Add support to zeroize plaintext in S3 record layer

Some applications want even all plaintext copies beeing
zeroized. However, currently plaintext residuals are kept in rbuf
within the s3 record layer.

This patch add the option SSL_OP_CLEANSE_PLAINTEXT to its friends to
optionally enable cleansing of decrypted plaintext data.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/12251)

CHANGES.md

@@ -1100,6 +1100,14 @@ OpenSSL 3.0
 
    *Boris Pismenny*
 
+ * The SSL option SSL_OP_CLEANSE_PLAINTEXT is introduced. If that
+   option is set, openssl cleanses (zeroize) plaintext bytes from
+   internal buffers after delivering them to the application. Note,
+   the application is still responsible for cleansing other copies
+   (e.g.: data received by SSL_read(3)).
+
+   *Martin Elshuber*
+
 OpenSSL 1.1.1
 -------------
 

doc/man3/SSL_CTX_set_options.pod

@@ -265,6 +265,20 @@ functionality is not required. Those applications can turn this feature off by
 setting this option. This is a server-side opton only. It is ignored by
 clients.
 
+=item SSL_OP_CLEANSE_PLAINTEXT
+
+By default TLS connections keep a copy of received plaintext
+application data in a static buffer until it is overwritten by the
+next portion of data. When enabling SSL_OP_CLEANSE_PLAINTEXT
+deciphered application data is cleansed by calling OPENSSL_cleanse(3)
+after passing data to the application. Data is also cleansed when
+releasing the connection (eg. L<SSL_free(3)>).
+
+Since OpenSSL only cleanses internal buffers, the application is still
+responsible for cleansing all other buffers. Most notably, this
+applies to buffers passed to functions like L<SSL_read(3)>,
+L<SSL_peek(3)> but also like L<SSL_write(3)>.
+
 =back
 
 The following options no longer have any effect but their identifiers are

include/openssl/ssl.h

@@ -321,7 +321,8 @@ typedef int (*SSL_async_callback_fn)(SSL *s, void *arg);
 /* Disable Extended master secret */
 # define SSL_OP_NO_EXTENDED_MASTER_SECRET                0x00000001U
 
-/* Reserved value (until OpenSSL 3.0.0)                  0x00000002U */
+/* Cleanse plaintext copies of data delivered to the application */
+# define SSL_OP_CLEANSE_PLAINTEXT                        0x00000002U
 
 /* Allow initial connection to servers that don't support RI */
 # define SSL_OP_LEGACY_SERVER_CONNECT                    0x00000004U

ssl/record/rec_layer_d1.c

@@ -74,6 +74,8 @@ void DTLS_RECORD_LAYER_clear(RECORD_LAYER *rl)
 
     while ((item = pqueue_pop(d->processed_rcds.q)) != NULL) {
         rdata = (DTLS1_RECORD_DATA *)item->data;
+        if (rl->s->options & SSL_OP_CLEANSE_PLAINTEXT)
+            OPENSSL_cleanse(rdata->rbuf.buf, rdata->rbuf.len);
         OPENSSL_free(rdata->rbuf.buf);
         OPENSSL_free(item->data);
         pitem_free(item);
@@ -81,6 +83,8 @@ void DTLS_RECORD_LAYER_clear(RECORD_LAYER *rl)
 
     while ((item = pqueue_pop(d->buffered_app_data.q)) != NULL) {
         rdata = (DTLS1_RECORD_DATA *)item->data;
+        if (rl->s->options & SSL_OP_CLEANSE_PLAINTEXT)
+            OPENSSL_cleanse(rdata->rbuf.buf, rdata->rbuf.len);
         OPENSSL_free(rdata->rbuf.buf);
         OPENSSL_free(item->data);
         pitem_free(item);
@@ -514,6 +518,8 @@ int dtls1_read_bytes(SSL *s, int type, int *recvd_type, unsigned char *buf,
             if (SSL3_RECORD_get_length(rr) == 0)
                 SSL3_RECORD_set_read(rr);
         } else {
+            if (s->options & SSL_OP_CLEANSE_PLAINTEXT)
+                OPENSSL_cleanse(&(SSL3_RECORD_get_data(rr)[SSL3_RECORD_get_off(rr)]), n);
             SSL3_RECORD_sub_length(rr, n);
             SSL3_RECORD_add_off(rr, n);
             if (SSL3_RECORD_get_length(rr) == 0) {

ssl/record/rec_layer_s3.c

@@ -1484,6 +1484,8 @@ int ssl3_read_bytes(SSL *s, int type, int *recvd_type, unsigned char *buf,
                 if (SSL3_RECORD_get_length(rr) == 0)
                     SSL3_RECORD_set_read(rr);
             } else {
+                if (s->options & SSL_OP_CLEANSE_PLAINTEXT)
+                    OPENSSL_cleanse(&(rr->data[rr->off]), n);
                 SSL3_RECORD_sub_length(rr, n);
                 SSL3_RECORD_add_off(rr, n);
                 if (SSL3_RECORD_get_length(rr) == 0) {

ssl/record/ssl3_buffer.c

@@ -180,6 +180,8 @@ int ssl3_release_read_buffer(SSL *s)
     SSL3_BUFFER *b;
 
