Turning normal socket used by CURL into SSL socket in C - c

Question

I'm dealing with code written by a 3rd party (which I don't want to radically change as I'll explain) which opens a tcp connection to talk json-rpc with a server. I want to add ssl to this as the server has an ssl port for the same comms. EDIT: I want to avoid using external proxies (like stunnel, which works).
Currently, the socket is opened by socket() and passed to curl via CURLOPT_OPENSOCKETFUNCTION and libcurl is told to stop after connecting (perform no data transfer) via CURLOPT_CONNECT_ONLY.
I tried enabling the SSL options in CURL starting with curl_easy_setopt(curl, CURLOPT_USE_SSL, CURLUSESSL_ALL); but they had no effect, i.e. the connection itself succeeded in that the function connect() returned true (CURL was happy) but the data is still being sent unencrypted.
My understanding is: the socket is created externally (and passed to CURL which is told to not handle data transfer) and then written to directly, not via CURL or OpenSSL, so nothing actually encrypts the transfer ... despite enabling SSL in CURL.
I also tried to point CURL to https:// by doing sprintf(sctx->curl_url, "https%s", strstr(url, "://")); and then I get:
SSL: couldn't create a context: error:140A90A1:lib(20):func(169):reason(161)
I've also tried using OpenSSL directly on the opened socket, like here https://stackoverflow.com/a/41321247/1676217, by initializing OpenSSL and setting its file descriptor to sctx->sock which is the same socket passed to CURL (i.e. calling SSL_set_fd(ssl, sctx->sock);) but SSL_Connect() fails.
I can't change the code entirely to remove CURL or to remove the external socket creation as both are used in other parts of the code, though I can attempt to adapt it.
Basically, I'd like to keep the function declarations for thread_recv_line() and thread_send_line() and make them use SSL.
If anyone proficient with OpenSSL, CURL and/or sockets could show me how to do it within this code snippet, I'd be very grateful.
#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/tcp.h>
#define socket_blocks() (errno == EAGAIN || errno == EWOULDBLOCK)
#define RBUFSIZE 2048
#define RECVSIZE (RBUFSIZE - 4)
struct ctx {
char *url;
CURL *curl;
char *curl_url;
char curl_err_str[CURL_ERROR_SIZE];
curl_socket_t sock;
size_t sockbuf_size;
char *sockbuf;
pthread_mutex_t sock_lock;
};
static curl_socket_t opensocket_grab_cb(void *clientp, curlsocktype purpose,
struct curl_sockaddr *addr)
{
curl_socket_t *sock = (curl_socket_t*) clientp;
*sock = socket(addr->family, addr->socktype, addr->protocol);
return *sock;
}
bool connect(struct ctx *sctx, const char *url)
{
CURL *curl;
int rc;
pthread_mutex_lock(&sctx->sock_lock);
if (sctx->curl)
curl_easy_cleanup(sctx->curl);
sctx->curl = curl_easy_init();
if (!sctx->curl) {
pthread_mutex_unlock(&sctx->sock_lock);
return false;
}
curl = sctx->curl;
if (!sctx->sockbuf) {
sctx->sockbuf = (char*) calloc(RBUFSIZE, 1);
sctx->sockbuf_size = RBUFSIZE;
}
sctx->sockbuf[0] = '\0';
pthread_mutex_unlock(&sctx->sock_lock);
if (url != sctx->url) {
free(sctx->url);
sctx->url = strdup(url);
}
free(sctx->curl_url);
sctx->curl_url = (char*) malloc(strlen(url));
sprintf(sctx->curl_url, "http%s", strstr(url, "://"));
curl_easy_setopt(curl, CURLOPT_VERBOSE, 1);
curl_easy_setopt(curl, CURLOPT_URL, sctx->curl_url);
curl_easy_setopt(curl, CURLOPT_FRESH_CONNECT, 1);
curl_easy_setopt(curl, CURLOPT_CONNECTTIMEOUT, 30);
curl_easy_setopt(curl, CURLOPT_ERRORBUFFER, sctx->curl_err_str);
curl_easy_setopt(curl, CURLOPT_NOSIGNAL, 1);
curl_easy_setopt(curl, CURLOPT_TCP_NODELAY, 1);
curl_easy_setopt(curl, CURLOPT_HTTPPROXYTUNNEL, 1);
curl_easy_setopt(curl, CURLOPT_SOCKOPTFUNCTION, sockopt_keepalive_cb);
curl_easy_setopt(curl, CURLOPT_OPENSOCKETFUNCTION, opensocket_grab_cb);
curl_easy_setopt(curl, CURLOPT_OPENSOCKETDATA, &sctx->sock);
curl_easy_setopt(curl, CURLOPT_CONNECT_ONLY, 1);
rc = curl_easy_perform(curl);
if (rc) {
curl_easy_cleanup(curl);
sctx->curl = NULL;
return false;
}
return true;
}
static bool send_line(curl_socket_t sock, char *s)
{
size_t sent = 0;
int len = (int) strlen(s);
s[len++] = '\n';
while (len > 0) {
struct timeval timeout = {0, 0};
int n;
fd_set wd;
FD_ZERO(&wd);
FD_SET(sock, &wd);
if (select((int) (sock + 1), NULL, &wd, NULL, &timeout) < 1)
return false;
n = send(sock, s + sent, len, 0);
if (n < 0) {
if (!socket_blocks())
return false;
n = 0;
}
sent += n;
len -= n;
}
return true;
}
bool thread_send_line(struct ctx *sctx, char *s)
{
bool ret = false;
pthread_mutex_lock(&sctx->sock_lock);
ret = send_line(sctx->sock, s);
pthread_mutex_unlock(&sctx->sock_lock);
return ret;
}
static bool socket_full(curl_socket_t sock, int timeout)
{
struct timeval tv;
fd_set rd;
FD_ZERO(&rd);
FD_SET(sock, &rd);
tv.tv_sec = timeout;
tv.tv_usec = 0;
if (select((int)(sock + 1), &rd, NULL, NULL, &tv) > 0)
return true;
return false;
}
static void buffer_append(struct ctx *sctx, const char *s)
{
size_t old, n;
old = strlen(sctx->sockbuf);
n = old + strlen(s) + 1;
if (n >= sctx->sockbuf_size) {
sctx->sockbuf_size = n + (RBUFSIZE - (n % RBUFSIZE));
sctx->sockbuf = (char*) realloc(sctx->sockbuf, sctx->sockbuf_size);
}
strcpy(sctx->sockbuf + old, s);
}
char *thread_recv_line(struct ctx *sctx)
{
ssize_t len, buflen;
char *tok, *sret = NULL;
if (!strstr(sctx->sockbuf, "\n")) {
bool ret = true;
time_t rstart;
time(&rstart);
if (!socket_full(sctx->sock, 60))
return sret;
do {
char s[RBUFSIZE];
ssize_t n;
memset(s, 0, RBUFSIZE);
n = recv(sctx->sock, s, RECVSIZE, 0);
if (!n) {
ret = false;
break;
}
if (n < 0) {
if (!socket_blocks() || !socket_full(sctx->sock, 1)) {
ret = false;
break;
}
} else
buffer_append(sctx, s);
} while (time(NULL) - rstart < 60 && !strstr(sctx->sockbuf, "\n"));
if (!ret)
return sret;
}
buflen = (ssize_t) strlen(sctx->sockbuf);
tok = strtok(sctx->sockbuf, "\n");
if (!tok)
return sret;
sret = strdup(tok);
len = (ssize_t) strlen(sret);
if (buflen > len + 1)
memmove(sctx->sockbuf, sctx->sockbuf + len + 1, buflen - len + 1);
else
sctx->sockbuf[0] = '\0';
}

