Distrust external OpenSSL clients; clear err queue

OpenSSL has an unfortunate tendency to mix per-session state error
handling with per-thread error handling.  This can cause problems when
programs that link to libpq with OpenSSL enabled have some other use of
OpenSSL; without care, one caller of OpenSSL may cause problems for the
other caller.  Backend code might similarly be affected, for example
when a third party extension independently uses OpenSSL without taking
the appropriate precautions.

To fix, don't trust other users of OpenSSL to clear the per-thread error
queue.  Instead, clear the entire per-thread queue ahead of certain I/O
operations when it appears that there might be trouble (these I/O
operations mostly need to call SSL_get_error() to check for success,
which relies on the queue being empty).  This is slightly aggressive,
but it's pretty clear that the other callers have a very dubious claim
to ownership of the per-thread queue.  Do this is both frontend and
backend code.

Finally, be more careful about clearing our own error queue, so as to
not cause these problems ourself.  It's possibly that control previously
did not always reach SSLerrmessage(), where ERR_get_error() was supposed
to be called to clear the queue's earliest code.  Make sure
ERR_get_error() is always called, so as to spare other users of OpenSSL
the possibility of similar problems caused by libpq (as opposed to
problems caused by a third party OpenSSL library like PHP's OpenSSL
extension).  Again, do this is both frontend and backend code.

See bug #12799 and https://bugs.php.net/bug.php?id=68276

Based on patches by Dave Vitek and Peter Eisentraut.

From: Peter Geoghegan <pg@bowt.ie>

Author: petere

src/backend/libpq/be-secure-openssl.c

@@ -78,7 +78,7 @@ static DH  *tmp_dh_cb(SSL *s, int is_export, int keylength);
 static int	verify_cb(int, X509_STORE_CTX *);
 static void info_cb(const SSL *ssl, int type, int args);
 static void initialize_ecdh(void);
-static const char *SSLerrmessage(void);
+static const char *SSLerrmessage(unsigned long ecode);
 
 static char *X509_NAME_to_cstring(X509_NAME *name);
 
@@ -182,7 +182,7 @@ be_tls_init(void)
 		if (!SSL_context)
 			ereport(FATAL,
 					(errmsg("could not create SSL context: %s",
-							SSLerrmessage())));
+							SSLerrmessage(ERR_get_error()))));
 
 		/*
 		 * Disable OpenSSL's moving-write-buffer sanity check, because it
@@ -198,7 +198,7 @@ be_tls_init(void)
 			ereport(FATAL,
 					(errcode(ERRCODE_CONFIG_FILE_ERROR),
 				  errmsg("could not load server certificate file \"%s\": %s",
-						 ssl_cert_file, SSLerrmessage())));
+						 ssl_cert_file, SSLerrmessage(ERR_get_error()))));
 
 		if (stat(ssl_key_file, &buf) != 0)
 			ereport(FATAL,
@@ -251,12 +251,12 @@ be_tls_init(void)
 										SSL_FILETYPE_PEM) != 1)
 			ereport(FATAL,
 					(errmsg("could not load private key file \"%s\": %s",
-							ssl_key_file, SSLerrmessage())));
+							ssl_key_file, SSLerrmessage(ERR_get_error()))));
 
 		if (SSL_CTX_check_private_key(SSL_context) != 1)
 			ereport(FATAL,
 					(errmsg("check of private key failed: %s",
-							SSLerrmessage())));
+							SSLerrmessage(ERR_get_error()))));
 	}
 
 	/* set up ephemeral DH keys, and disallow SSL v2/v3 while at it */
@@ -285,7 +285,7 @@ be_tls_init(void)
 			(root_cert_list = SSL_load_client_CA_file(ssl_ca_file)) == NULL)
 			ereport(FATAL,
 					(errmsg("could not load root certificate file \"%s\": %s",
-							ssl_ca_file, SSLerrmessage())));
+							ssl_ca_file, SSLerrmessage(ERR_get_error()))));
 	}
 
 	/*----------
@@ -316,7 +316,7 @@ be_tls_init(void)
 			else
 				ereport(FATAL,
 						(errmsg("could not load SSL certificate revocation list file \"%s\": %s",
-								ssl_crl_file, SSLerrmessage())));
+								ssl_crl_file, SSLerrmessage(ERR_get_error()))));
 		}
 	}
 
@@ -353,6 +353,7 @@ be_tls_open_server(Port *port)
 	int			r;
 	int			err;
 	int			waitfor;
+	unsigned long ecode;
 
 	Assert(!port->ssl);
 	Assert(!port->peer);
@@ -362,24 +363,44 @@ be_tls_open_server(Port *port)
 		ereport(COMMERROR,
 				(errcode(ERRCODE_PROTOCOL_VIOLATION),
 				 errmsg("could not initialize SSL connection: %s",
-						SSLerrmessage())));
+						SSLerrmessage(ERR_get_error()))));
 		return -1;
 	}
 	if (!my_SSL_set_fd(port, port->sock))
 	{
 		ereport(COMMERROR,
 				(errcode(ERRCODE_PROTOCOL_VIOLATION),
 				 errmsg("could not set SSL socket: %s",
-						SSLerrmessage())));
+						SSLerrmessage(ERR_get_error()))));
 		return -1;
 	}
 	port->ssl_in_use = true;
 
