Allow security_level to mestastasize into the verifier

The tentacles are everywhere.  This checks that all certs in a chain
have keys and signature algorithms matching the requirements of the
security_level configured in the verify parameters.

ok beck jsing

4 files changed, 156 insertions, 4 deletions

diff --git a/src/lib/libcrypto/x509/x509_internal.h b/src/lib/libcrypto/x509/x509_internal.h
index c6ce5229ad..030f24c470 100644
--- a/src/lib/libcrypto/x509/x509_internal.h
+++ b/src/lib/libcrypto/x509/x509_internal.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: x509_internal.h,v 1.18 2022/03/14 21:15:49 tb Exp $ */
+/* $OpenBSD: x509_internal.h,v 1.19 2022/06/27 14:10:22 tb Exp $ */
 /*
  * Copyright (c) 2020 Bob Beck <beck@openbsd.org>
  *
@@ -135,6 +135,7 @@ int x509_constraints_check(struct x509_constraints_names *names,
 int x509_constraints_chain(STACK_OF(X509) *chain, int *error,
     int *depth);
 void x509_verify_cert_info_populate(X509 *cert);
+int x509_vfy_check_security_level(X509_STORE_CTX *ctx);
 
 __END_HIDDEN_DECLS
 
diff --git a/src/lib/libcrypto/x509/x509_verify.c b/src/lib/libcrypto/x509/x509_verify.c
index f6959d1f3a..83030672ef 100644
--- a/src/lib/libcrypto/x509/x509_verify.c
+++ b/src/lib/libcrypto/x509/x509_verify.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: x509_verify.c,v 1.56 2022/06/25 20:01:43 beck Exp $ */
+/* $OpenBSD: x509_verify.c,v 1.57 2022/06/27 14:10:22 tb Exp $ */
 /*
  * Copyright (c) 2020-2021 Bob Beck <beck@openbsd.org>
  *
@@ -415,6 +415,9 @@ x509_verify_ctx_validate_legacy_chain(struct x509_verify_ctx *ctx,
                 goto err;
 #endif
 
+        if (!x509_vfy_check_security_level(ctx->xsc))
+                goto err;
+
         if (!x509_constraints_chain(ctx->xsc->chain,
                 &ctx->xsc->error, &ctx->xsc->error_depth)) {
                 X509 *cert = sk_X509_value(ctx->xsc->chain, depth);
diff --git a/src/lib/libcrypto/x509/x509_vfy.c b/src/lib/libcrypto/x509/x509_vfy.c
index 9a929963f3..18cf08a07c 100644
--- a/src/lib/libcrypto/x509/x509_vfy.c
+++ b/src/lib/libcrypto/x509/x509_vfy.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: x509_vfy.c,v 1.101 2022/01/22 00:36:46 inoguchi Exp $ */
+/* $OpenBSD: x509_vfy.c,v 1.102 2022/06/27 14:10:22 tb Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -144,6 +144,8 @@ static int X509_cmp_time_internal(const ASN1_TIME *ctm, time_t *cmp_time,
 
 static int internal_verify(X509_STORE_CTX *ctx);
 static int get_issuer_sk(X509 **issuer, X509_STORE_CTX *ctx, X509 *x);
+static int check_key_level(X509_STORE_CTX *ctx, X509 *cert);
+static int verify_cb_cert(X509_STORE_CTX *ctx, X509 *x, int depth, int err);
 
 int ASN1_time_tm_clamp_notafter(struct tm *tm);
 
@@ -542,6 +544,11 @@ X509_verify_cert_legacy(X509_STORE_CTX *ctx)
         if (!ok)
                 goto end;
 
+        /* Check that the chain satisfies the security level. */
+        ok = x509_vfy_check_security_level(ctx);
+        if (!ok)
+                goto end;
+
         /* Check name constraints */
         ok = check_name_constraints(ctx);
         if (!ok)
@@ -629,6 +636,14 @@ X509_verify_cert(X509_STORE_CTX *ctx)
         }
 
