16:50 08 Apr 2012

OpenSSL as a Filter (or non-blocking OpenSSL)

OpenSSL as a library at first glance is complicated, and then you realise that a lot of the documentation seems to be incomplete or missing. Generally the individual man pages for the various functions are not bad, and they will give you relevant information to help you along, but it can be hard to find.

The idea is to write a filter in such a way that OpenSSL can easily be disabled or removed. We don't want to rely on OpenSSL specific functionality in our code, for instance in the future we may want to change to using Botan or another SSL/TLS library without having to change a lot of core functionality.

Possible methods

There are various ways of doing non-blocking OpenSSL, the main one is to simply set the underlying socket to non-blocking and pass it into OpenSSL, at that point the functions SSL_write() and SSL_read() will enter various error states that can be read by SSL_get_error(), more specifically the function will return either SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE.

The other method, the one used below, was described by Marc Lehmann in a post to the libev mailling list and uses memory BIO objects. Specifically he linked to some Perl sample code; the OpenSSL setup, and the function called for new incoming data/data to be written: dotls function.

The code is very new and might still be buggy, but it outlines the principles: use a memory stream, which will avoid all issues with blocking in openssl

It shows the idea pretty clearly but unless you know Perl it can be really confusing as to what is going on.

The SSL Filter implementation

The SSL_CTX that is passed in contains all of the initialisation that is done for OpenSSL in general, which is not shown here. The SSLFilter is created for each time you want to have a socket start doing SSL.

// openssl_filter.h

class SSLFilter {
    public:
        SSLFilter(SSL_CTX* ctxt,
                  std::string* nread,
                  std::string* nwrite,
                  std::string* aread,
                  std::string* awrite);
        virtual ~SSLFilter();

        void update();

    private:
        bool continue_ssl_(int function_return);

        SSL * ssl;
        BIO * rbio;
        BIO * wbio;

        std::string* nread;
        std::string* nwrite;
        std::string* aread;
        std::string* awrite;
};

The class contains mainly various different bits of state, when you create the filter you pass in a pointer to four different strings, they are used as follows:

nread: This contains data to be processed by the filter, from the network
nwrite: This contains data that has been processed by the filter, and has to be sent to the network
aread: This contains data that has been processed by the filter, and is ready to be processed by the application
awrite: This contains data that the application wants to send out to the network and is ready for processing by the filter.

The other private variables contain various different pieces of state and are created by the filters constructors.

ssl: Contains the state for the current SSL connection.
rbio: Contains the data that the SSL functions will read from, data put in here is copied from nread.
wbio: Contains the data that the SSL functions will write to, data from this BIO is copied to nwrite.

The implementation is the really interesting part.

// openssl_filter.cc

#include <stdexcept>

#include "openssl_filter.h"

SSLFilter::SSLFilter(SSL_CTX* ctxt, 
                     std::string* nread, 
                     std::string* nwrite,
                     std::string* aread,
                     std::string* awrite)
                      :
                     nread(nread), 
                     nwrite(nwrite), 
                     aread(aread), 
                     awrite(awrite)

    rbio = BIO_new(BIO_s_mem());
    wbio = BIO_new(BIO_s_mem());

    ssl = SSL_new(ctxt);

    SSL_set_accept_state(ssl);
    SSL_set_bio(ssl, rbio, wbio);
}

SSLFilter::~SSLFilter() {
    SSL_free(_ssl);
}

void SSLFilter::update(Filter::FilterDirection) {
    // If we have data from the network to process, put it the memory BIO for OpenSSL
    if (!nread->empty()) {
        int written = BIO_write(rbio, nread->c_str(), nread->length());
        if (written > 0) nread->erase(0, written);
    }

    // If the application wants to write data out to the network, process it with SSL_write
    if (!awrite->empty()) {
        int written = SSL_write(ssl, awrite->c_str(), awrite->length());

        if (!continue_ssl_()) {
            throw std::runtime_error("An SSL error occured.");
        }

        if (written > 0) awrite->erase(0, written);
    }

    // Read data for the application from the encrypted connection and place it in the string for the app to read
    while (1) {
        char *readto = new char[1024];
        int read = SSL_read(ssl, readto, 1024);

        if (!continue_ssl_()) {
            delete readto;
            throw std::runtime_error("An SSL error occured.");
        }