 aloop:
+	/*
+	 * Prepare to call SSL_get_error() by clearing thread's OpenSSL error
+	 * queue.  In general, the current thread's error queue must be empty
+	 * before the TLS/SSL I/O operation is attempted, or SSL_get_error()
+	 * will not work reliably.  An extension may have failed to clear the
+	 * per-thread error queue following another call to an OpenSSL I/O
+	 * routine.
+	 */
+	ERR_clear_error();
 	r = SSL_accept(port->ssl);
 	if (r <= 0)
 	{
 		err = SSL_get_error(port->ssl, r);
+
+		/*
+		 * Other clients of OpenSSL in the backend may fail to call
+		 * ERR_get_error(), but we always do, so as to not cause problems
+		 * for OpenSSL clients that don't call ERR_clear_error()
+		 * defensively.  Be sure that this happens by calling now.
+		 * SSL_get_error() relies on the OpenSSL per-thread error queue
+		 * being intact, so this is the earliest possible point
+		 * ERR_get_error() may be called.
+		 */
+		ecode = ERR_get_error();
 		switch (err)
 		{
 			case SSL_ERROR_WANT_READ:
@@ -413,7 +434,7 @@ be_tls_open_server(Port *port)
 				ereport(COMMERROR,
 						(errcode(ERRCODE_PROTOCOL_VIOLATION),
 						 errmsg("could not accept SSL connection: %s",
-								SSLerrmessage())));
+								SSLerrmessage(ecode))));
 				break;
 			case SSL_ERROR_ZERO_RETURN:
 				ereport(COMMERROR,
@@ -522,10 +543,13 @@ be_tls_read(Port *port, void *ptr, size_t len, int *waitfor)
 {
 	ssize_t		n;
 	int			err;
+	unsigned long ecode;
 
 	errno = 0;
+	ERR_clear_error();
 	n = SSL_read(port->ssl, ptr, len);
 	err = SSL_get_error(port->ssl, n);
+	ecode = (err != SSL_ERROR_NONE || n < 0) ? ERR_get_error() : 0;
 	switch (err)
 	{
 		case SSL_ERROR_NONE:
@@ -552,7 +576,7 @@ be_tls_read(Port *port, void *ptr, size_t len, int *waitfor)
 		case SSL_ERROR_SSL:
 			ereport(COMMERROR,
 					(errcode(ERRCODE_PROTOCOL_VIOLATION),
-					 errmsg("SSL error: %s", SSLerrmessage())));
+					 errmsg("SSL error: %s", SSLerrmessage(ecode))));
 			/* fall through */
 		case SSL_ERROR_ZERO_RETURN:
 			errno = ECONNRESET;
@@ -579,10 +603,13 @@ be_tls_write(Port *port, void *ptr, size_t len, int *waitfor)
 {
 	ssize_t		n;
 	int			err;
+	unsigned long ecode;
 
 	errno = 0;
+	ERR_clear_error();
 	n = SSL_write(port->ssl, ptr, len);
 	err = SSL_get_error(port->ssl, n);
+	ecode = (err != SSL_ERROR_NONE || n < 0) ? ERR_get_error() : 0;
 	switch (err)
 	{
 		case SSL_ERROR_NONE:
@@ -609,7 +636,7 @@ be_tls_write(Port *port, void *ptr, size_t len, int *waitfor)
 		case SSL_ERROR_SSL:
 			ereport(COMMERROR,
 					(errcode(ERRCODE_PROTOCOL_VIOLATION),
-					 errmsg("SSL error: %s", SSLerrmessage())));
+					 errmsg("SSL error: %s", SSLerrmessage(ecode))));
 			/* fall through */
 		case SSL_ERROR_ZERO_RETURN:
 			errno = ECONNRESET;
@@ -765,7 +792,8 @@ load_dh_file(int keylength)
 	{
 		if (DH_check(dh, &codes) == 0)
 		{
-			elog(LOG, "DH_check error (%s): %s", fnbuf, SSLerrmessage());
+			elog(LOG, "DH_check error (%s): %s", fnbuf,
+				 SSLerrmessage(ERR_get_error()));
 			return NULL;
 		}
 		if (codes & DH_CHECK_P_NOT_PRIME)
@@ -805,7 +833,7 @@ load_dh_buffer(const char *buffer, size_t len)
 	if (dh == NULL)
 		ereport(DEBUG2,
 				(errmsg_internal("DH load buffer: %s",
-								 SSLerrmessage())));
+								 SSLerrmessage(ERR_get_error()))));
 	BIO_free(bio);
 
 	return dh;
@@ -971,26 +999,26 @@ initialize_ecdh(void)
 }
 
 /*
- * Obtain reason string for last SSL error
+ * Obtain reason string for passed SSL errcode
+ *
+ * ERR_get_error() is used by caller to get errcode to pass here.
  *
  * Some caution is needed here since ERR_reason_error_string will
  * return NULL if it doesn't recognize the error code.  We don't
  * want to return NULL ever.
  */
 static const char *
-SSLerrmessage(void)
+SSLerrmessage(unsigned long ecode)
 {
-	unsigned long errcode;
 	const char *errreason;
 	static char errbuf[32];
 
-	errcode = ERR_get_error();
-	if (errcode == 0)
+	if (ecode == 0)
 		return _("no SSL error reported");
-	errreason = ERR_reason_error_string(errcode);
+	errreason = ERR_reason_error_string(ecode);
 	if (errreason != NULL)
 		return errreason;
-	snprintf(errbuf, sizeof(errbuf), _("SSL error code %lu"), errcode);
+	snprintf(errbuf, sizeof(errbuf), _("SSL error code %lu"), ecode);
 	return errbuf;
 }