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>

1 files changed, 54 insertions, 24 deletions

diff --git a/src/interfaces/libpq/fe-secure.c b/src/interfaces/libpq/fe-secure.c
index 73a0b0cf078..f8a5f6e7a24 100644
--- a/src/interfaces/libpq/fe-secure.c
+++ b/src/interfaces/libpq/fe-secure.c
@@ -88,7 +88,7 @@ static int	initialize_SSL(PGconn *conn);
 static void destroySSL(void);
 static PostgresPollingStatusType open_client_SSL(PGconn *);
 static void close_SSL(PGconn *);
-static char *SSLerrmessage(void);
+static char *SSLerrmessage(unsigned long ecode);
 static void SSLerrfree(char *buf);
 
 static bool pq_init_ssl_lib = true;
@@ -277,7 +277,7 @@ pqsecure_open_client(PGconn *conn)
 			!SSL_set_app_data(conn->ssl, conn) ||
 			!SSL_set_fd(conn->ssl, conn->sock))
 		{
-			char	   *err = SSLerrmessage();
+			char	   *err = SSLerrmessage(ERR_get_error());
 
 			printfPQExpBuffer(&conn->errorMessage,
 				   libpq_gettext("could not establish SSL connection: %s\n"),
@@ -342,6 +342,7 @@ pqsecure_read(PGconn *conn, void *ptr, size_t len)
 	if (conn->ssl)
 	{
 		int			err;
+		unsigned long ecode;
 
 		DECLARE_SIGPIPE_INFO(spinfo);
 
@@ -349,9 +350,30 @@ pqsecure_read(PGconn *conn, void *ptr, size_t len)
 		DISABLE_SIGPIPE(conn, spinfo, return -1);
 
 rloop:
+		/*
+		 * 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.  Since the possibility
+		 * exists that other OpenSSL clients running in the same thread but
+		 * not under our control will fail to call ERR_get_error()
+		 * themselves (after their own I/O operations), pro-actively clear
+		 * the per-thread error queue now.
+		 */
 		SOCK_ERRNO_SET(0);
+		ERR_clear_error();
 		n = SSL_read(conn->ssl, ptr, len);
 		err = SSL_get_error(conn->ssl, n);
+
+		/*
+		 * Other clients of OpenSSL 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 != SSL_ERROR_NONE || n < 0) ? ERR_get_error() : 0;
 		switch (err)
 		{
 			case SSL_ERROR_NONE:
@@ -405,7 +427,7 @@ rloop:
 				break;
 			case SSL_ERROR_SSL:
 				{
-					char	   *errm = SSLerrmessage();
+					char	   *errm = SSLerrmessage(ecode);
 
 					printfPQExpBuffer(&conn->errorMessage,
 									  libpq_gettext("SSL error: %s\n"), errm);
@@ -506,12 +528,15 @@ pqsecure_write(PGconn *conn, const void *ptr, size_t len)
 	if (conn->ssl)
 	{
 		int			err;
+		unsigned long ecode;
 
 		DISABLE_SIGPIPE(conn, spinfo, return -1);
 
 		SOCK_ERRNO_SET(0);
+		ERR_clear_error();
 		n = SSL_write(conn->ssl, ptr, len);
 		err = SSL_get_error(conn->ssl, n);
+		ecode = (err != SSL_ERROR_NONE || n < 0) ? ERR_get_error() : 0;
 		switch (err)
 		{
 			case SSL_ERROR_NONE:
@@ -565,7 +590,7 @@ pqsecure_write(PGconn *conn, const void *ptr, size_t len)
 				break;
 			case SSL_ERROR_SSL:
 				{
-					char	   *errm = SSLerrmessage();
+					char	   *errm = SSLerrmessage(ecode);
 
