Likes: 
Greetings,
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
------brk_fix.c------------
#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
*
* WARNING WARNING WARNING
* 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@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
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* 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;
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,4,9)
#ifdef MODULE_LICENSE
MODULE_LICENSE("GPL");
#endif
#endif
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;
cptr+=4;
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=(newptr-cptr);
count++;
}
}
else{
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);
*lptr=(long)(newptr);
}
}
}
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 */
}
-------end---------
------------------------------------------------------------------------
Shane Canon
National Energy Research Scientific
Computing Center
------------------------------------------------------------------------
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