Suppress DANE TLSA reflection when verification fails

As documented both SSL_get0_dane_authority() and SSL_get0_dane_tlsa()
are expected to return a negative match depth and nothing else when
verification fails.  However, this only happened when verification
failed during chain construction.  Errors in verification of the
constructed chain did not have the intended effect on these functions.

This commit updates the functions to check for verify_result ==
X509_V_OK, and no longer erases any accumulated match information
when chain construction fails.  Sophisticated developers can, with
care, use SSL_set_verify_result(ssl, X509_V_OK) to "peek" at TLSA
info even when verification fail.  They must of course first check
and save the real error, and restore the original error as quickly
as possible.  Hiding by default seems to be the safer interface.

Introduced X509_V_ERR_DANE_NO_MATCH code to signal failure to find
matching TLSA records.  Previously reported via X509_V_ERR_CERT_UNTRUSTED.

This also changes the "-brief" output from s_client to include
verification results and TLSA match information.

Mentioned session resumption in code example in SSL_CTX_dane_enable(3).
Also mentioned that depths returned are relative to the verified chain
which is now available via SSL_get0_verified_chain(3).

Added a few more test-cases to danetest, that exercise the new
code.

Resolved thread safety issue in use of static buffer in
X509_verify_cert_error_string().

Fixed long-stating issue in apps/s_cb.c which always sets verify_error
to either X509_V_OK or "chain to long", code elsewhere (e.g.
s_time.c), seems to expect the actual error.  [ The new chain
construction code is expected to correctly generate "chain
too long" errors, so at some point we need to drop the
work-arounds, once SSL_set_verify_depth() is also fixed to
propagate the depth to X509_STORE_CTX reliably. ]

Reviewed-by: Rich Salz <rsalz@openssl.org>

2 files changed, 9 insertions, 9 deletions

diff --git a/crypto/x509/x509_txt.c b/crypto/x509/x509_txt.c
index 0e480644e5..884c3afdd4 100644
--- a/crypto/x509/x509_txt.c
+++ b/crypto/x509/x509_txt.c
@@ -69,11 +69,11 @@
 
 const char *X509_verify_cert_error_string(long n)
 {
-    static char buf[100];
-
     switch ((int)n) {
     case X509_V_OK:
         return ("ok");
+    case X509_V_ERR_UNSPECIFIED:
+        return ("unspecified certificate verification error");
     case X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT:
         return ("unable to get issuer certificate");
     case X509_V_ERR_UNABLE_TO_GET_CRL:
@@ -202,9 +202,11 @@ const char *X509_verify_cert_error_string(long n)
         return ("Email address mismatch");
     case X509_V_ERR_IP_ADDRESS_MISMATCH:
         return ("IP address mismatch");
+    case X509_V_ERR_DANE_NO_MATCH:
+        return ("No matching DANE TLSA records");
 
     default:
-        BIO_snprintf(buf, sizeof buf, "error number %ld", n);
-        return (buf);
+        /* Printing an error number into a static buffer is not thread-safe */
+        return ("unknown certificate verification error");
     }
 }
diff --git a/crypto/x509/x509_vfy.c b/crypto/x509/x509_vfy.c
index 113c116ef6..f8b9b503ac 100644
--- a/crypto/x509/x509_vfy.c
+++ b/crypto/x509/x509_vfy.c
@@ -2631,7 +2631,7 @@ static int check_dane_pkeys(X509_STORE_CTX *ctx)
             X509_verify(cert, t->spki) <= 0)
             continue;
 
-        /* Clear PKIX-?? matches that failed to panned out to a full chain */
+        /* Clear any PKIX-?? matches that failed to extend to a full chain */
         X509_free(dane->mcert);
         dane->mcert = NULL;
 
@@ -2711,7 +2711,7 @@ static int dane_verify(X509_STORE_CTX *ctx)
             return 0;
         ctx->current_cert = cert;
         ctx->error_depth = 0;
-        ctx->error = X509_V_ERR_CERT_UNTRUSTED;
+        ctx->error = X509_V_ERR_DANE_NO_MATCH;
         return ctx->verify_cb(0, ctx);
     }
 
@@ -3026,7 +3026,7 @@ static int build_chain(X509_STORE_CTX *ctx)
             ctx->error = X509_V_ERR_CERT_CHAIN_TOO_LONG;
         else if (DANETLS_ENABLED(dane) &&
                  (!DANETLS_HAS_PKIX(dane) || dane->pdpth >= 0))
-            ctx->error = X509_V_ERR_CERT_UNTRUSTED;
+            ctx->error = X509_V_ERR_DANE_NO_MATCH;
         else if (ss && sk_X509_num(ctx->chain) == 1)
             ctx->error = X509_V_ERR_DEPTH_ZERO_SELF_SIGNED_CERT;
         else if (ss)
@@ -3035,8 +3035,6 @@ static int build_chain(X509_STORE_CTX *ctx)
             ctx->error = X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT_LOCALLY;
         else
             ctx->error = X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT;
-        if (DANETLS_ENABLED(dane))
-            dane_reset(dane);
         return ctx->verify_cb(0, ctx);
     }
 }