         /*
+         * If the certificate's public key is too weak, don't bother
+         * continuing.
+         */
+        if (!check_key_level(ctx, ctx->cert) &&
+            !verify_cb_cert(ctx, ctx->cert, 0, X509_V_ERR_EE_KEY_TOO_SMALL))
+                return 0;
+
+        /*
          * If flags request legacy, use the legacy verifier. If we
          * requested "no alt chains" from the age of hammer pants, use
          * the legacy verifier because the multi chain verifier really
@@ -2596,3 +2611,129 @@ X509_STORE_CTX_set0_param(X509_STORE_CTX *ctx, X509_VERIFY_PARAM *param)
                 X509_VERIFY_PARAM_free(ctx->param);
         ctx->param = param;
 }
+
+/*
+ * Check if |bits| are adequate for |security level|.
+ * Returns 1 if ok, 0 otherwise.
+ */
+static int
+enough_bits_for_security_level(int bits, int level)
+{
+        /*
+         * Sigh. OpenSSL does this silly squashing, so we will
+         * too. Derp for Derp compatibility being important.
+         */
+        if (level < 0)
+                level = 0;
+        if (level > 5)
+                level = 5;
+
+        switch (level) {
+        case 0:
+                return 1;
+        case 1:
+                return bits >= 80;
+        case 2:
+                return bits >= 112;
+        case 3:
+                return bits >= 128;
+        case 4:
+                return bits >= 192;
+        case 5:
+                return bits >= 256;
+        default:
+                return 0;
+        }
+}
+
+/*
+ * Check whether the public key of |cert| meets the security level of |ctx|.
+ *
+ * Returns 1 on success, 0 otherwise.
+ */
+static int
+check_key_level(X509_STORE_CTX *ctx, X509 *cert)
+{
+        EVP_PKEY *pkey;
+        int bits;
+
+        /* Unsupported or malformed keys are not secure */
+        if ((pkey = X509_get0_pubkey(cert)) == NULL)
+                return 0;
+
+        if ((bits = EVP_PKEY_security_bits(pkey)) <= 0)
+                return 0;
+
+        return enough_bits_for_security_level(bits, ctx->param->security_level);
+}
+
+/*
+ * Check whether the signature digest algorithm of |cert| meets the security
+ * level of |ctx|.  Do not check trust anchors (self-signed or not).
+ *
+ * Returns 1 on success, 0 otherwise.
+ */
+static int
+check_sig_level(X509_STORE_CTX *ctx, X509 *cert)
+{
+        const EVP_MD *md;
+        int bits, nid, md_nid;
+
+        if ((nid = X509_get_signature_nid(cert)) == NID_undef)
+                return 0;
+
+        /*
+         * Look up signature algorithm digest.
+         */
+
+        if (!OBJ_find_sigid_algs(nid, &md_nid, NULL))
+                return 0;
+
+        if (md_nid == NID_undef)
+                return 0;
+
+        if ((md = EVP_get_digestbynid(md_nid)) == NULL)
+                return 0;
+
+        /* Assume 4 bits of collision resistance for each hash octet. */
+        bits = EVP_MD_size(md) * 4;
+
+        return enough_bits_for_security_level(bits, ctx->param->security_level);
+}
+
+int
+x509_vfy_check_security_level(X509_STORE_CTX *ctx)
+{
+        int num = sk_X509_num(ctx->chain);
+        int i;
+
+        if (ctx->param->security_level <= 0)
+                return 1;
+
+        for (i = 0; i < num; i++) {
+                X509 *cert = sk_X509_value(ctx->chain, i);
+
+                /*
+                 * We've already checked the security of the leaf key, so here
+                 * we only check the security of issuer keys.
+                 */
+                if (i > 0) {
+                        if (!check_key_level(ctx, cert) &&
+                            !verify_cb_cert(ctx, cert, i,
+                            X509_V_ERR_CA_KEY_TOO_SMALL))
+                                return 0;
+                }
+
+                /*
+                 * We also check the signature algorithm security of all certs
+                 * except those of the trust anchor at index num - 1.
+                 */
+                if (i == num - 1)
+                        break;
+
+                if (!check_sig_level(ctx, cert) &&
+                    !verify_cb_cert(ctx, cert, i, X509_V_ERR_CA_MD_TOO_WEAK))
+                        return 0;
+        }
+        return 1;
+}
diff --git a/src/lib/libcrypto/x509/x509_vfy.h b/src/lib/libcrypto/x509/x509_vfy.h
index a007fe4795..d31e779aae 100644
--- a/src/lib/libcrypto/x509/x509_vfy.h
+++ b/src/lib/libcrypto/x509/x509_vfy.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: x509_vfy.h,v 1.51 2022/06/27 14:00:09 tb Exp $ */
+/* $OpenBSD: x509_vfy.h,v 1.52 2022/06/27 14:10:22 tb Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -205,6 +205,13 @@ void X509_STORE_CTX_set_depth(X509_STORE_CTX *ctx, int depth);
 /* Issuer lookup error */
 #define         X509_V_ERR_STORE_LOOKUP                         66
 
+#if defined(LIBRESSL_INTERNAL)
+/* Security level errors */
+#define         X509_V_ERR_EE_KEY_TOO_SMALL                     67
+#define         X509_V_ERR_CA_KEY_TOO_SMALL                     68
+#define         X509_V_ERR_CA_MD_TOO_WEAK                       69
+#endif
+
 /* Certificate verify flags */
 
 /* Send issuer+subject checks to verify_cb */