Always use 2048 bit DH parameters for OpenSSL ephemeral DH ciphers.

1024 bits is considered weak these days, but OpenSSL always passes 1024 as
the key length to the tmp_dh callback. All the code to handle other key
lengths is, in fact, dead.

To remedy those issues:

* Only include hard-coded 2048-bit parameters.
* Set the parameters directly with SSL_CTX_set_tmp_dh(), without the
  callback
* The name of the file containing the DH parameters is now a GUC. This
  replaces the old hardcoded "dh1024.pem" filename. (The files for other
  key lengths, dh512.pem, dh2048.pem, etc. were never actually used.)

This is not a new problem, but it doesn't seem worth the risk and churn to
backport. If you care enough about the strength of the DH parameters on
old versions, you can create custom DH parameters, with as many bits as you
wish, and put them in the "dh1024.pem" file.

Per report by Nicolas Guini and Damian Quiroga. Reviewed by Michael Paquier.

Discussion: https://www.postgresql.org/message-id/CAMxBoUyjOOautVozN6ofzym828aNrDjuCcOTcCquxjwS-L2hGQ@mail.gmail.com

Author: hlinnaka

doc/src/sgml/config.sgml

@@ -1203,6 +1203,30 @@ include_dir 'conf.d'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-ssl-dh-params-file" xreflabel="ssl_dh_params_file">
+      <term><varname>ssl_dh_params_file</varname> (<type>string</type>)
+      <indexterm>
+       <primary><varname>ssl_dh_params_file</> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Specifies the name of the file containing Diffie-Hellman parameters
+        used for so-called ephemeral DH family of SSL ciphers. The default is
+        empty, in which case compiled-in default DH parameters used. Using
+        custom DH parameters reduces the exposure if an attacker manages to
+        crack the well-known compiled-in DH parameters. You can create your own
+        DH parameters file with the command
+        <command>openssl dhparam -out dhparams.pem 2048</command>.
+       </para>
+
+       <para>
+        This parameter can only be set in the <filename>postgresql.conf</>
+        file or on the server command line.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-krb-server-keyfile" xreflabel="krb_server_keyfile">
       <term><varname>krb_server_keyfile</varname> (<type>string</type>)
       <indexterm>

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

@@ -61,6 +61,7 @@
 #include "libpq/libpq.h"
 #include "miscadmin.h"
 #include "pgstat.h"
+#include "storage/fd.h"
 #include "storage/latch.h"
 #include "tcop/tcopprot.h"
 #include "utils/memutils.h"
@@ -71,13 +72,12 @@ static int	my_sock_write(BIO *h, const char *buf, int size);
 static BIO_METHOD *my_BIO_s_socket(void);
 static int	my_SSL_set_fd(Port *port, int fd);
 
-static DH  *load_dh_file(int keylength);
+static DH  *load_dh_file(char *filename, bool isServerStart);
 static DH  *load_dh_buffer(const char *, size_t);
-static DH  *generate_dh_parameters(int prime_len, int generator);
-static DH  *tmp_dh_cb(SSL *s, int is_export, int keylength);
 static int	ssl_passwd_cb(char *buf, int size, int rwflag, void *userdata);
 static int	verify_cb(int, X509_STORE_CTX *);
 static void info_cb(const SSL *ssl, int type, int args);
+static bool initialize_dh(SSL_CTX *context, bool isServerStart);
 static bool initialize_ecdh(SSL_CTX *context, bool isServerStart);
 static const char *SSLerrmessage(unsigned long ecode);
 
@@ -96,37 +96,21 @@ static bool ssl_passwd_cb_called = false;
  *	As discussed above, EDH protects the confidentiality of
  *	sessions even if the static private key is compromised,
  *	so we are *highly* motivated to ensure that we can use
- *	EDH even if the DBA... or an attacker... deletes the
- *	$DataDir/dh*.pem files.
+ *	EDH even if the DBA has not provided custom DH parameters.
  *
  *	We could refuse SSL connections unless a good DH parameter
  *	file exists, but some clients may quietly renegotiate an
  *	unsecured connection without fully informing the user.
- *	Very uncool.
- *
- *	Alternatively, the backend could attempt to load these files
- *	on startup if SSL is enabled - and refuse to start if any
- *	do not exist - but this would tend to piss off DBAs.
+ *	Very uncool. Alternatively, the system could refuse to start
+ *	if a DH parameters is not specified, but this would tend to
+ *	piss off DBAs.
  *
  *	If you want to create your own hardcoded DH parameters
  *	for fun and profit, review "Assigned Number for SKIP
  *	Protocols" (http://www.skip-vpn.org/spec/numbers.html)
  *	for suggestions.
  */
 
