Be aware of insecure compiler optimization when dealing with sensitive data

Insecure compiler optimizations can occur leaving sensitive data (such as passwords or cryptographic keys) in memory. 

Non-Compliant Code Example (memset())

void getPassword() {
  char pwd[64];
  if (GetPassword(pwd, sizeof(pwd))) {
    /* checking of password, secure operations, etc */
  }
  memset(pwd, 0, sizeof(pwd));
}

Some compiler optimization modes may remove code sections if the optimizer determines that doing so will not alter the behavior of the program. In this example, this can cause the call to memset() (which the programmer had hoped would clear sensitive memory) to be removed because after the store to pwd, pwd is never accessed again. Check compiler documentation for information about this compiler specific behavior and which optimization levels can cause this behavior to occur.

For all of the below listed compliant code examples, it is strongly recommended that the programmer inspect the generated assembly code to ensure that memory is actually zeroed and none of the function calls were optimized out.

Non-Compliant Code Example (Touching Memory)

This non-compliant code example accesses the buffer again after the call to memset(). This prevents some compilers from optimizing out the call to memset() but does not work for all implementaitons. Check compiler documentation to guarantee this behavior for a specific platform.

void getPassword() {
  char pwd[64];
  if (GetPassword(pwd, sizeof(pwd))) {
    /*checking of password, secure operations, etc */
  }
  memset(pwd, 0, sizeof(pwd));
  *(volatile char*)pwd= *(volatile char*)pwd;
}

Implementation Details

The MIPSpro compiler and versions 3 and later of GCC cleverly nullify only the first byte and leave the rest intact.

Non-Compliant Code Example (Windows)

This compliant solution uses a ZeroMemory() function provided by many versions of the Microsoft Visual Studio compiler.

void getPassword() {
  char pwd[64];
  if (GetPassword(pwd, sizeof(pwd))) {
    /* checking of password, secure operations, etc */
  }
  ZeroMemory(pwd, sizeof(pwd));
}

A call to ZeroMemory() may be optimized out in similar manner as a call to memset().

Compliant Code Example (Windows)

This compliant solution uses a SecureZeroMemory() function provided by many versions of the Microsoft Visual Studio compiler. The documentation for the SecureZeroMemory() function guarantees that the compiler will not optimize out this call when zeroing memory.

void getPassword() {
  char pwd[64];
  if(GetPassword(pwd, sizeof(pwd))) {
    /* checking of password, secure operations, etc */
  }
  SecureZeroMemory(pwd, sizeof(pwd));
}

Compliant Solution (Windows)

The #pragma directives here instructs the compiler to avoid optimizing the enclosed code. This #pragma directive is supported on some versions of Microsoft Visual Studio, and may be supported on other compilers. Check compiler documentation to ensure its availability and its optimization guarantees.

void getPassword() {
  char pwd[64];
  if(GetPassword(pwd, sizeof(pwd))) {
    /* checking of password, secure operations, etc */
  }
#pragma optimize("", off)
  memset(pwd, 0, sizeof(pwd));
#pragma optimize("", on)
}

Compliant Solution (POSIX)

This compliant solution guarantees, via the volatile type qualifier, that memory is actually overwritten and the compiler will not optimize out the call to the memset_s() function. Unfortunately, this compliant solution may not be as efficient as possible due to the nature of the volatile type qualifier preventing the compiler from optimizing the code at all. Typically, some compilers are smart enough to replace calls to memset() with equivalent assembly instructions which are much more efficient then the memset() implementation. Implementing a memset_s() function as below may prevent the compiler from using the optimal assembly instructions and may result in less efficient code. Check compiler documentation and the assembly output from the compiler.

void *memset_s(void \*v, int c, size_t n) {
	volatile char *p = v;
	while(n--)
		*p++ = c;

	return v;
}

void getPassword() {
	char pwd\[64\];
	if(GetPassword(pwd, sizeof(pwd))) {
		/*checking of password, secure operations, etc \*/
	}
	pwd = memset_s(pwd, 0, sizeof(pwd));
}

Risk Assessment

References

https://buildsecurityin.us-cert.gov/daisy/bsi-rules/home/g1/771.html
http://msdn2.microsoft.com/en-us/library/aa366877.aspx
http://msdn2.microsoft.com/en-us/library/chh3fb0k(VS.80).aspx
David Wheeler
C99