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Bugtraq: Hot fix for do_brk bug

From: canon_at_nersc.gov
Date: Dec 04 2003


I've written a linux kernel module that can be used to hot fix a
Linux system for the bug in do_brk. It scans the
kernel space and replaces jmp and calls to do_brk
to point to a wrapper routine instead. It also maps
the symbol table to point to the wrapper. This only
works on x86 and it has only been tested with RH kernels
2.4.18-27.7.xsmp and 2.4.20-20.7smp. It is quite possible
this could crash or screw-up a system, so use at your own
risk. I've tested the module against the proof of concept code
written and posted by Christophe Devine. The module catches
the exploit and logs the attempt.

--Shane Canon

#define MODULE
#define __KERNEL__

/* some constants used in our module */
#define MODULE_NAME "brk_fix"
#define MODULE_VERSION "0.01"

/* brk_fix
 * Verions: 0.01
 * This module intentionally modifies kernel memory.
 * This module could potentially crash or damage your system.
 * Use at your own risks.
 * Hot fix for integer overflow in do_brk
 * This module scans kernel code space looking for jumps and calls
 * to do_brk. It then changes these calls to instead call my_brk,
 * which is a wrapper routine that does some checks before calling do_brk.
 * The module also scans for the address of do_brk and replaces it
 * with my_brk. This would mainly occur in the exported symbols table.
 * This has been tested with RH 2.4.20-20.7smp and RH 2.4.18-27.7.xsmp on i686
 * Sample use...
 * gcc -O3 -I/lib/modules/`uname -r`/build/include -c -o brk_fix.o brk_fix.c
 * insmod ./brk_fix.o
 * Written by Shane Canon <canon_at_nersc.gov>
 * (c) Copyright 2003 Regents of the University of California
 * All rights reserved.
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * GNU General Public License for more details.
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA

#include <linux/kernel.h>
#include <linux/errno.h>
#include <linux/version.h>
#include <linux/module.h>
#include <linux/slab.h>

extern void * __start___kallsyms;


unsigned long my_brk(unsigned long addr, unsigned long len);

int init_module(void)
  unsigned char *ptr;
  unsigned char *newptr;
  long *lptr;
  unsigned char *cptr;
  long diff;
  int count=0;
  unsigned char *start=(unsigned char *) (TASK_SIZE); /* Start at the
beginning of kernel space */
  unsigned char *end=(unsigned char *)&__start___kallsyms;
                                             /* Want something that is well
past the kernel test */
                                             /* This seems to fit the bill

  ptr=(unsigned char *)(do_brk); /* We are looking for
calls/jumps to this function */
  newptr=(unsigned char *)(my_brk); /* This is what we will change
it to */
  for (cptr=start;cptr<end;cptr++){ /* Lets scan all of kernel space
    if (*cptr==0xe8||*cptr==0xe9){ /* Look for calls or jumps
      cptr++; /* If you find one look at the
next dword */
      lptr=(long *)cptr;
      if ((cptr+*lptr)==(ptr)){ /* See if the offset would point
to do_brk */
        printk("fixing 0x%08lx\n",lptr); /* If so, change it to our new
routine */
      lptr=(long *)cptr; /* Look for the address as well.
 This would show */
      if ((unsigned char*)(*lptr)==ptr){ /* up in the symbol table.
        printk("Fixing address at 0x%08lx\n",lptr);
  printk ("Fix brk installed..\n"); /* All done. */
  MOD_INC_USE_COUNT; /* We can't unload this one. So
lets inc the mod */
                                              /* count and leave it there. */
  return 0; /* success */


void cleanup_module(void)
  /* Can't remove it */

/* These are the first couple of lines from the patched mmap.c */
/* Do the new checks and then call the original do_brk */

unsigned long my_brk(unsigned long addr, unsigned long len)
        len = PAGE_ALIGN(len);
        if (!len)
                return addr;

        if ((addr + len) > TASK_SIZE || (addr + len) < addr){ /* Let's make
sure its in bounds */
                printk("caught do_brk exploit!!!\n");
                return -EINVAL;
        return do_brk(addr,len); /* Call the real
do_brk */


Shane Canon
National Energy Research Scientific
  Computing Center

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