Add mbedtls_ssl_cf_memcpy_offset() with tests

The tests are supposed to be failing now (in all.sh component
test_memsan_constant_flow), but they don't as apparently MemSan doesn't
complain when the src argument of memcpy() is uninitialized, see
https://github.com/google/sanitizers/issues/1296

The next commit will add an option to test constant flow with valgrind, which
will hopefully correctly flag the current non-constant-flow implementation.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>

library/ssl_invasive.h

@@ -73,6 +73,30 @@ int mbedtls_ssl_cf_hmac(
         const unsigned char *data, size_t data_len_secret,
         size_t min_data_len, size_t max_data_len,
         unsigned char *output );
+
+/** \brief Copy data from a secret position with constant flow.
+ *
+ * This function copies \p len bytes from \p src_base + \p offset_secret to \p
+ * dst, with a code flow and memory access pattern that does not depend on \p
+ * offset_secret, but only on \p offset_min, \p offset_max and \p len.
+ *
+ * \param dst           The destination buffer. This must point to a writable
+ *                      buffer of at least \p len bytes.
+ * \param src_base      The base of the source buffer. This must point to a
+ *                      readable buffer of at least \p offset_max + \p len
+ *                      bytes.
+ * \param offset_secret The offset in the source buffer from which to copy.
+ *                      This must be no less than \p offset_min and no greater
+ *                      than \p offset_max.
+ * \param offset_min    The minimal value of \p offset_secret.
+ * \param offset_max    The maximal value of \p offset_secret.
+ * \param len           The number of bytes to copy.
+ */
+void mbedtls_ssl_cf_memcpy_offset( unsigned char *dst,
+                                   const unsigned char *src_base,
+                                   size_t offset_secret,
+                                   size_t offset_min, size_t offset_max,
+                                   size_t len );
 #endif /* MBEDTLS_TEST_HOOKS && MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC */
 
 #endif /* MBEDTLS_SSL_INVASIVE_H */