Timing fix mitigation for FIPS mode.

We have to use EVP in FIPS mode so we can only partially mitigate
timing differences.

Make an extra call to EVP_DigestSignUpdate to hash additonal blocks
to cover any timing differences caused by removal of padding.

diff --git a/ssl/s3_cbc.c b/ssl/s3_cbc.c
index e9b112c1b589510aff18ef39cace825b4529ce2b..c3305eac06d914c42d660530e8fd5da48fe1043a 100644 (file)
--- a/ssl/s3_cbc.c
+++ b/ssl/s3_cbc.c
@@ -368,6 +368,10 @@ static void tls1_sha512_final_raw(void* ctx, unsigned char *md_out)
  * which ssl3_cbc_digest_record supports. */
 char ssl3_cbc_record_digest_supported(const EVP_MD_CTX *ctx)
        {
+#ifdef OPENSSL_FIPS
+       if (FIPS_mode())
+               return 0;
+#endif
        switch (ctx->digest->type)
                {
                case NID_md5:
@@ -694,3 +698,50 @@ void ssl3_cbc_digest_record(
                *md_out_size = md_out_size_u;
        EVP_MD_CTX_cleanup(&md_ctx);
        }
+
+#ifndef OPENSSL_FIPS
+
+/* Due to the need to use EVP in FIPS mode we can't reimplement digests but
+ * we can ensure the number of blocks processed is equal for all cases
+ * by digesting additional data.
+ */
+
+void tls_fips_digest_extra(
+       const EVP_CIPHER_CTX *cipher_ctx, EVP_MD_CTX *mac_ctx,
+       const unsigned char *data, size_t data_len, size_t orig_len)
+       {
+       size_t block_size, digest_pad, blocks_data, blocks_orig;
+       if (EVP_CIPHER_CTX_mode(cipher_ctx) != EVP_CIPH_CBC_MODE)
+               return;
+       block_size = EVP_MD_CTX_block_size(mac_ctx);
+       /* We are in FIPS mode if we get this far so we know we have only SHA*
+        * digests and TLS to deal with.
+        * Minimum digest padding length is 17 for SHA384/SHA512 and 9
+        * otherwise.
+        * Additional header is 13 bytes. To get the number of digest blocks
+        * processed round up the amount of data plus padding to the nearest
+        * block length. Block length is 128 for SHA384/SHA512 and 64 otherwise.
+        * So we have:
+        * blocks = (payload_len + digest_pad + 13 + block_size - 1)/block_size
+        * equivalently:
+        * blocks = (payload_len + digest_pad + 12)/block_size + 1
+        * HMAC adds a constant overhead.
+        * We're ultimately only interested in differences so this becomes
+        * blocks = (payload_len + 29)/128
+        * for SHA384/SHA512 and
+        * blocks = (payload_len + 21)/64
+        * otherwise.
+        */
+       digest_pad = block_size == 64 ? 21 : 29;
+       blocks_orig = (orig_len + digest_pad)/block_size;
+       blocks_data = (data_len + digest_pad)/block_size;
+       /* MAC enough blocks to make up the difference between the original
+        * and actual lengths plus one extra block to ensure this is never a
+        * no op. The "data" pointer should always have enough space to
+        * perform this operation as it is large enough for a maximum
+        * length TLS buffer. 
+        */
+       EVP_DigestSignUpdate(mac_ctx, data,
+                               (blocks_orig - blocks_data + 1) * block_size);
+       }
+#endif

diff --git a/ssl/ssl_locl.h b/ssl/ssl_locl.h
index dd8388cace4783b6418e301e5995aeb185bddd7d..e9430cd3dbfe7be7c39d29d04f0f50c243dcdce5 100644 (file)
--- a/ssl/ssl_locl.h
+++ b/ssl/ssl_locl.h
@@ -1167,4 +1167,8 @@ void ssl3_cbc_digest_record(
        unsigned mac_secret_length,
        char is_sslv3);
 
+void tls_fips_digest_extra(
+       const EVP_CIPHER_CTX *cipher_ctx, EVP_MD_CTX *mac_ctx,
+       const unsigned char *data, size_t data_len, size_t orig_len);
+
 #endif

diff --git a/ssl/t1_enc.c b/ssl/t1_enc.c
index bb46f7f9ba5b7e5bceb183787f569db003d264fc..6fbe2c33aa7ed88beaa7a8a2d413a5c86705d302 100644 (file)
--- a/ssl/t1_enc.c
+++ b/ssl/t1_enc.c
@@ -1025,6 +1025,13 @@ int tls1_mac(SSL *ssl, unsigned char *md, int send)
                EVP_DigestSignUpdate(mac_ctx,rec->input,rec->length);
                t=EVP_DigestSignFinal(mac_ctx,md,&md_size);
                OPENSSL_assert(t > 0);
+#ifdef OPENSSL_FIPS
+               if (!send && FIPS_mode())
+                       tls_fips_digest_extra(
+                                       ssl->enc_read_ctx,
+                                       mac_ctx, rec->input,
+                                       rec->length, rec->orig_len);
+#endif
                }
                
        if (!stream_mac)