Fix bug 160207. Make TLS implementation resistant to timing attacks on

CBC block mode cipher suites in TLS.  See bug for details.

security/nss/lib/ssl/ssl3con.c

@@ -33,7 +33,7 @@
  * may use your version of this file under either the MPL or the
  * GPL.
  *
- * $Id: ssl3con.c,v 1.47 2003-02-15 01:21:23 relyea%netscape.com Exp $
+ * $Id: ssl3con.c,v 1.48 2003-02-21 23:00:16 nelsonb%netscape.com Exp $
  */
 
 #include "nssrenam.h"
@@ -7413,9 +7413,10 @@ const ssl3BulkCipherDef *cipher_def;
     ssl3State *          ssl3 			= ss->ssl3;
     ssl3CipherSpec *     crSpec;
     SECStatus            rv;
-    unsigned int         hashBytes;
+    unsigned int         hashBytes		= MAX_MAC_LENGTH + 1;
     unsigned int         padding_length;
     PRBool               isTLS;
+    PRBool               padIsBad               = PR_FALSE;
     SSL3ContentType      rType;
     SSL3Opaque           hash[MAX_MAC_LENGTH];
 
@@ -7456,6 +7457,7 @@ const ssl3BulkCipherDef *cipher_def;
 	    SSL_DBG(("%d: SSL3[%d]: HandleRecord, tried to get %d bytes",
 		     SSL_GETPID(), ss->fd, MAX_FRAGMENT_LENGTH + 2048));
 	    /* sslBuffer_Grow has set a memory error code. */
+	    /* Perhaps we should send an alert. (but we have no memory!) */
 	    return SECFailure;
 	}
     }
@@ -7481,11 +7483,11 @@ const ssl3BulkCipherDef *cipher_def;
 
     PRINT_BUF(80, (ss, "cleartext:", databuf->buf, databuf->len));
     if (rv != SECSuccess) {
+	int err = ssl_MapLowLevelError(SSL_ERROR_DECRYPTION_FAILURE);
 	ssl_ReleaseSpecReadLock(ss);
-	ssl_MapLowLevelError(SSL_ERROR_DECRYPTION_FAILURE);
-	SSL3_SendAlert(ss, alert_fatal, 
-		       isTLS ? decryption_failed : bad_record_mac);
-	ssl_MapLowLevelError(SSL_ERROR_DECRYPTION_FAILURE);
+	SSL3_SendAlert(ss, alert_fatal,
+	               isTLS ? decryption_failed : bad_record_mac);
+	PORT_SetError(err);
 	return SECFailure;
     }
 
@@ -7493,48 +7495,45 @@ const ssl3BulkCipherDef *cipher_def;
     if (cipher_def->type == type_block) {
 	padding_length = *(databuf->buf + databuf->len - 1);
 	/* TLS permits padding to exceed the block size, up to 255 bytes. */
-	if (padding_length + crSpec->mac_size >= databuf->len)
-	    goto bad_pad;
+	if (padding_length + 1 + crSpec->mac_size > databuf->len)
+	    padIsBad = PR_TRUE;
 	/* if TLS, check value of first padding byte. */
-	if (padding_length && isTLS && padding_length != 
-		*(databuf->buf + databuf->len - 1 - padding_length))
-	    goto bad_pad;
-	databuf->len -= padding_length + 1;
-	if (databuf->len <= 0) {
-bad_pad:
-	    /* must not hold spec lock when calling SSL3_SendAlert. */
-	    ssl_ReleaseSpecReadLock(ss);
-	    /* SSL3 & TLS must send bad_record_mac if padding check fails. */
-	    SSL3_SendAlert(ss, alert_fatal, bad_record_mac);
-	    PORT_SetError(SSL_ERROR_BAD_BLOCK_PADDING);
-	    return SECFailure;
-	}
+	else if (padding_length && isTLS && 
+	         padding_length != 
+	                 *(databuf->buf + databuf->len - (padding_length + 1)))
+	    padIsBad = PR_TRUE;
+	else
+	    databuf->len -= padding_length + 1;
     }
 
-    /* Check the MAC. */
-    if (databuf->len < crSpec->mac_size) {
-    	/* record is too short to have a valid mac. */
-	goto bad_mac;
-    }
-    databuf->len -= crSpec->mac_size;
+    /* Remove the MAC. */
+    if (databuf->len >= crSpec->mac_size)
+	databuf->len -= crSpec->mac_size;
+    else
+    	padIsBad = PR_TRUE;	/* really macIsBad */
+
+    /* compute the MAC */
     rType = cText->type;
     rv = ssl3_ComputeRecordMAC(
-    	crSpec, (ss->sec.isServer) ? crSpec->client.write_mac_context
+	crSpec, (ss->sec.isServer) ? crSpec->client.write_mac_context
 				    : crSpec->server.write_mac_context,
 	rType, cText->version, crSpec->read_seq_num, 
 	databuf->buf, databuf->len, hash, &hashBytes);
     if (rv != SECSuccess) {
+	int err = ssl_MapLowLevelError(SSL_ERROR_MAC_COMPUTATION_FAILURE);
 	ssl_ReleaseSpecReadLock(ss);
-	ssl_MapLowLevelError(SSL_ERROR_MAC_COMPUTATION_FAILURE);
+	SSL3_SendAlert(ss, alert_fatal, bad_record_mac);
+	PORT_SetError(err);
 	return rv;
     }
 
-    if (hashBytes != (unsigned)crSpec->mac_size ||
+    /* Check the MAC */
+    if (hashBytes != (unsigned)crSpec->mac_size || padIsBad || 
 	PORT_Memcmp(databuf->buf + databuf->len, hash, crSpec->mac_size) != 0) {
-bad_mac:
 	/* must not hold spec lock when calling SSL3_SendAlert. */
 	ssl_ReleaseSpecReadLock(ss);
 	SSL3_SendAlert(ss, alert_fatal, bad_record_mac);
+	/* always log mac error, in case attacker can read server logs. */
 	PORT_SetError(SSL_ERROR_BAD_MAC_READ);
 
 	SSL_DBG(("%d: SSL3[%d]: mac check failed", SSL_GETPID(), ss->fd));