     b = RECORD_LAYER_get_rbuf(&s->rlayer);
+    if (s->options & SSL_OP_CLEANSE_PLAINTEXT)
+        OPENSSL_cleanse(b->buf, b->len);
     OPENSSL_free(b->buf);
     b->buf = NULL;
     return 1;

test/sslapitest.c

@@ -1595,6 +1595,119 @@ static int test_large_message_dtls(void)
 }
 #endif
 
+static int execute_cleanse_plaintext(const SSL_METHOD *smeth,
+                                     const SSL_METHOD *cmeth,
+                                     int min_version, int max_version)
+{
+    size_t i;
+    SSL_CTX *cctx = NULL, *sctx = NULL;
+    SSL *clientssl = NULL, *serverssl = NULL;
+    int testresult = 0;
+    SSL3_RECORD *rr;
+    void *zbuf;
+
+    static unsigned char cbuf[16000];
+    static unsigned char sbuf[16000];
+
+    if (!TEST_true(create_ssl_ctx_pair(libctx,
+                                       smeth, cmeth,
+                                       min_version, max_version,
+                                       &sctx, &cctx, cert,
+                                       privkey)))
+        goto end;
+
+    if (!TEST_true(create_ssl_objects(sctx, cctx, &serverssl, &clientssl,
+                                      NULL, NULL)))
+        goto end;
+
+    if (!TEST_true(SSL_set_options(serverssl, SSL_OP_CLEANSE_PLAINTEXT)))
+        goto end;
+
+    if (!TEST_true(create_ssl_connection(serverssl, clientssl,
+                                         SSL_ERROR_NONE)))
+        goto end;
+
+    for (i = 0; i < sizeof(cbuf); i++) {
+        cbuf[i] = i & 0xff;
+    }
+
+    if (!TEST_int_eq(SSL_write(clientssl, cbuf, sizeof(cbuf)), sizeof(cbuf)))
+        goto end;
+
+    if (!TEST_int_eq(SSL_peek(serverssl, &sbuf, sizeof(sbuf)), sizeof(sbuf)))
+        goto end;
+
+    if (!TEST_mem_eq(cbuf, sizeof(cbuf), sbuf, sizeof(sbuf)))
+        goto end;
+
+    /*
+     * Since we called SSL_peek(), we know the data in the record
+     * layer is a plaintext record. We can gather the pointer to check
+     * for zeroization after SSL_read().
+     */
+    rr = serverssl->rlayer.rrec;
+    zbuf = &rr->data[rr->off];
+    if (!TEST_int_eq(rr->length, sizeof(cbuf)))
+        goto end;
+
+    /*
+     * After SSL_peek() the plaintext must still be stored in the
+     * record.
+     */
+    if (!TEST_mem_eq(cbuf, sizeof(cbuf), zbuf, sizeof(cbuf)))
+        goto end;
+
+    memset(sbuf, 0, sizeof(sbuf));
+    if (!TEST_int_eq(SSL_read(serverssl, &sbuf, sizeof(sbuf)), sizeof(sbuf)))
+        goto end;
+
+    if (!TEST_mem_eq(cbuf, sizeof(cbuf), sbuf, sizeof(cbuf)))
+        goto end;
+
+    /* Check if rbuf is cleansed */
+    memset(cbuf, 0, sizeof(cbuf));
+    if (!TEST_mem_eq(cbuf, sizeof(cbuf), zbuf, sizeof(cbuf)))
+        goto end;
+
+    testresult = 1;
+ end:
+    SSL_free(serverssl);
+    SSL_free(clientssl);
+    SSL_CTX_free(sctx);
+    SSL_CTX_free(cctx);
+
+    return testresult;
+}
+
+static int test_cleanse_plaintext(void)
+{
+#if !defined(OPENSSL_NO_TLS1_2)
+    if (!TEST_true(execute_cleanse_plaintext(TLS_server_method(),
+                                             TLS_client_method(),
+                                             TLS1_2_VERSION,
+                                             TLS1_2_VERSION)))
+        return 0;
+
+#endif
+
+#if !defined(OPENSSL_NO_TLS1_3)
+    if (!TEST_true(execute_cleanse_plaintext(TLS_server_method(),
+                                             TLS_client_method(),
+                                             TLS1_3_VERSION,
+                                             TLS1_3_VERSION)))
+        return 0;
+#endif
+
+#if !defined(OPENSSL_NO_DTLS)
+    if (!TEST_true(execute_cleanse_plaintext(DTLS_server_method(),
+                                             DTLS_client_method(),
+                                             DTLS1_VERSION,
+                                             0)))
+        return 0;
+#endif
+    return 1;
+}
+
 #ifndef OPENSSL_NO_OCSP
 static int ocsp_server_cb(SSL *s, void *arg)
 {
@@ -8324,6 +8437,7 @@ int setup_tests(void)
 #ifndef OPENSSL_NO_DTLS
     ADD_TEST(test_large_message_dtls);
 #endif
+    ADD_TEST(test_cleanse_plaintext);
 #ifndef OPENSSL_NO_OCSP
     ADD_TEST(test_tlsext_status_type);
 #endif