user_unix_auth.c

#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <pwd.h>
#include <shadow.h>
#include <crypt.h>
#include <unistd.h>
#include <stdlib.h>
#include <termios.h>

struct passwd *auth_sys_user(const char*username, const char *password);
int echo_set(int fd, int flag);

int main(int argc, char *argv[])
{
    char user[80], pass[80];
    struct passwd *pw;
    
    if (setuid(0) == -1) {
        printf("Can't get root access\n"
               "Probably you need to set SUID:\n"
               "    # chown root:root %s\n"
               "    # chmod u+s %s\n",
               argv[0], argv[0]);
        exit(-1);
    }
    
    while (1) {
        printf("User: ");
        gets(user);
        
        printf("Password: ");
        echo_set(STDIN_FILENO, 0);
        gets(pass);
        echo_set(STDIN_FILENO, 1);
        putchar('\n');
        
        pw = auth_sys_user(user, pass);
        memset(pass, 0, 80);
        
        if (pw) {
            printf("Auth OK: uid %d\n", pw->pw_uid);
            break;
        } else {
            printf("Auth FAIL\n");
            sleep(2);
            putchar('\n');
        }
    }

    setuid(pw->pw_uid);
    
    char *bash_argv[] = { "/bin/bash", NULL };
    execvp("/bin/bash", bash_argv);
    
    return 0;
}

struct passwd *auth_sys_user(const char *username,
                                const char *password) {
    struct passwd *pw;
    struct spwd *sp;
    char *encrypted, *correct;
    
    /* Cheack user existance */
    pw = getpwnam(username);
    endpwent();

    if (!pw)
        return NULL;

    /* Get shadow entry */
    sp = getspnam(pw->pw_name);
    endspent();

    /* Encrypt password */
    correct = sp ? sp->sp_pwdp : pw->pw_passwd;
    encrypted = crypt(password, correct);
    
    /* Compare with correct password */
    return strcmp(encrypted, correct) ? NULL : pw;
}

int echo_set(int fd, int flag)
{
    struct termios ts;
    
    /* Get terminal attributes */
    if (tcgetattr(fd, &ts) != 0)
        return -1;

    /* Set/unset echo flag */
    if (flag)
        ts.c_lflag |=  ECHO;
    else
        ts.c_lflag &= ~ECHO;
    
    /* Set modified attributes */
    if (tcsetattr(fd, TCSAFLUSH, &ts) != 0)
        return -1;
    
    return 0;
}