rsh.c File Reference

restricted shell to run bash as root More...

#include <cm.h>
#include <libssh/libssh.h>
#include <limits.h>
#include <stdbool.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/resource.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <syslog.h>
#include <termios.h>
#include <unistd.h>

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Functions
void	ABEND (int e, char const *)
	Abnormal termination - print an error message and exit.
int	main (int argc, char **argv)
	Main function for rsh.

Variables
bool	f_verbose = false

Detailed Description

restricted shell to run bash as root

Author

Bill Waller Copyright (c) 2025 MIT License billx.nosp@m.wall.nosp@m.er@gm.nosp@m.ail..nosp@m.com

Date

2026-02-09

Definition in file rsh.c.

Function Documentation

ABEND()

void ABEND	(	int	e,
		char const *	s )

Abnormal termination - print an error message and exit.

Parameters

e	Error code
s	Error message

Note

This function prints the provided error message along with the error code and its corresponding string representation, then exits the program with a failure status.

Definition at line 160 of file rsh.c.

                                 {
    fprintf(stderr, "%s: %d %s\n", s, e, strerror(e));
    exit(EXIT_FAILURE);
}

main()

int main	(	int	argc,
		char **	argv )

Main function for rsh.

Parameters

argc	Argument count
argv	Argument vector

Note

If executed as 'rsh', this program sets the user ID and group ID to 0 (root) and then executes the user's default shell (or /usr/bin/bash if SHELL is not set) with the provided arguments. If no arguments are given, it runs the shell in interactive mode.

To work properly, this program must be compiled and set with the setuid bit:

$ sudo -s
cc rsh.c -o rsh
sudo chown root:root rsh
sudo chmod 4755 rsh
exit

to verify proper operation:

$ rsh
$ whoami
root

Definition at line 67 of file rsh.c.

                                {
    char *cargv[30];
    char exec_cmd[MAXLEN] = "/usr/bin/bash";
    char rsh_user[MAXLEN];
    char *p;
    int c, a;
    int rc;
    int status;
    pid_t pid;
#ifdef RSH_SSH
    ssh_session _ssh_session = ssh_new();
    if (_ssh_session == NULL)
        exit(EXIT_FAILURE);
    // "$HOME"/.ssh/authorized_keys
    ssh_options_set(_ssh_session, SSH_OPTIONS_HOST, "localhost");
    if ((rc = ssh_connect(_ssh_session)) != SSH_OK) {
        fprintf(stderr, "Error: %s\n", ssh_get_error(_ssh_session));
        ssh_free(_ssh_session);
        exit(EXIT_FAILURE);
    }
    // Authenticate using public key
    if (ssh_userauth_publickey_auto(_ssh_session, NULL, NULL) !=
        SSH_AUTH_SUCCESS) {
        fprintf(stderr, "SSH Public key auth failed: %s\n",
                ssh_get_error(_ssh_session));
        syslog(LOG_ERR, "SSH Error: %s", ssh_get_error(_ssh_session));
        exit(EXIT_FAILURE);
    }
    ssh_disconnect(_ssh_session);
    ssh_free(_ssh_session);
#endif
#ifdef RSH_LOG
    char ttyname[MAXLEN];
    p = getenv("USER");
    strncpy(rsh_user, p ? p : "unknown", sizeof(rsh_user));
    openlog("rsh", LOG_PID | LOG_CONS, LOG_AUTH);
    if (ttyname_r(STDERR_FILENO, ttyname, sizeof(ttyname)) == 0)
        syslog(LOG_INFO, "rsh started by user '%s' on terminal '%s'", rsh_user,
               ttyname);
    closelog();
#endif
    if ((p = getenv("SHELL")))
        strncpy(exec_cmd, p, MAXLEN - 1);
    cargv[0] = strdup(exec_cmd);
    c = 1;
    a = 1;
    if (argc == 1)
        cargv[c++] = "-i";
    while (a < argc)
        cargv[c++] = strdup(argv[a++]);
    cargv[c] = (char *)'\0';
    pid = fork();
    switch (pid) {
    case -1:
        ABEND(EXIT_FAILURE, "fork() fatal error");
        break;
    case 0: // Child
        if (argv[0] && strstr(argv[0], "rsh")) {
            if (setuid(0) || setgid(0))
                ABEND(EXIT_FAILURE, "setuid(0) fatal error");
            struct rlimit rl;
            getrlimit(RLIMIT_FSIZE, &rl);
            rl.rlim_cur = RLIM_INFINITY;
            rl.rlim_max = RLIM_INFINITY;
            setrlimit(RLIMIT_FSIZE, &rl);
        }
        execvp(exec_cmd, cargv);
        ABEND(EXIT_FAILURE, "execvp() fatal error");
        break;
    default: // Parent
 
        waitpid(pid, &status, 0);
        if (f_verbose) {
            if (WIFEXITED(status)) {
                rc = WEXITSTATUS(status);
                if (rc != 0)
                    ABEND(rc, "Child process exited");
            } else {
                if (WIFSIGNALED(status)) {
                    rc = WTERMSIG(status);
                    ABEND(rc, "Child process terminated by signal");
                } else
                    ABEND(EXIT_FAILURE, "Child process terminated abnormally");
            }
        }
        break;
    }
    openlog("rsh", LOG_PID | LOG_CONS, LOG_AUTH);
    syslog(LOG_INFO, "rsh exited by user '%s'", rsh_user);
    closelog();
    exit(EXIT_SUCCESS);
}

References ABEND(), and f_verbose.

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Variable Documentation

f_verbose

bool f_verbose = false

Definition at line 32 of file rsh.c.

Referenced by main().