Improve server certificate selection for TLSv1.3.

This allows an EC certificate to be selected and used, if the client
sigalgs would allow it.

With feedback from tb@

ok inoguchi@ tb@

2 files changed, 94 insertions, 23 deletions

diff --git a/src/lib/libssl/ssl_locl.h b/src/lib/libssl/ssl_locl.h
index 046c4bba52..e7e3e56154 100644
--- a/src/lib/libssl/ssl_locl.h
+++ b/src/lib/libssl/ssl_locl.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssl_locl.h,v 1.276 2020/05/29 17:39:42 jsing Exp $ */
+/* $OpenBSD: ssl_locl.h,v 1.277 2020/05/29 18:00:10 jsing Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -435,6 +435,12 @@ typedef struct ssl_handshake_st {
         uint8_t *sigalgs;
 } SSL_HANDSHAKE;
 
+typedef struct cert_pkey_st {
+        X509 *x509;
+        EVP_PKEY *privatekey;
+        STACK_OF(X509) *chain;
+} CERT_PKEY;
+
 typedef struct ssl_handshake_tls13_st {
         uint16_t min_version;
         uint16_t max_version;
@@ -443,6 +449,10 @@ typedef struct ssl_handshake_tls13_st {
         int use_legacy;
         int hrr;
 
+        /* Certificate and sigalg selected for use (static pointers). */
+        const CERT_PKEY *cpk;
+        const struct ssl_sigalg *sigalg;
+
         /* Version proposed by peer server. */
         uint16_t server_version;
 
@@ -985,12 +995,6 @@ typedef struct dtls1_state_internal_st {
 } DTLS1_STATE_INTERNAL;
 #define D1I(s) (s->d1->internal)
 
-typedef struct cert_pkey_st {
-        X509 *x509;
-        EVP_PKEY *privatekey;
-        STACK_OF(X509) *chain;
-} CERT_PKEY;
-
 typedef struct cert_st {
         /* Current active set */
         CERT_PKEY *key; /* ALWAYS points to an element of the pkeys array
diff --git a/src/lib/libssl/tls13_server.c b/src/lib/libssl/tls13_server.c
index 181ba583a0..e9fecdee26 100644
--- a/src/lib/libssl/tls13_server.c
+++ b/src/lib/libssl/tls13_server.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: tls13_server.c,v 1.54 2020/05/29 17:47:30 jsing Exp $ */
+/* $OpenBSD: tls13_server.c,v 1.55 2020/05/29 18:00:10 jsing Exp $ */
 /*
  * Copyright (c) 2019, 2020 Joel Sing <jsing@openbsd.org>
  * Copyright (c) 2020 Bob Beck <beck@openbsd.org>
@@ -16,6 +16,8 @@
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
+#include <openssl/x509v3.h>
+
 #include "ssl_locl.h"
 #include "ssl_tlsext.h"
 
@@ -448,19 +450,85 @@ tls13_server_certificate_request_send(struct tls13_ctx *ctx, CBB *cbb)
         return 0;
 }
 