 					printfPQExpBuffer(&conn->errorMessage,
 									  libpq_gettext("SSL error: %s\n"), errm);
@@ -974,7 +999,7 @@ init_ssl_system(PGconn *conn)
 		SSL_context = SSL_CTX_new(SSLv23_method());
 		if (!SSL_context)
 		{
-			char	   *err = SSLerrmessage();
+			char	   *err = SSLerrmessage(ERR_get_error());
 
 			printfPQExpBuffer(&conn->errorMessage,
 						 libpq_gettext("could not create SSL context: %s\n"),
@@ -1139,7 +1164,7 @@ initialize_SSL(PGconn *conn)
 #endif
 		if (SSL_CTX_use_certificate_chain_file(SSL_context, fnbuf) != 1)
 		{
-			char	   *err = SSLerrmessage();
+			char	   *err = SSLerrmessage(ERR_get_error());
 
 			printfPQExpBuffer(&conn->errorMessage,
 			   libpq_gettext("could not read certificate file \"%s\": %s\n"),
@@ -1154,7 +1179,7 @@ initialize_SSL(PGconn *conn)
 
 		if (SSL_use_certificate_file(conn->ssl, fnbuf, SSL_FILETYPE_PEM) != 1)
 		{
-			char	   *err = SSLerrmessage();
+			char	   *err = SSLerrmessage(ERR_get_error());
 
 			printfPQExpBuffer(&conn->errorMessage,
 			   libpq_gettext("could not read certificate file \"%s\": %s\n"),
@@ -1209,7 +1234,7 @@ initialize_SSL(PGconn *conn)
 			conn->engine = ENGINE_by_id(engine_str);
 			if (conn->engine == NULL)
 			{
-				char	   *err = SSLerrmessage();
+				char	   *err = SSLerrmessage(ERR_get_error());
 
 				printfPQExpBuffer(&conn->errorMessage,
 					 libpq_gettext("could not load SSL engine \"%s\": %s\n"),
@@ -1221,7 +1246,7 @@ initialize_SSL(PGconn *conn)
 
 			if (ENGINE_init(conn->engine) == 0)
 			{
-				char	   *err = SSLerrmessage();
+				char	   *err = SSLerrmessage(ERR_get_error());
 
 				printfPQExpBuffer(&conn->errorMessage,
 				libpq_gettext("could not initialize SSL engine \"%s\": %s\n"),
@@ -1237,7 +1262,7 @@ initialize_SSL(PGconn *conn)
 										   NULL, NULL);
 			if (pkey == NULL)
 			{
-				char	   *err = SSLerrmessage();
+				char	   *err = SSLerrmessage(ERR_get_error());
 
 				printfPQExpBuffer(&conn->errorMessage,
 								  libpq_gettext("could not read private SSL key \"%s\" from engine \"%s\": %s\n"),
@@ -1251,7 +1276,7 @@ initialize_SSL(PGconn *conn)
 			}
 			if (SSL_use_PrivateKey(conn->ssl, pkey) != 1)
 			{
-				char	   *err = SSLerrmessage();
+				char	   *err = SSLerrmessage(ERR_get_error());
 
 				printfPQExpBuffer(&conn->errorMessage,
 								  libpq_gettext("could not load private SSL key \"%s\" from engine \"%s\": %s\n"),
@@ -1307,7 +1332,7 @@ initialize_SSL(PGconn *conn)
 
 		if (SSL_use_PrivateKey_file(conn->ssl, fnbuf, SSL_FILETYPE_PEM) != 1)
 		{
-			char	   *err = SSLerrmessage();
+			char	   *err = SSLerrmessage(ERR_get_error());
 
 			printfPQExpBuffer(&conn->errorMessage,
 			   libpq_gettext("could not load private key file \"%s\": %s\n"),
@@ -1321,7 +1346,7 @@ initialize_SSL(PGconn *conn)
 	if (have_cert &&
 		SSL_check_private_key(conn->ssl) != 1)
 	{
-		char	   *err = SSLerrmessage();
+		char	   *err = SSLerrmessage(ERR_get_error());
 