-static const char file_dh512[] =
-"-----BEGIN DH PARAMETERS-----\n\
-MEYCQQD1Kv884bEpQBgRjXyEpwpy1obEAxnIByl6ypUM2Zafq9AKUJsCRtMIPWak\n\
-XUGfnHy9iUsiGSa6q6Jew1XpKgVfAgEC\n\
------END DH PARAMETERS-----\n";
-
-static const char file_dh1024[] =
-"-----BEGIN DH PARAMETERS-----\n\
-MIGHAoGBAPSI/VhOSdvNILSd5JEHNmszbDgNRR0PfIizHHxbLY7288kjwEPwpVsY\n\
-jY67VYy4XTjTNP18F1dDox0YbN4zISy1Kv884bEpQBgRjXyEpwpy1obEAxnIByl6\n\
-ypUM2Zafq9AKUJsCRtMIPWakXUGfnHy9iUsiGSa6q6Jew1XpL3jHAgEC\n\
------END DH PARAMETERS-----\n";
-
 static const char file_dh2048[] =
 "-----BEGIN DH PARAMETERS-----\n\
 MIIBCAKCAQEA9kJXtwh/CBdyorrWqULzBej5UxE5T7bxbrlLOCDaAadWoxTpj0BV\n\
@@ -137,21 +121,6 @@ Q6MdGGzeMyEstSr/POGxKUAYEY18hKcKctaGxAMZyAcpesqVDNmWn6vQClCbAkbT\n\
 CD1mpF1Bn5x8vYlLIhkmuquiXsNV6TILOwIBAg==\n\
 -----END DH PARAMETERS-----\n";
 
-static const char file_dh4096[] =
-"-----BEGIN DH PARAMETERS-----\n\
-MIICCAKCAgEA+hRyUsFN4VpJ1O8JLcCo/VWr19k3BCgJ4uk+d+KhehjdRqNDNyOQ\n\
-l/MOyQNQfWXPeGKmOmIig6Ev/nm6Nf9Z2B1h3R4hExf+zTiHnvVPeRBhjdQi81rt\n\
-Xeoh6TNrSBIKIHfUJWBh3va0TxxjQIs6IZOLeVNRLMqzeylWqMf49HsIXqbcokUS\n\
-Vt1BkvLdW48j8PPv5DsKRN3tloTxqDJGo9tKvj1Fuk74A+Xda1kNhB7KFlqMyN98\n\
-VETEJ6c7KpfOo30mnK30wqw3S8OtaIR/maYX72tGOno2ehFDkq3pnPtEbD2CScxc\n\
-alJC+EL7RPk5c/tgeTvCngvc1KZn92Y//EI7G9tPZtylj2b56sHtMftIoYJ9+ODM\n\
-sccD5Piz/rejE3Ome8EOOceUSCYAhXn8b3qvxVI1ddd1pED6FHRhFvLrZxFvBEM9\n\
-ERRMp5QqOaHJkM+Dxv8Cj6MqrCbfC4u+ZErxodzuusgDgvZiLF22uxMZbobFWyte\n\
-OvOzKGtwcTqO/1wV5gKkzu1ZVswVUQd5Gg8lJicwqRWyyNRczDDoG9jVDxmogKTH\n\
-AaqLulO7R8Ifa1SwF2DteSGVtgWEN8gDpN3RBmmPTDngyF2DHb5qmpnznwtFKdTL\n\
-KWbuHn491xNO25CQWMtem80uKw+pTnisBRF/454n1Jnhub144YRBoN8CAQI=\n\
------END DH PARAMETERS-----\n";
-
 
 /* ------------------------------------------------------------ */
 /*						 Public interface						*/
@@ -316,13 +285,14 @@ be_tls_init(bool isServerStart)
 		goto error;
 	}
 
-	/* set up ephemeral DH keys, and disallow SSL v2/v3 while at it */
-	SSL_CTX_set_tmp_dh_callback(context, tmp_dh_cb);
+	/* disallow SSL v2/v3 */
 	SSL_CTX_set_options(context,
 						SSL_OP_SINGLE_DH_USE |
 						SSL_OP_NO_SSLv2 | SSL_OP_NO_SSLv3);
 
-	/* set up ephemeral ECDH keys */
+	/* set up ephemeral DH and ECDH keys */
+	if (!initialize_dh(context, isServerStart))
+		goto error;
 	if (!initialize_ecdh(context, isServerStart))
 		goto error;
 
