这是两个多月前的一篇笔记，一直没有贴出来。而现在霓虹小兄弟已经把逆向出的代码扔出来很久了。但是写这篇笔记的时候除了towelroot V1以外啥也没有，所以~~当时其实我很快就掌握了利用的细节，主要是一开始我就没想逆向towelroot，直接靠trace定位了利用方法。

转载请注明 http://retme.net/index.php/2014/09/19/cve-2014-3153.html

膜拜geohot~以下是当时的笔记：

一，首先看补丁

https://github.com/torvalds/linux/commit/e9c243a5a6de0be8e584c604d353412584b592f8

    if (requeue_pi) {
       /*
 +      * Requeue PI only works on two distinct uaddrs. This
 +      * check is only valid for private futexes. See below.
 +      */
 +     if (uaddr1 == uaddr2)
 +         return -EINVAL;
 +
 +     /*

补丁要求两个 futex地址不能相同。如果相同会发生什么呢？

二，相关数据结构

实际上每个 futex进入内核中会计算一个 key（ get_futex_key）并且被插入哈希表futext_queues， futext_queues的结构如下：

static struct futex_hash_bucket futex_queues[1<<FUTEX_HASHBITS];

static struct futex_hash_bucket *hash_futex(union futex_key *key)
{
    u32 hash = jhash2((u32*)&key->both.word,
             (sizeof(key->both.word)+sizeof(key->both.ptr))/4,
             key->both.offset);
    return &futex_queues[hash & ((1 << FUTEX_HASHBITS)-1)];
}

futex_hash_bucket是哈希表中的一个节点，结构如下

struct futex_hash_bucket {
    spinlock_t lock;
    struct plist_head chain;
};

其内部也是一个自旋锁，和一个队列。 chain 是一个优先级队列，等待线程的优先级越高，该线程在队列中越靠前。



plist_head链表中的成员是futex_q，代表了一个 futex的内核对象

/**
 * struct futex_q - The hashed futex queue entry, one per waiting task
 * @list:     priority-sorted list of tasks waiting on this futex
 * @task:     the task waiting on the futex
 * @lock_ptr:     the hash bucket lock
 * @key:      the key the futex is hashed on
 * @pi_state:     optional priority inheritance state
 * @rt_waiter:       rt_waiter storage for use with requeue_pi
 * @requeue_pi_key:  the requeue_pi target futex key
 * @bitset:       bitset for the optional bitmasked wakeup
 *
 * We use this hashed waitqueue, instead of a normal wait_queue_t, so
 * we can wake only the relevant ones (hashed queues may be shared).
 *
 * A futex_q has a woken state, just like tasks have TASK_RUNNING.
 * It is considered woken when plist_node_empty(&q->list) || q->lock_ptr == 0.
 * The order of wakeup is always to make the first condition true, then
 * the second.
 *
 * PI futexes are typically woken before they are removed from the hash list via
 * the rt_mutex code. See unqueue_me_pi().
 */
struct futex_q {
    struct plist_node list;

    struct task_struct *task;
    spinlock_t *lock_ptr;
    union futex_key key;
    struct futex_pi_state *pi_state;
    struct rt_mutex_waiter *rt_waiter;
    union futex_key *requeue_pi_key;
    u32 bitset;
};

看到了里面与 PI有关的东西，现在还不明白 ,一会儿通过几个函数了解一下



现在只要知道 futex 有 PI 和 non-PI之分， PI futex的 futex_q结构会有额外的几个成员， futex-> pi_state->pi_mutex会是一个rt_mutex ，而 rt_mutex_waiter是等待他的一个结构，通常分配在等待线程的栈上

三 函数执行流程
1. futex_lock_pi

实际上会将一个栈上的 rt_mutex_waiter插入到链表futex_q.pi_state->pi_mutex 中，这是一个rt_mutex的结构

调用流程： futex_lock_pi->rt_mutex_timed_lock-> rt_mutex_timed_fastlock->rt_mutex_slowlock->task_blocks_on_rt_mutex

debug_rt_mutex_init_waiter(&waiter);  rt_waiter 是rt_mutex_slowlock 在栈上的临时分配的结构

随后futex_lock_pi->rt_mutex_timed_lock-> rt_mutex_timed_fastlock->rt_mutex_slowlock->__rt_mutex_slowlock