 		printfPQExpBuffer(&conn->errorMessage,
 						  libpq_gettext("certificate does not match private key file \"%s\": %s\n"),
@@ -1359,7 +1384,7 @@ initialize_SSL(PGconn *conn)
 #endif
 		if (SSL_CTX_load_verify_locations(SSL_context, fnbuf, NULL) != 1)
 		{
-			char	   *err = SSLerrmessage();
+			char	   *err = SSLerrmessage(ERR_get_error());
 
 			printfPQExpBuffer(&conn->errorMessage,
 							  libpq_gettext("could not read root certificate file \"%s\": %s\n"),
@@ -1389,7 +1414,7 @@ initialize_SSL(PGconn *conn)
 				X509_STORE_set_flags(cvstore,
 						  X509_V_FLAG_CRL_CHECK | X509_V_FLAG_CRL_CHECK_ALL);
 #else
-				char	   *err = SSLerrmessage();
+				char	   *err = SSLerrmessage(ERR_get_error());
 
 				printfPQExpBuffer(&conn->errorMessage,
 								  libpq_gettext("SSL library does not support CRL certificates (file \"%s\")\n"),
@@ -1452,11 +1477,14 @@ open_client_SSL(PGconn *conn)
 {
 	int			r;
 
+	ERR_clear_error();
 	r = SSL_connect(conn->ssl);
 	if (r <= 0)
 	{
 		int			err = SSL_get_error(conn->ssl, r);
+		unsigned long ecode;
 
+		ecode = ERR_get_error();
 		switch (err)
 		{
 			case SSL_ERROR_WANT_READ:
@@ -1481,7 +1509,7 @@ open_client_SSL(PGconn *conn)
 				}
 			case SSL_ERROR_SSL:
 				{
-					char	   *err = SSLerrmessage();
+					char	   *err = SSLerrmessage(ecode);
 
 					printfPQExpBuffer(&conn->errorMessage,
 									  libpq_gettext("SSL error: %s\n"),
@@ -1509,7 +1537,9 @@ open_client_SSL(PGconn *conn)
 	conn->peer = SSL_get_peer_certificate(conn->ssl);
 	if (conn->peer == NULL)
 	{
-		char	   *err = SSLerrmessage();
+		char	   *err;
+
+		err = SSLerrmessage(ERR_get_error());
 
 		printfPQExpBuffer(&conn->errorMessage,
 					libpq_gettext("certificate could not be obtained: %s\n"),
@@ -1585,7 +1615,9 @@ close_SSL(PGconn *conn)
 }
 
 /*
- * 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
@@ -1596,28 +1628,26 @@ static char ssl_nomem[] = "out of memory allocating error description";
 #define SSL_ERR_LEN 128
 
 static char *
-SSLerrmessage(void)
+SSLerrmessage(unsigned long ecode)
 {
-	unsigned long errcode;
 	const char *errreason;
 	char	   *errbuf;
 
 	errbuf = malloc(SSL_ERR_LEN);
 	if (!errbuf)
 		return ssl_nomem;
-	errcode = ERR_get_error();
-	if (errcode == 0)
+	if (ecode == 0)
 	{
 		snprintf(errbuf, SSL_ERR_LEN, libpq_gettext("no SSL error reported"));
 		return errbuf;
 	}
-	errreason = ERR_reason_error_string(errcode);
+	errreason = ERR_reason_error_string(ecode);
 	if (errreason != NULL)
 	{
 		strlcpy(errbuf, errreason, SSL_ERR_LEN);
 		return errbuf;
 	}
-	snprintf(errbuf, SSL_ERR_LEN, libpq_gettext("SSL error code %lu"), errcode);
+	snprintf(errbuf, SSL_ERR_LEN, libpq_gettext("SSL error code %lu"), ecode);
 	return errbuf;
 }