bastion/src/session.c

#include <libssh/callbacks.h>
#include <libssh/server.h>
#include <poll.h>
#include <pty.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/wait.h>

#include "pty.h"
#include "session.h"

#define USER "julien"
#define PASS "graou"
#define BUF_SIZE 1048576
#define SESSION_END (SSH_CLOSED | SSH_CLOSED_ERROR)
#define SFTP_SERVER_PATH "/usr/lib/sftp-server"

// callback function for channel data and exceptions
static int data_function(ssh_session session, ssh_channel channel, void *data,
                         uint32_t len, int is_stderr, void *userdata) {
    struct channel_data_struct *cdata = (struct channel_data_struct *) userdata;

    (void) session;
    (void) channel;
    (void) is_stderr;

    if (len == 0 || cdata->pid < 1 || kill(cdata->pid, 0) < 0) {
        return 0;
    }

    return write(cdata->child_stdin, (char *) data, len);
}

static int exec_request(ssh_session session, ssh_channel channel,
                        const char *command, void *userdata) {
    struct channel_data_struct *cdata = (struct channel_data_struct *) userdata;


    (void) session;
    (void) channel;

    if(cdata->pid > 0) {
        return SSH_ERROR;
    }

    if (cdata->pty_master != -1 && cdata->pty_slave != -1) {
        return exec_pty("-c", command, cdata);
    }
    return exec_nopty(command, cdata);
}

static int shell_request(ssh_session session, ssh_channel channel,
                         void *userdata) {
    struct channel_data_struct *cdata = (struct channel_data_struct *) userdata;

    (void) session;
    (void) channel;

    if(cdata->pid > 0) {
        return SSH_ERROR;
    }

    if (cdata->pty_master != -1 && cdata->pty_slave != -1) {
        return exec_pty("-l", NULL, cdata);
    }
    /* Client requested a shell without a pty, let's pretend we allow that */
    return SSH_OK;
}

static int subsystem_request(ssh_session session, ssh_channel channel,
                             const char *subsystem, void *userdata) {
    /* subsystem requests behave simillarly to exec requests. */
    if (strcmp(subsystem, "sftp") == 0) {
        return exec_request(session, channel, SFTP_SERVER_PATH, userdata);
    }
    return SSH_ERROR;
}

static int auth_password(ssh_session session, const char *user,
                         const char *pass, void *userdata) {
    struct session_data_struct *sdata = (struct session_data_struct *) userdata;

    (void) session;

    if (strcmp(user, USER) == 0 && strcmp(pass, PASS) == 0) {
        sdata->authenticated = 1;
        return SSH_AUTH_SUCCESS;
    }

    sdata->auth_attempts++;
    return SSH_AUTH_DENIED;
}

static ssh_channel channel_open(ssh_session session, void *userdata) {
    struct session_data_struct *sdata = (struct session_data_struct *) userdata;

    sdata->channel = ssh_channel_new(session);
    return sdata->channel;
}

static int process_stdout(socket_t fd, int revents, void *userdata) {
    char buf[BUF_SIZE];
    int n = -1;
    ssh_channel channel = (ssh_channel) userdata;

    if (channel != NULL && (revents & POLLIN) != 0) {
        n = read(fd, buf, BUF_SIZE);
        if (n > 0) {
            ssh_channel_write(channel, buf, n);
            // TODO implement logging here
        }
    }

    return n;
}

static int process_stderr(socket_t fd, int revents, void *userdata) {
    char buf[BUF_SIZE];
    int n = -1;
    ssh_channel channel = (ssh_channel) userdata;

    if (channel != NULL && (revents & POLLIN) != 0) {
        n = read(fd, buf, BUF_SIZE);
        if (n > 0) {
            ssh_channel_write_stderr(channel, buf, n);
        }
    }

    return n;
}

void handle_session(ssh_event event, ssh_session session) {
    int n, rc;

    /* Structure for storing the pty size. */
    struct winsize wsize = {
        .ws_row = 0,
        .ws_col = 0,
        .ws_xpixel = 0,
        .ws_ypixel = 0
    };

    /* Our struct holding information about the channel. */
    struct channel_data_struct cdata = {
        .pid = 0,
        .pty_master = -1,
        .pty_slave = -1,
        .child_stdin = -1,
        .child_stdout = -1,
        .child_stderr = -1,
        .event = NULL,
        .winsize = &wsize
    };

