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.
(cherry picked from commit b908e88ec15aa0a74805e3f2236fc4f83f2789c2)

1 files changed, 51 insertions, 0 deletions

diff --git a/ssl/s3_cbc.c b/ssl/s3_cbc.c
index e9b112c1b5..c3305eac06 100644
--- 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