns-transaction.c
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+++ b/src/resolve/resolved-dns-transaction.c
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@@ -3140,11 +3140,14 @@ static int dnssec_validate_records(
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DnsTransaction *t,
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Phase phase,
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bool *have_nsec,
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+ unsigned *nvalidations,
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DnsAnswer **validated) {
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DnsResourceRecord *rr;
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int r;
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+ assert(nvalidations);
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+
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/* Returns negative on error, 0 if validation failed, 1 to restart validation, 2 when finished. */
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DNS_ANSWER_FOREACH(rr, t->answer) {
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@@ -3186,6 +3189,7 @@ static int dnssec_validate_records(
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&rrsig);
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if (r < 0)
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return r;
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+ *nvalidations += r;
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log_debug("Looking at %s: %s", strna(dns_resource_record_to_string(rr)), dnssec_result_to_string(result));
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@@ -3383,7 +3387,8 @@ static int dnssec_validate_records(
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DNSSEC_SIGNATURE_EXPIRED,
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DNSSEC_NO_SIGNATURE))
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manager_dnssec_verdict(t->scope->manager, DNSSEC_BOGUS, rr->key);
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- else /* DNSSEC_MISSING_KEY or DNSSEC_UNSUPPORTED_ALGORITHM */
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+ else /* DNSSEC_MISSING_KEY, DNSSEC_UNSUPPORTED_ALGORITHM,
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+ or DNSSEC_TOO_MANY_VALIDATIONS */
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manager_dnssec_verdict(t->scope->manager, DNSSEC_INDETERMINATE, rr->key);
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/* This is a primary response to our question, and it failed validation.
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@@ -3476,13 +3481,21 @@ int dns_transaction_validate_dnssec(DnsTransaction *t) {
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return r;
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phase = DNSSEC_PHASE_DNSKEY;
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- for (;;) {
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+ for (unsigned nvalidations = 0;;) {
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bool have_nsec = false;
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- r = dnssec_validate_records(t, phase, &have_nsec, &validated);
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+ r = dnssec_validate_records(t, phase, &have_nsec, &nvalidations, &validated);
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if (r <= 0)
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return r;
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+ if (nvalidations > DNSSEC_VALIDATION_MAX) {
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+ /* This reply requires an onerous number of signature validations to verify. Let's
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+ * not waste our time trying, as this shouldn't happen for well-behaved domains
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+ * anyway. */
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+ t->answer_dnssec_result = DNSSEC_TOO_MANY_VALIDATIONS;
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+ return 0;
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+ }
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+
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/* Try again as long as we managed to achieve something */
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if (r == 1)
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continue;
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