        if (read > 0) {
            size_t cur_size = aread->length();
            aread->resize(cur_size + read);
            std::copy(readto, readto + read, aread->begin() + cur_size);
        }

        delete readto;

        if (static_cast<size_t>(read) != 1024 || written == 0) break;
    }

    // Read any data to be written to the network from the memory BIO and copy it to nwrite
    while (1) {
        char *readto = new char[1024];
        int read = BIO_read(wbio, readto, 1024);

        if (read > 0) {
            size_t cur_size = nwrite->length();
            nwrite->resize(cur_size + read);
            std::copy(readto, readto + read, nwrite->begin() + cur_size);
        }

        delete readto;

        if (static_cast<size_t>(read) != 1024 || read == 0) break;
    }
}

bool SSLFilter::continue_ssl_(int function_return) {
    int err = SSL_get_error(ssl, function_return);

    if (err == SSL_ERROR_NONE || err == SSL_ERROR_WANT_READ) {
        return true;
    }

    if (err == SSL_ERROR_SYSCALL) {
        ERR_print_errors_fp(stderr);
        perror("syscall error: ");
        return false;
    }

    if (err == SSL_ERROR_SSL) {
        ERR_print_errors_fp(stderr);
        return false;
    }
    return true;
}

Explanation of the SSL Filter class

Line 7 - 25, we create the two memory BIO's and then call SSL_new() using the passed in SSL_CTX, set the accept set for SSL and then we hook up the two BIO objects to the ssl object.

Line 27 - 29, All we do in the destructor is SSL_free() the SSL state, which automatically takes care of freeing the two memory BIOs that were given to it.

Line 33 - 36, if nread is not empty we copy the data into rbio. rbio is used by the SSL functions to read from (as if it were reading from a socket). We read into the memory BIO as much as possible, technically this should be everything, but it is possible it won't read everything due to memory constraints, next time SSLFilter::update() is called it will get emptied as much as possible again.

Line 39 - 47, we see if there is anything that the app wants to write out to the remote client, if there is we call SSL_write(). After the call to SSL_write() we call continue_ssl_() to see if we can safely continue using SSL.

Line 50 - 68, this is where try to read as much data from SSL_read() as we can and place it in aread. This is data that has been decrypted by OpenSSL and can be used by the application. After the call to SSL_read() we call continue_ssl_() to see if we can safely continue using SSL.

Line 71 - 84, if OpenSSL has to write something to the network, either directly due to something the app did (by filling awrite) or because of something the remote client sent, it will place it in its memory buffer (wbio), what we do here is drain that memory buffer and fill nwrite.

Line 87 - 105, the continue_ssl_() function gets the SSL errors using SSL_get_error and checks that either it is SSL_ERROR_NONE or SSL_ERROR_WANT_READ both of which are acceptable errors, any other errors and we check to see if it is an error due to a system call or due to an SSL error, we print out the errors to standard error and return false. At that point the calling code is free to go about its business as it pleases (most likely throwing an error).

Notes regarding this sample code ...

The only thing missing in this example code is checking to see if the SSL connection has gone into an SSL_shutdown() mode. This should be added so that you cleanly shut down an SSL connection, but it is not absolutely required, especially since it seems to be shaky as to whether or not it will work, especially if you are using SSLv3 or SSLv2.

How to use the SSL Filter

Here are the steps to using this filter:

At program startup set up the SSL_CTX structure as required, so that it be used in the SSLFilter
Set up socket/event loop and accept() new socket.
Create four new std::string's and create a new SSLFilter, pass in the SSL_CTX you created, and the four buffers.
Add the new socket to the event loop to wait for reading.

Now upon receiving a read ready status from the event loop, the following should be done:

Read data into nread buffer.
call SSLFilter::update()
See if aread is not empty, process the data as you please
Write data to awrite as needed, if data is written to awrite, call SSLFilter::update(WRITE) to process the data
See if nwrite is not empty, if so add socket to event loop for write readiness.

Once you receive a write ready status from the event loop, you should do the following:

Write data in nwrite to the socket.
If nwrite is empty, remove the socket from the event loop for writing (so that you don't have the event loop notifying you of the ability to write, even-though you have nothing to write).

Now it becomes simple to add OpenSSL to any open socket, in a way that is easy to refactor later and or disable depending on parameters passed in as arguments to the program. If you set nread to the same string as aread and nwrite to awrite and don't create an SSLFilter you have bypassed the filter...

typedef int (*funcptr)();

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