[Openvpn-devel,v2,1/3] Use crypto library functions for const time memcmp when possible

Message ID 20200417075302.6924-1-arne@rfc2549.org
State Accepted
Headers show
Series
  • [Openvpn-devel,v2,1/3] Use crypto library functions for const time memcmp when possible
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Commit Message

Arne Schwabe April 17, 2020, 7:53 a.m.
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
---
 src/openvpn/crypto.h         | 16 +---------------
 src/openvpn/crypto_mbedtls.c | 20 ++++++++++++++++++++
 src/openvpn/crypto_openssl.c |  5 +++++
 3 files changed, 26 insertions(+), 15 deletions(-)

Patch

diff --git a/src/openvpn/crypto.h b/src/openvpn/crypto.h
index 18a86ceb..dadf0a90 100644
--- a/src/openvpn/crypto.h
+++ b/src/openvpn/crypto.h
@@ -528,21 +528,7 @@  void crypto_read_openvpn_key(const struct key_type *key_type,
  * As memcmp(), but constant-time.
  * Returns 0 when data is equal, non-zero otherwise.
  */
-static inline 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;
-}
+int memcmp_constant_time(const void *a, const void *b, size_t size);
 
 static inline bool
 key_ctx_bi_defined(const struct key_ctx_bi *key)
diff --git a/src/openvpn/crypto_mbedtls.c b/src/openvpn/crypto_mbedtls.c
index 3e77fa9e..1f6a23f8 100644
--- a/src/openvpn/crypto_mbedtls.c
+++ b/src/openvpn/crypto_mbedtls.c
@@ -972,4 +972,24 @@  hmac_ctx_final(mbedtls_md_context_t *ctx, uint8_t *dst)
     ASSERT(0 == mbedtls_md_hmac_finish(ctx, dst));
 }
 
+int
+memcmp_constant_time(const void *a, const void *b, size_t size)
+{
+    /* mbed TLS has a no const time memcmp function.
+     * Adapt the function mbedtls_safer_memcmp that mbedtls
+     * internally uses as it considers that to be safe. */
+    volatile const unsigned char *A = (volatile const unsigned char *)a;
+    volatile const unsigned char *B = (volatile const unsigned char *)b;
+    volatile unsigned char diff = 0;
+
+    for (size_t i = 0; i < size; i++)
+    {
+        /* this conversion was introduced by mbedTLS to suppress a IAR
+         * compiler warning. We keep it as it is. */
+        unsigned char x = A[i], y = B[i];
+        diff |= x ^ y;
+    }
+
+    return diff;
+}
 #endif /* ENABLE_CRYPTO_MBEDTLS */
diff --git a/src/openvpn/crypto_openssl.c b/src/openvpn/crypto_openssl.c
index a81dcfd8..9e7ea0ff 100644
--- a/src/openvpn/crypto_openssl.c
+++ b/src/openvpn/crypto_openssl.c
@@ -1066,4 +1066,9 @@  hmac_ctx_final(HMAC_CTX *ctx, uint8_t *dst)
     HMAC_Final(ctx, dst, &in_hmac_len);
 }
 
+int
+memcmp_constant_time(const void *a, const void *b, size_t size)
+{
+    return CRYPTO_memcmp(a, b, size);
+}
 #endif /* ENABLE_CRYPTO_OPENSSL */