将进入无限等待，除非被唤醒

static int __sched
__rt_mutex_slowlock(struct rt_mutex *lock, int state,
           struct hrtimer_sleeper *timeout,
           struct rt_mutex_waiter *waiter)
{
    int ret = 0;

    for (;;) {
       /* Try to acquire the lock: */
       if (try_to_take_rt_mutex(lock, current, waiter))
           break;

       /*
        * TASK_INTERRUPTIBLE checks for signals and
        * timeout. Ignored otherwise.
        */
       if (unlikely(state == TASK_INTERRUPTIBLE)) {
           /* Signal pending? */
           if (signal_pending(current))
              ret = -EINTR;
           if (timeout && !timeout->task)
              ret = -ETIMEDOUT;
           if (ret)
              break;
       }

2.futex_wait_requeue_pi

    /*
     * The waiter is allocated on our stack, manipulated by the requeue
     * code while we sleep on uaddr.
     */
    debug_rt_mutex_init_waiter(&rt_waiter);// 临时分配一个rt_waiter，与 futex_lock_pi类似
    rt_waiter.task = NULL;

    ret = get_futex_key(uaddr2, flags & FLAGS_SHARED, &key2, VERIFY_WRITE);
    if (unlikely(ret != 0))
       goto out;

    q.bitset = bitset;
    q.rt_waiter = &rt_waiter;   //for use with requeue_pi
    q.requeue_pi_key = &key2;  //requeue pi target key

    if(is_my_process){
       printk("[%d] futex_wait_requeue_pi:Prepare to wait on uaddr.\n",
           task_pid_vnr(current_task));

       futex_dump_futex_q(&q);
    }
    /*
     * Prepare to wait on uaddr. On success, increments q.key (key1) ref
     * count.
     *//等待从addr1 被唤醒
    ret = futex_wait_setup(uaddr, val, flags, &q, &hb);
    if (ret)
       goto out_key2;

    if(is_my_process){
       printk("[%d] futex_wait_requeue_pi:before Queue the futex_q.\n",
           task_pid_vnr(current_task));

       futex_dump_futex_q(&q);
    }

    /* Queue the futex_q, drop the hb lock, wait for wakeup. */
    futex_wait_queue_me(hb, &q, to);   //将本线程插入futex2的队列中，这里是将 rt_waiter插入去等待

3  futex_requeue_pi（futex1 ，futex2 ）会将futex1上面的 waiter唤醒并插入 futex2

如果这两个值相等，那么唤醒 futex1上的 waiter会使得 futex_wait_queue_me线程被唤醒，但是这个值又会被插入到 futex2中



由于futex_wait_requeue_pi的线程被唤醒并退出，那么 futex2的 rt_mutex队列上面便挂了一个已经被释放掉的 rt_mutex_waiter，这就是内核栈空间的use after free

四。如何利用？

futex_wait_requeue_pi所在的线程内核栈出现的 UAF问题，该线程利用 sendmmsg可以对内核堆栈进行控制

我们选择控制 rt_mutex_waiter结构中，这个结构有两个链表， UAF之后链表将被我们控制

struct rt_mutex_waiter {
    struct plist_node list_entry;
    struct plist_node pi_list_entry;
    struct task_struct   *task;
    struct rt_mutex      *lock;
}

于是我们调用 futex_lock_pi会走到task_blocks_on_rt_mutex 触发一个plist_add操作，造成内核栈信息泄漏，并且给了我们一次机会进行任意地址写



我们选择写内核栈上的 thread_info->addr_limit，一个栈上面的地址将会被写入到 addr_limit，导致我们有了从用户态写内核态的方法



这相当于造出了 CVE-2013-6282，读写任意地址



注意：该方法不能退出进程，否则释放被利用的线程将让内核崩溃