Answer 1

This is not what you asked for, but you should consider placing a separate TCP-to-TLS proxy (such as HAProxy) between your code and remote server.
You will have to neither mess with TLS protocol details, nor modify existing code. Such setup will likely be safer and faster than anything you could write yourself.

Answer 2

Libcurl establishes an SSL connection with the server, even with CURLOPT_CONNECT_ONLY. It is not possible to establish another SSL connection on top of the same TCP connection. The only way to continue is to use the same SSL connection that libcurl has already established.
Fortunately it is possible to retrieve the SSL connection data from the curl handle using curl_easy_getinfo and CURLINFO_TLS_SSL_PTR. It should return a pointer of type SSL*.
The thread_send_line and thread_recv_line functions should not use the socket directly, but call SSL_read and SSL_write on the retrieved SSL pointer instead.
See CURLINFO_TLS_SSL_PTR for more information. Make sure you are getting an SSL pointer and not an SSL_CTX pointer.
Something like this should work:
bool thread_send_line(struct ctx *sctx, char *s)
{
bool ret = false;
struct curl_tlssessioninfo* session;
curl_easy_getinfo (sctx->curl, CURLINFO_TLS_SSL_PTR, &session);
SSL* ssl = session->internals;
pthread_mutex_lock(&sctx->sock_lock);
int len = (int) strlen(s);
s[len++] = '\n';
ret = SSL_write(ssl, s, len);
pthread_mutex_unlock(&sctx->sock_lock);
return ret == len;
}