Andrey Karpov

Jan 17 2012

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#Cpp #Knowledge

Overwriting memory - why?

We decided to publish this article in our knowledge base to show programmers how easily private data can get out of the program handling them. There is the V597 diagnostic rule in PVS-Studio that allows you to detect those calls of the memset() function which fail to clear the memory. But the danger looks unconvincing and improbable. This article shows well that the danger is real and must not be ignored.

There is SecureZeroMemory() function in the depths of Win32 API. Its description is rather concise and reads that this function overwrites a memory region with zeroes and is designed in such way that the compiler never eliminates a call of this function during code optimization. The description further says this function should be used for overwriting memory that was previously used to store passwords and cryptokeys.

One question remains - why is that needed? One can find some abstract speculations about the risk of application's memory being written into swap file, hibernate file or crash dump where an intruder could find it. It looks like paranoia - definitely not every intruder can get access to these files.

There are much more possibilities to get access to data a program has forgotten to overwrite, actually - sometimes even access to the computer is not needed. Next we will consider an example, and everyone will decide for himself/herself whether this paranoia is reasonable.

All the examples are in pseudocode that suspiciously resembles C++. Below are lots of text and not very clean code, and later you will see that things are not much better in clean code.

So. In a far-away function we get a cryptokey, a password or a PIN (further called simply "the secret"), use it and do not overwrite it:

{
    const int secretLength = 1024;
    WCHAR secret[secretLength] = {};
    obtainSecret( secret, secretLength );
    processWithSecret( what, secret, secretLength );
}

In another function that is completely unrelated to the previous one, our application's instance asks another instance for a file with a specified name. This is done using RPC - a dinosaur-age technology present in many platforms and widely used by Windows for interprocess and intercomputer communication.

Usually you have to write an interface specification in IDL to use RPC. It will have a method specification similar to this:

//MAX_FILE_PATH == 1024
error_status_t rpcRetrieveFile(
    [in] const WCHAR fileName[MAX_FILE_PATH],
    [out] BYTE_PIPE filePipe );

The second parameter here has a special type that facilitates passing data streams of arbitrary lengths. The first parameter is a character array for the filename.

This specification is compiled by the MIDL compiler, and the latter produces a header file (.h) with this function

error_status_t rpcRetrieveFile (
  handle_t IDL_handle, 
  const WCHAR fileName[1024], 
  BYTE_PIPE filePipe);

MIDL has added a service parameter here, and the second and the third parameters are the same as in the previous specification.

We call that function:

void retrieveFile( handle_t binding )
{
  WCHAR remoteFileName[MAX_FILE_PATH];
  retrieveFileName( remoteFileName, MAX_FILE_PATH );
  CBytePipeImplementation pipe;
  rpcRetrieveFile( binding, remoteFileName, pipe );           
}

Everything is fine - retrieveFileName() gets a null-terminated (no, the terminating null character wasn't omitted) string, the called party receives the string and handles it, i.e. gets the full path to the file, opens it and passes data from it.

Everyone is optimistic, and several product releases are shipped with this code, but nobody has noticed the elephant yet. Here it is. From C++ point of view, the following function's parameter

const WCHAR fileName[1024]

is not an array, but a pointer to the first array element. The rpcRetrieveFile() function is just a thunk also generated by MIDL. It packages all its parameters and calls the same WinAPI NdrClientCall2() function each time which semantics is "Windows, could you please execute an RPC-call with theeese parameters?" and passes the parameters list to the NdrClientCall2() function. One of the first parameters being passed is the format string generated by MIDL according to the specification in IDL. Looks much like the good old printf().

NdrClientCall2() looks carefully at the received format string and packages the parameters for passing them to the other party (this is called marshalling). Each parameter is accompanied by a type specifier, so each parameter is packaged according to its type. In our case, the address of the first array element is passed for the fileName parameter and "an array of 1024 items of the WCHAR type" specifier is passed for its type.

Now we have two successive calls in code:

processWithSecret( whatever );
retrieveFile( binding );

The processWithSecret() function occupies 2 Kbytes on the stack to store the secret and forgets about them on return. The retrieveFile() function is then called, and it retrieves the filename which length is 18 characters (18 characters plus terminating null - 19 characters total, i.e. 38 bytes). The filename is again stored on the stack and most likely it will be the same memory region as the one used to store the secret in the first function.

Then a remote call occurs and the packing function dutifully packages the whole array (2048 bytes, not 38 bytes) into a packet, and then this packet is sent over the network.

QUITE SUDDENLY

the secret is passed over the network. The application did not even intend to ever pass the secret over the network, but the secret is passed. This defect is much more convenient to "use" than even looking into the swap file. Who is paranoid now?

The example above looks rather complicated. Here is similar code that you can try on codepad.org

const int bufferSize = 32;

void first()
{
  char buffer[bufferSize];
  memset( buffer, 'A', sizeof( buffer ) );
}

void second()
{
  char buffer[bufferSize];
  memset( buffer, 'B', bufferSize / 2 );
  printf( "%s", buffer );
}

int main()
{
  first();
  second();
}

The code yields undefined behavior. At the moment of writing this post, the results are the following: a string of 16 'B' characters followed by 16 'A' characters.

Now it's just the right time for brandishing pitchforks and torches and angry shouts that no sane person uses simple arrays and that we must use std::vector, std::string and the CanDoEverything class that handle memory "correctly", and for a holy war worth no fewer than 9 thousand comments.

All that wouldn't actually help in the above case because the packing function in the depths of RPC would still read more data than previously written by the calling code. As a result, it would read the data at the adjacent addresses or (in some cases) the application would crash on illegal memory access. Those adjacent addresses could again store data that must not be sent over the network.

Whose fault is it? As usual, it's the developer's fault - it is he/she who misunderstood how the rpcRetrieveFile() function handles received parameters. This results in undefined behavior which leads to uncontrolled transmission of data over the network. This can be fixed either by changing the RPC-interface and altering the code on the both sides, or by using an array of large enough size and fully overwriting it before copying a parameter into the array.

This is a situation where the SecureZeroMemory() function would help: should the first function overwrite the secret before returning, an error in the second function would at least cause transmission of an overwritten array. Getting a Darwin Award gets harder this way.

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