    /* Our struct holding information about the session. */
    struct session_data_struct sdata = {
        .channel = NULL,
        .auth_attempts = 0,
        .authenticated = 0
    };

    struct ssh_channel_callbacks_struct channel_cb = {
        .userdata = &cdata,
        .channel_pty_request_function = pty_request,
        .channel_pty_window_change_function = pty_resize,
        .channel_shell_request_function = shell_request,
        .channel_exec_request_function = exec_request,
        .channel_data_function = data_function,
        .channel_subsystem_request_function = subsystem_request
    };

    struct ssh_server_callbacks_struct server_cb = {
        .userdata = &sdata,
        .auth_password_function = auth_password,
        .channel_open_request_session_function = channel_open,
    };

    ssh_callbacks_init(&server_cb);
    ssh_callbacks_init(&channel_cb);

    ssh_set_server_callbacks(session, &server_cb);

    if (ssh_handle_key_exchange(session) != SSH_OK) {
        fprintf(stderr, "%s\n", ssh_get_error(session));
        return;
    }

    ssh_set_auth_methods(session, SSH_AUTH_METHOD_PASSWORD);
    ssh_event_add_session(event, session);

    n = 0;
    while (sdata.authenticated == 0 || sdata.channel == NULL) {
        /* If the user has used up all attempts, or if he hasn't been able to
         * authenticate in 10 seconds (n * 100ms), disconnect. */
        if (sdata.auth_attempts >= 3 || n >= 100) {
            return;
        }

        if (ssh_event_dopoll(event, 100) == SSH_ERROR) {
            fprintf(stderr, "%s\n", ssh_get_error(session));
            return;
        }
        n++;
    }

    ssh_set_channel_callbacks(sdata.channel, &channel_cb);

    do {
        /* Poll the main event which takes care of the session, the channel and
         * even our child process's stdout/stderr (once it's started). */
        if (ssh_event_dopoll(event, -1) == SSH_ERROR) {
          ssh_channel_close(sdata.channel);
        }

        /* If child process's stdout/stderr has been registered with the event,
         * or the child process hasn't started yet, continue. */
        if (cdata.event != NULL || cdata.pid == 0) {
            continue;
        }
        /* Executed only once, once the child process starts. */
        cdata.event = event;
        /* If stdout valid, add stdout to be monitored by the poll event. */
        if (cdata.child_stdout != -1) {
            if (ssh_event_add_fd(event, cdata.child_stdout, POLLIN, process_stdout,
                                 sdata.channel) != SSH_OK) {
                fprintf(stderr, "Failed to register stdout to poll context\n");
                ssh_channel_close(sdata.channel);
            }
        }

        /* If stderr valid, add stderr to be monitored by the poll event. */
        if (cdata.child_stderr != -1){
            if (ssh_event_add_fd(event, cdata.child_stderr, POLLIN, process_stderr,
                                 sdata.channel) != SSH_OK) {
                fprintf(stderr, "Failed to register stderr to poll context\n");
                ssh_channel_close(sdata.channel);
            }
        }
    } while(ssh_channel_is_open(sdata.channel) &&
            (cdata.pid == 0 || waitpid(cdata.pid, &rc, WNOHANG) == 0));

    close(cdata.pty_master);
    close(cdata.child_stdin);
    close(cdata.child_stdout);
    close(cdata.child_stderr);

    /* Remove the descriptors from the polling context, since they are now
     * closed, they will always trigger during the poll calls. */
    ssh_event_remove_fd(event, cdata.child_stdout);
    ssh_event_remove_fd(event, cdata.child_stderr);

    /* If the child process exited. */
    if (kill(cdata.pid, 0) < 0 && WIFEXITED(rc)) {
        rc = WEXITSTATUS(rc);
        ssh_channel_request_send_exit_status(sdata.channel, rc);
    /* If client terminated the channel or the process did not exit nicely,
     * but only if something has been forked. */
    } else if (cdata.pid > 0) {
        kill(cdata.pid, SIGKILL);
    }

    ssh_channel_send_eof(sdata.channel);
    ssh_channel_close(sdata.channel);

    /* Wait up to 5 seconds for the client to terminate the session. */
    for (n = 0; n < 50 && (ssh_get_status(session) & SESSION_END) == 0; n++) {
        ssh_event_dopoll(event, 100);
    }
}