+static int
+tls13_server_check_certificate(struct tls13_ctx *ctx, CERT_PKEY *cpk,
+    int *ok, const struct ssl_sigalg **out_sigalg)
+{
+        const struct ssl_sigalg *sigalg;
+        SSL *s = ctx->ssl;
+
+        *ok = 0;
+        *out_sigalg = NULL;
+
+        if (cpk->x509 == NULL || cpk->privatekey == NULL)
+                goto done;
+
+        if (!X509_check_purpose(cpk->x509, -1, 0))
+                return 0;
+
+        /*
+         * The digitalSignature bit MUST be set if the Key Usage extension is
+         * present as per RFC 8446 section 4.4.2.2.
+         */ 
+        if ((cpk->x509->ex_flags & EXFLAG_KUSAGE) &&
+            !(cpk->x509->ex_kusage & X509v3_KU_DIGITAL_SIGNATURE))
+                goto done;
+
+        if ((sigalg = ssl_sigalg_select(s, cpk->privatekey)) == NULL)
+                goto done;
+
+        *ok = 1;
+        *out_sigalg = sigalg;
+
+ done:
+        return 1;
+}
+ 
+static int
+tls13_server_select_certificate(struct tls13_ctx *ctx, CERT_PKEY **out_cpk,
+    const struct ssl_sigalg **out_sigalg)
+{
+        SSL *s = ctx->ssl;
+        const struct ssl_sigalg *sigalg;
+        CERT_PKEY *cpk;
+        int cert_ok;
+
+        *out_cpk = NULL;
+        *out_sigalg = NULL;
+
+        cpk = &s->cert->pkeys[SSL_PKEY_ECC];
+        if (!tls13_server_check_certificate(ctx, cpk, &cert_ok, &sigalg))
+                return 0;
+        if (cert_ok)
+                goto done;
+
+        cpk = &s->cert->pkeys[SSL_PKEY_RSA];
+        if (!tls13_server_check_certificate(ctx, cpk, &cert_ok, &sigalg))
+                return 0;
+        if (cert_ok)
+                goto done;
+
+        return 0;
+
+ done:
+        *out_cpk = cpk;
+        *out_sigalg = sigalg;
+
+        return 1;
+}
+
 int
 tls13_server_certificate_send(struct tls13_ctx *ctx, CBB *cbb)
 {
         SSL *s = ctx->ssl;
         CBB cert_request_context, cert_list;
+        const struct ssl_sigalg *sigalg;
         STACK_OF(X509) *chain;
         CERT_PKEY *cpk;
         X509 *cert;
         int i, ret = 0;
 
-        /* XXX - Need to revisit certificate selection. */
-        cpk = &s->cert->pkeys[SSL_PKEY_RSA];
-        if (cpk->x509 == NULL) {
+        if (!tls13_server_select_certificate(ctx, &cpk, &sigalg)) {
                 /* A server must always provide a certificate. */
                 ctx->alert = TLS13_ALERT_HANDSHAKE_FAILURE;
                 tls13_set_errorx(ctx, TLS13_ERR_NO_CERTIFICATE, 0,
@@ -468,6 +536,9 @@ tls13_server_certificate_send(struct tls13_ctx *ctx, CBB *cbb)
                 goto err;
         }
 
+        ctx->hs->cpk = cpk;
+        ctx->hs->sigalg = sigalg;
+
         if ((chain = cpk->chain) == NULL)
                 chain = s->ctx->extra_certs;
 
@@ -502,27 +573,23 @@ tls13_server_certificate_send(struct tls13_ctx *ctx, CBB *cbb)
 int
 tls13_server_certificate_verify_send(struct tls13_ctx *ctx, CBB *cbb)
 {
-        SSL *s = ctx->ssl;
-        const struct ssl_sigalg *sigalg = NULL;
+        const struct ssl_sigalg *sigalg;
         uint8_t *sig = NULL, *sig_content = NULL;
         size_t sig_len, sig_content_len;
         EVP_MD_CTX *mdctx = NULL;
         EVP_PKEY_CTX *pctx;
         EVP_PKEY *pkey;
-        CERT_PKEY *cpk;
+        const CERT_PKEY *cpk;
         CBB sig_cbb;
         int ret = 0;
 
         memset(&sig_cbb, 0, sizeof(sig_cbb));
 
-        /* XXX - Need to revisit certificate selection. */
-        cpk = &s->cert->pkeys[SSL_PKEY_RSA];
-        pkey = cpk->privatekey;
-
-        if ((sigalg = ssl_sigalg_select(s, pkey)) == NULL) {
-                /* XXX - SSL_R_SIGNATURE_ALGORITHMS_ERROR */
+        if ((cpk = ctx->hs->cpk) == NULL)
+                goto err;
+        if ((sigalg = ctx->hs->sigalg) == NULL)
                 goto err;
-        }
+        pkey = cpk->privatekey;
 
         if (!CBB_init(&sig_cbb, 0))
                 goto err;
@@ -829,9 +896,9 @@ tls13_client_certificate_verify_recv(struct tls13_ctx *ctx, CBS *cbs)
         ret = 1;
 
  err:
-        if (!ret && ctx->alert == 0) {
+        if (!ret && ctx->alert == 0)
                 ctx->alert = TLS13_ALERT_DECODE_ERROR;
-        }
+
         CBB_cleanup(&cbb);
         EVP_MD_CTX_free(mdctx);
         free(sig_content);