From: =?utf-8?b?VmxhZGltw61yIMSMdW7DoXQ=?= Date: Tue, 2 Jan 2024 11:18:31 +0100 Subject: validator: similarly also limit excessive NSEC3 salt length Limit combination of iterations and salt length, based on estimated expense of the computation. Note that the result only differs for salt length > 44 which is rather nonsensical and very rare: https://chat.dns-oarc.net/community/pl/h58qx9sjkbgt9dajb7x988p78a --- lib/cache/api.c | 2 +- lib/cache/nsec3.c | 2 +- lib/dnssec/nsec3.c | 4 ++-- lib/dnssec/nsec3.h | 32 ++++++++++++++++++++++++++++---- lib/layer/validate.c | 7 ++++--- 5 files changed, 36 insertions(+), 11 deletions(-) diff --git a/lib/cache/api.c b/lib/cache/api.c index 116d775..bb627ea 100644 --- a/lib/cache/api.c +++ b/lib/cache/api.c @@ -500,7 +500,7 @@ static ssize_t stash_rrset(struct kr_cache *cache, const struct kr_query *qry, return kr_ok(); } if (rr->type == KNOT_RRTYPE_NSEC3 && rr->rrs.count - && knot_nsec3_iters(rr->rrs.rdata) > KR_NSEC3_MAX_ITERATIONS) { + && kr_nsec3_limited_rdata(rr->rrs.rdata)) { /* This shouldn't happen often, thanks to downgrades during validation. */ VERBOSE_MSG(qry, "=> skipping NSEC3 with too many iterations\n"); return kr_ok(); diff --git a/lib/cache/nsec3.c b/lib/cache/nsec3.c index 0b70775..9832630 100644 --- a/lib/cache/nsec3.c +++ b/lib/cache/nsec3.c @@ -84,7 +84,7 @@ static knot_db_val_t key_NSEC3_name(struct key *k, const knot_dname_t *name, .data = (uint8_t *)/*const-cast*/name, }; - if (kr_fails_assert(nsec_p->libknot.iterations <= KR_NSEC3_MAX_ITERATIONS)) { + if (kr_fails_assert(!kr_nsec3_limited_params(&nsec_p->libknot))) { /* This is mainly defensive; it shouldn't happen thanks to downgrades. */ return VAL_EMPTY; } diff --git a/lib/dnssec/nsec3.c b/lib/dnssec/nsec3.c index 037d5bd..e4d314b 100644 --- a/lib/dnssec/nsec3.c +++ b/lib/dnssec/nsec3.c @@ -71,7 +71,7 @@ static int hash_name(dnssec_binary_t *hash, const dnssec_nsec3_params_t *params, return kr_error(EINVAL); if (!name) return kr_error(EINVAL); - if (kr_fails_assert(params->iterations <= KR_NSEC3_MAX_ITERATIONS)) { + if (kr_fails_assert(!kr_nsec3_limited_params(params))) { /* This if is mainly defensive; it shouldn't happen. */ return kr_error(EINVAL); } @@ -565,7 +565,7 @@ int kr_nsec3_wildcard_answer_response_check(const knot_pkt_t *pkt, knot_section_ const knot_rrset_t *rrset = knot_pkt_rr(sec, i); if (rrset->type != KNOT_RRTYPE_NSEC3) continue; - if (knot_nsec3_iters(rrset->rrs.rdata) > KR_NSEC3_MAX_ITERATIONS) { + if (kr_nsec3_limited_rdata(rrset->rrs.rdata)) { /* Avoid hashing with too many iterations. * If we get here, the `sname` wildcard probably ends up bogus, * but it gets downgraded to KR_RANK_INSECURE when validator diff --git a/lib/dnssec/nsec3.h b/lib/dnssec/nsec3.h index 723dc4a..76ef2e9 100644 --- a/lib/dnssec/nsec3.h +++ b/lib/dnssec/nsec3.h @@ -5,15 +5,39 @@ #pragma once #include +#include +#include + + +static inline unsigned int kr_nsec3_price(unsigned int iterations, unsigned int salt_len) +{ + // SHA1 works on 64-byte chunks. + // On iterating we hash the salt + 20 bytes of the previous hash. + int chunks_per_iter = (20 + salt_len - 1) / 64 + 1; + return (iterations + 1) * chunks_per_iter; +} /** High numbers in NSEC3 iterations don't really help security * - * ...so we avoid doing all the work. The value is a current compromise; - * zones shooting over get downgraded to insecure status. + * ...so we avoid doing all the work. The limit is a current compromise; + * answers using NSEC3 over kr_nsec3_limited* get downgraded to insecure status. * https://datatracker.ietf.org/doc/html/rfc9276#name-recommendation-for-validati */ -#define KR_NSEC3_MAX_ITERATIONS 50 +static inline bool kr_nsec3_limited(unsigned int iterations, unsigned int salt_len) +{ + const int MAX_ITERATIONS = 50; // limit with short salt length + return kr_nsec3_price(iterations, salt_len) > MAX_ITERATIONS + 1; +} +static inline bool kr_nsec3_limited_rdata(const knot_rdata_t *rd) +{ + return kr_nsec3_limited(knot_nsec3_iters(rd), knot_nsec3_salt_len(rd)); +} +static inline bool kr_nsec3_limited_params(const dnssec_nsec3_params_t *params) +{ + return kr_nsec3_limited(params->iterations, params->salt.size); +} + /** * Name error response check (RFC5155 7.2.2). @@ -36,7 +60,7 @@ int kr_nsec3_name_error_response_check(const knot_pkt_t *pkt, knot_section_t sec * KNOT_ERANGE - NSEC3 RR that covers a wildcard * has been found, but has opt-out flag set; * otherwise - error. - * Records over KR_NSEC3_MAX_ITERATIONS are skipped, so you probably get kr_error(ENOENT). + * Too expensive NSEC3 records are skipped, so you probably get kr_error(ENOENT). */ int kr_nsec3_wildcard_answer_response_check(const knot_pkt_t *pkt, knot_section_t section_id, const knot_dname_t *sname, int trim_to_next); diff --git a/lib/layer/validate.c b/lib/layer/validate.c index 93f1d4f..5fea99d 100644 --- a/lib/layer/validate.c +++ b/lib/layer/validate.c @@ -128,14 +128,15 @@ static bool maybe_downgrade_nsec3(const ranked_rr_array_entry_t *e, struct kr_qu const knot_rdataset_t *rrs = &e->rr->rrs; knot_rdata_t *rd = rrs->rdata; for (int j = 0; j < rrs->count; ++j, rd = knot_rdataset_next(rd)) { - if (knot_nsec3_iters(rd) > KR_NSEC3_MAX_ITERATIONS) + if (kr_nsec3_limited_rdata(rd)) goto do_downgrade; } return false; do_downgrade: // we do this deep inside calls because of having signer name available - VERBOSE_MSG(qry, "<= DNSSEC downgraded due to NSEC3 iterations %d > %d\n", - (int)knot_nsec3_iters(rd), (int)KR_NSEC3_MAX_ITERATIONS); + VERBOSE_MSG(qry, + "<= DNSSEC downgraded due to expensive NSEC3: %d iterations, %d salt length\n", + (int)knot_nsec3_iters(rd), (int)knot_nsec3_salt_len(rd)); qry->flags.DNSSEC_WANT = false; qry->flags.DNSSEC_INSECURE = true; rank_records(qry, true, KR_RANK_INSECURE, vctx->zone_name);