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 67d0ce8)

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);
 

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);
 

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;