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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>
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1 parent 9a3f670 commit 11d21349a4e7e38a025849479b36ace7c2eec2ee @syzzer syzzer committed with cron2 Mar 19, 2013
Showing with 27 additions and 1 deletion.
  1. +8 −0 src/openvpn/buffer.h
  2. +19 −1 src/openvpn/crypto.c
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@@ -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)
{
View
@@ -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));

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