Use constant time memcmp when comparing HMACs in openvpn_decrypt.

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Signed-off-by: Gert Doering <gert@greenie.muc.de>

src/openvpn/buffer.h

@@ -668,6 +668,10 @@ buf_read_u32 (struct buffer *buf, bool *good)
    }
}

/**
 * Compare src buffer contents with match.
 * *NOT* constant time. Do not use when comparing HMACs.
 */
static inline bool
buf_string_match (const struct buffer *src, const void *match, int size)
{
@@ -676,6 +680,10 @@ buf_string_match (const struct buffer *src, const void *match, int size)
  return memcmp (BPTR (src), match, size) == 0;
}

/**
 * Compare first size bytes of src buffer contents with match.
 * *NOT* constant time. Do not use when comparing HMACs.
 */
static inline bool
buf_string_match_head (const struct buffer *src, const void *match, int size)
{

src/openvpn/crypto.c

@@ -65,6 +65,24 @@
#define CRYPT_ERROR(format) \
  do { msg (D_CRYPT_ERRORS, "%s: " format, error_prefix); goto error_exit; } while (false)

/**
 * As memcmp(), but constant-time.
 * Returns 0 when data is equal, non-zero otherwise.
 */
static int
memcmp_constant_time (const void *a, const void *b, size_t size) {
  const uint8_t * a1 = a;
  const uint8_t * b1 = b;
  int ret = 0;
  size_t i;

  for (i = 0; i < size; i++) {
      ret |= *a1++ ^ *b1++;
  }

  return ret;
}

void
openvpn_encrypt (struct buffer *buf, struct buffer work,
		 const struct crypto_options *opt,
@@ -244,7 +262,7 @@ openvpn_decrypt (struct buffer *buf, struct buffer work,
	  hmac_ctx_final (ctx->hmac, local_hmac);

	  /* Compare locally computed HMAC with packet HMAC */
	  if (memcmp (local_hmac, BPTR (buf), hmac_len))
	  if (memcmp_constant_time (local_hmac, BPTR (buf), hmac_len))
	    CRYPT_ERROR ("packet HMAC authentication failed");

	  ASSERT (buf_advance (buf, hmac_len));