/*
 * Verify established TLS connections.
 *
 * Copyright (C) 2011  Simon Ruderich
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#include "tlsproxy.h"
#include "verify.h"

/* errno */
#include <errno.h>
/* gnutls_x509_*() */
#include <gnutls/x509.h>


int verify_tls_connection(gnutls_session_t session, const char *hostname) {
    int result;
    char path[1024];

    size_t size;
    unsigned int status;
    gnutls_x509_crt_t cert;
    const gnutls_datum_t *cert_list;
    unsigned int cert_list_size;
    FILE *file;
    char buffer[66]; /* one line in a PEM file is 64 bytes + '\n' + '\0' */
    char server_cert[8192];
    char stored_cert[8192];

    result = gnutls_certificate_verify_peers2(session, &status);
    /* Verification failed (!= invalid certificate but worse), no need for any
     * more checks. */
    if (0 > result) {
        LOG(LOG_WARNING,
            "verify_tls_connection(): gnutls_certificate_verify_peers2() failed: %s",
            gnutls_strerror(result));
        return -1;
    }
    /* Definitely an invalid certificate, abort. */
    if (status & GNUTLS_CERT_EXPIRED
            || status & GNUTLS_CERT_REVOKED
            || status & GNUTLS_CERT_NOT_ACTIVATED
            || status & GNUTLS_CERT_INSECURE_ALGORITHM) {
        LOG(LOG_WARNING,
            "verify_tls_connection(): invalid server certificate");
        return -1;
    }

    /* We only handle X509 certificates for now. Let validation fail to
     * prevent an attacker from changing the certificate type to prevent
     * detection. */
    if (GNUTLS_CRT_X509 != gnutls_certificate_type_get(session)) {
        LOG(LOG_WARNING,
            "verify_tls_connection(): no X509 server certificate");
        return -1;
    }

    /* Get server certificate. */

    if (0 > (result = gnutls_x509_crt_init(&cert))) {
        LOG(LOG_WARNING,
            "verify_tls_connection(): gnutls_x509_crt_init() failed: %s",
            gnutls_strerror(result));
        return -1;
    }

    cert_list = gnutls_certificate_get_peers(session, &cert_list_size);
    if (NULL == cert_list) {
        LOG(LOG_WARNING,
            "verify_tls_connection(): gnutls_certificate_get_peers() failed");
        gnutls_x509_crt_deinit(cert);
        return -1;
    }

    if (0 > (result = gnutls_x509_crt_import(cert, &cert_list[0],
                                             GNUTLS_X509_FMT_DER))) {
        LOG(LOG_WARNING,
            "verify_tls_connection(): gnutls_x509_crt_import() failed: %s",
            gnutls_strerror(result));
        gnutls_x509_crt_deinit(cert);
        return -1;
    }

    /* Export the certificate as PEM to compare it with the known certificate
     * stored on disk. */
    size = sizeof(server_cert);
    result = gnutls_x509_crt_export(cert, GNUTLS_X509_FMT_PEM,
                                    server_cert, &size);
    if (GNUTLS_E_SUCCESS != result) {
        LOG(LOG_WARNING,
            "verify_tls_connection(): gnutls_x509_crt_export() failed: %s",
            gnutls_strerror(result));
        gnutls_x509_crt_deinit(cert);
        return -1;
    }
    /* We got the certificate as PEM, free it. */
    gnutls_x509_crt_deinit(cert);

    /* Open stored server certificate file. */
    if (0 != server_certificate_path(&file, hostname, path, sizeof(path))) {
        return -1;
    }

    size = 1; /* '\0' */
    stored_cert[0] = '\0'; /* for strcat() */

    while (NULL != fgets(buffer, sizeof(buffer), file)) {
        size += strlen(buffer);
        /* Make sure the buffer is big enough. */
        if (sizeof(stored_cert) <= size) {
            LOG(LOG_WARNING, "verify_tls_connection(): '%s' too big",
                path);
            fclose(file);
            return -1;
        }

        strcat(stored_cert, buffer);
    }
    if (ferror(file)) {
        fclose(file);
        LOG(LOG_WARNING,
            "verify_tls_connection(): failed to read from '%s': %s",
            path, strerror(errno));
        return -1;
    }
    fclose(file);

    /* Check if the server certificate matches our stored certificate. */
    if (0 != strcmp(stored_cert, server_cert)) {
        LOG(LOG_ERROR,
            "verify_tls_connection(): server certificate changed!",
            path, strerror(errno));
        return -2;
    }

    return 0;
}

int server_certificate_path(FILE **file, const char *hostname, char *path, size_t size) {
    /* Hostname too long. */
    if (size - strlen(STORED_SERVER_CERT_FORMAT) <= strlen(hostname)) {
        LOG(LOG_WARNING,
            "server_certificate_path(): hostname too long: '%s'",
            hostname);
        return -1;
    }
    /* Try to prevent path traversals in hostnames. */
    if (NULL != strstr(hostname, "..")) {
        LOG(LOG_WARNING,
            "server_certificate_path(): possible path traversal: '%s'",
            hostname);
        return -1;
    }
    snprintf(path, size, STORED_SERVER_CERT_FORMAT, hostname);

    /* Open the stored certificate file. */
    *file = fopen(path, "rb");
    if (NULL == *file) {
        LOG(global_passthrough_unknown ? LOG_DEBUG : LOG_WARNING,
            "server_certificate_path(): failed to open '%s': %s",
            path, strerror(errno));
        /* Couldn't open the file, special case. */
        return -2;
    }

    return 0;
}