@@ -918,53 +888,57 @@ my_SSL_set_fd(Port *port, int fd)
  *	what we expect it to contain.
  */
 static DH  *
-load_dh_file(int keylength)
+load_dh_file(char *filename, bool isServerStart)
 {
 	FILE	   *fp;
-	char		fnbuf[MAXPGPATH];
 	DH		   *dh = NULL;
 	int			codes;
 
 	/* attempt to open file.  It's not an error if it doesn't exist. */
-	snprintf(fnbuf, sizeof(fnbuf), "dh%d.pem", keylength);
-	if ((fp = fopen(fnbuf, "r")) == NULL)
+	if ((fp = AllocateFile(filename, "r")) == NULL)
+	{
+		ereport(isServerStart ? FATAL : LOG,
+				(errcode_for_file_access(),
+				 errmsg("could not open DH parameters file \"%s\": %m",
+						filename)));
 		return NULL;
+	}
 
-/*	flock(fileno(fp), LOCK_SH); */
 	dh = PEM_read_DHparams(fp, NULL, NULL, NULL);
-/*	flock(fileno(fp), LOCK_UN); */
-	fclose(fp);
+	FreeFile(fp);
 
-	/* is the prime the correct size? */
-	if (dh != NULL && 8 * DH_size(dh) < keylength)
+	if (dh == NULL)
 	{
-		elog(LOG, "DH errors (%s): %d bits expected, %d bits found",
-			 fnbuf, keylength, 8 * DH_size(dh));
-		dh = NULL;
+		ereport(isServerStart ? FATAL : LOG,
+				(errcode(ERRCODE_CONFIG_FILE_ERROR),
+				 errmsg("could not load DH parameters file: %s",
+						SSLerrmessage(ERR_get_error()))));
+		return NULL;
 	}
 
 	/* make sure the DH parameters are usable */
-	if (dh != NULL)
+	if (DH_check(dh, &codes) == 0)
 	{
-		if (DH_check(dh, &codes) == 0)
-		{
-			elog(LOG, "DH_check error (%s): %s", fnbuf,
-				 SSLerrmessage(ERR_get_error()));
-			return NULL;
-		}
-		if (codes & DH_CHECK_P_NOT_PRIME)
-		{
-			elog(LOG, "DH error (%s): p is not prime", fnbuf);
-			return NULL;
-		}
-		if ((codes & DH_NOT_SUITABLE_GENERATOR) &&
-			(codes & DH_CHECK_P_NOT_SAFE_PRIME))
-		{
-			elog(LOG,
-				 "DH error (%s): neither suitable generator or safe prime",
-				 fnbuf);
-			return NULL;
-		}
+		ereport(isServerStart ? FATAL : LOG,
+				(errcode(ERRCODE_CONFIG_FILE_ERROR),
+				 errmsg("invalid DH parameters: %s",
+						SSLerrmessage(ERR_get_error()))));
+		return NULL;
+	}
+	if (codes & DH_CHECK_P_NOT_PRIME)
+	{
+		ereport(isServerStart ? FATAL : LOG,
+				(errcode(ERRCODE_CONFIG_FILE_ERROR),
+				 errmsg("invalid DH parameters: p is not prime")));
+		return NULL;
+	}
+	if ((codes & DH_NOT_SUITABLE_GENERATOR) &&
+		(codes & DH_CHECK_P_NOT_SAFE_PRIME))
+	{
+		ereport(isServerStart ? FATAL : LOG,
+				(errcode(ERRCODE_CONFIG_FILE_ERROR),
+				 errmsg("invalid DH parameters: neither suitable generator or safe prime")));
+		return NULL;
 	}
 
 	return dh;
@@ -995,102 +969,6 @@ load_dh_buffer(const char *buffer, size_t len)
 	return dh;
 }
 
