BUG OF THE MONTH | Dangerous optimization (a vulnerability)

V597 The compiler could delete the ‘memset’ function call, which is used to flush ‘temp_result’ buffer. The RtlSecureZeroMemory() function should be used to erase the private data. sha256-crypt.c 385

char *
__sha256_crypt_r (key, salt, buffer, buflen)
     const char *key;
     const char *salt;
     char *buffer;
     int buflen;
{
  ....
  unsigned char temp_result[32]
  ....
  memset (temp_result, '\0', sizeof (temp_result));
  ....
  .... // temp_result not used further on
}

The compiler is allowed to remove the call of the memset() function when compiling the Release version. Well, it is actually obliged to do so for the sake of optimization. Since the ‘temp_result’ buffer is not used anywhere after calling the memset() function, the function call itself is not needed too.

This is a vulnerability because the private data will not be erased. The memset() function should be replaced with a more appropriate one. The analyzer suggests RtlSecureZeroMemory(), which is absent in Linux of course. But there are other alternatives.

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