-/*
- *	Generate DH parameters.
- *
- *	Last resort if we can't load precomputed nor hardcoded
- *	parameters.
- */
-static DH  *
-generate_dh_parameters(int prime_len, int generator)
-{
-	DH		   *dh;
-
-	if ((dh = DH_new()) == NULL)
-		return NULL;
-
-	if (DH_generate_parameters_ex(dh, prime_len, generator, NULL))
-		return dh;
-
-	DH_free(dh);
-	return NULL;
-}
-
-/*
- *	Generate an ephemeral DH key.  Because this can take a long
- *	time to compute, we can use precomputed parameters of the
- *	common key sizes.
- *
- *	Since few sites will bother to precompute these parameter
- *	files, we also provide a fallback to the parameters provided
- *	by the OpenSSL project.
- *
- *	These values can be static (once loaded or computed) since
- *	the OpenSSL library can efficiently generate random keys from
- *	the information provided.
- */
-static DH  *
-tmp_dh_cb(SSL *s, int is_export, int keylength)
-{
-	DH		   *r = NULL;
-	static DH  *dh = NULL;
-	static DH  *dh512 = NULL;
-	static DH  *dh1024 = NULL;
-	static DH  *dh2048 = NULL;
-	static DH  *dh4096 = NULL;
-
-	switch (keylength)
-	{
-		case 512:
-			if (dh512 == NULL)
-				dh512 = load_dh_file(keylength);
-			if (dh512 == NULL)
-				dh512 = load_dh_buffer(file_dh512, sizeof file_dh512);
-			r = dh512;
-			break;
-
-		case 1024:
-			if (dh1024 == NULL)
-				dh1024 = load_dh_file(keylength);
-			if (dh1024 == NULL)
-				dh1024 = load_dh_buffer(file_dh1024, sizeof file_dh1024);
-			r = dh1024;
-			break;
-
-		case 2048:
-			if (dh2048 == NULL)
-				dh2048 = load_dh_file(keylength);
-			if (dh2048 == NULL)
-				dh2048 = load_dh_buffer(file_dh2048, sizeof file_dh2048);
-			r = dh2048;
-			break;
-
-		case 4096:
-			if (dh4096 == NULL)
-				dh4096 = load_dh_file(keylength);
-			if (dh4096 == NULL)
-				dh4096 = load_dh_buffer(file_dh4096, sizeof file_dh4096);
-			r = dh4096;
-			break;
-
-		default:
-			if (dh == NULL)
-				dh = load_dh_file(keylength);
-			r = dh;
-	}
-
-	/* this may take a long time, but it may be necessary... */
-	if (r == NULL || 8 * DH_size(r) < keylength)
-	{
-		ereport(DEBUG2,
-				(errmsg_internal("DH: generating parameters (%d bits)",
-								 keylength)));
-		r = generate_dh_parameters(keylength, DH_GENERATOR_2);
-	}
-
-	return r;
-}
-
 /*
  *	Passphrase collection callback
  *
@@ -1172,6 +1050,54 @@ info_cb(const SSL *ssl, int type, int args)
 	}
 }
 
+/*
+ * Set DH parameters for generating ephemeral DH keys.  The
+ * DH parameters can take a long time to compute, so they must be
+ * precomputed.
+ *
+ * Since few sites will bother to create a parameter file, we also
+ * also provide a fallback to the parameters provided by the
+ * OpenSSL project.
+ *
+ * These values can be static (once loaded or computed) since the
+ * OpenSSL library can efficiently generate random keys from the
+ * information provided.
+ */
+static bool
+initialize_dh(SSL_CTX *context, bool isServerStart)
+{
+	DH		   *dh = NULL;
+
+	SSL_CTX_set_options(context, SSL_OP_SINGLE_DH_USE);
+
+	if (ssl_dh_params_file[0])
+		dh = load_dh_file(ssl_dh_params_file, isServerStart);
+	if (!dh)
+		dh = load_dh_buffer(file_dh2048, sizeof file_dh2048);
+	if (!dh)
+	{
+		ereport(isServerStart ? FATAL : LOG,
+				(errcode(ERRCODE_CONFIG_FILE_ERROR),
+				 (errmsg("DH: could not load DH parameters"))));
+		return false;
+	}
+
+	if (SSL_CTX_set_tmp_dh(context, dh) != 1)
+	{
+		ereport(isServerStart ? FATAL : LOG,
+				(errcode(ERRCODE_CONFIG_FILE_ERROR),
+				 (errmsg("DH: could not set DH parameters: %s",
+						 SSLerrmessage(ERR_get_error())))));
+		return false;
+	}
+	return true;
+}
+
+/*
+ * Set ECDH parameters for generating ephemeral Elliptic Curve DH
+ * keys.  This is much simpler than the DH parameters, as we just
+ * need to provide the name of the curve to OpenSSL.
+ */
 static bool
 initialize_ecdh(SSL_CTX *context, bool isServerStart)
 {

src/backend/libpq/be-secure.c

@@ -44,6 +44,7 @@ char	   *ssl_cert_file;
 char	   *ssl_key_file;
 char	   *ssl_ca_file;
 char	   *ssl_crl_file;
+char	   *ssl_dh_params_file;
 
 #ifdef USE_SSL
 bool		ssl_loaded_verify_locations = false;