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影响所有Nexus手机的漏洞,浅析CVE-2015-1805

By @少仲

From 360 VulpeckerTeam

0x0 漏洞信息

影响所有Nexus手机和部分Android手机的漏洞,Google于2016/03/18发布了公告修复,具体请看链接.

http://www.cvedetails.com/cve-details.php?t=1&cve_id=cve-2015-1805X

http://source.android.com/security/advisory/2016-03-18.html

0x1 漏洞描述

在linux 内核3.16版本之前的fs/pipe.c当中,由于pipe_read和pipe_write没有考虑到拷贝过程中数据没有同步的一些临界情况,造成了拷贝越界的问题,因此有可能导致系统crash以及系统权限提升.这种漏洞又称之为” I/O vector array overrun”

0x2 代码分析

//摘自fs/pipe.c:

static ssize_t

pipe_read(struct kiocb *iocb, const struct iovec *_iov,

unsigned long nr_segs, loff_t pos)

{

struct file *filp = iocb->ki_filp;

struct pipe_inode_info *pipe = filp->private_data;

int do_wakeup;

ssize_t ret;

struct iovec *iov = (struct iovec *)_iov;

size_t total_len;

total_len = iov_length(iov, nr_segs);

/* Null read succeeds. */

if (unlikely(total_len == 0))

return 0;

do_wakeup = 0;

ret = 0;

__pipe_lock(pipe);

for (;;) {

int bufs = pipe->nrbufs;

if (bufs) {

int curbuf = pipe->curbuf;

struct pipe_buffer *buf = pipe->bufs + curbuf;

const struct pipe_buf_operations *ops = buf->ops;

void *addr;

size_t chars = buf->len;

int error, atomic;

if (chars > total_len)

chars = total_len;

error = ops->confirm(pipe, buf);

if (error) {

if (!ret)

ret = error;

break;

}

//(1)

atomic = !iov_fault_in_pages_write(iov, chars);

redo:

addr = ops->map(pipe, buf, atomic);

//(2)

error = pipe_iov_copy_to_user(iov, addr + buf->offset, chars, atomic);

ops->unmap(pipe, buf, addr);

if (unlikely(error)) {

* Just retry with the slow path if we failed.

//(3)

if (atomic) {

atomic = 0;

goto redo;

}

if (!ret)

ret = error;

break;

}

ret += chars;

buf->offset += chars;

buf->len -= chars;

/* Was it a packet buffer? Clean up and exit */

if (buf->flags & PIPE_BUF_FLAG_PACKET) {

total_len = chars;

buf->len = 0;

}

if (!buf->len) {

buf->ops = NULL;

ops->release(pipe, buf);

curbuf = (curbuf + 1) & (pipe->buffers - 1);

pipe->curbuf = curbuf;

pipe->nrbufs = --bufs;

do_wakeup = 1;

}

(5)//在这里更新total_len

total_len -= chars;

if (!total_len)

break; /* common path: read succeeded */

}

if (bufs) /* More to do? */

continue;

if (!pipe->writers)

break;

if (!pipe->waiting_writers) {

/* syscall merging: Usually we must not sleep

* if O_NONBLOCK is set, or if we got some data.

* But if a writer sleeps in kernel space, then

* we can wait for that data without violating POSIX.

if (ret)

break;

if (filp->f_flags & O_NONBLOCK) {

ret = -EAGAIN;

break;

}

if (signal_pending(current)) {

if (!ret)

ret = -ERESTARTSYS;

break;

}

if (do_wakeup) {

wake_up_interruptible_sync_poll(&pipe->wait, POLLOUT | POLLWRNORM);

kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);

}

pipe_wait(pipe);

}

__pipe_unlock(pipe);

/* Signal writers asynchronously that there is more room. */

if (do_wakeup) {

wake_up_interruptible_sync_poll(&pipe->wait, POLLOUT | POLLWRNORM);

kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);

}

if (ret > 0)

file_accessed(filp);

return ret;

}

(1).首先pipe_read()函数会先循环读取iovec结构,并且通过iov_fault_in_pages_write()函数判断iov->len是否大于0,且iov->base指向的地址是否可写且处于用户态,之后返回atomic.

(2)如果atomic=1,则pipe_iov_copy_to_user -> __copy_to_user_inatomic ->

__copy_to_user_nocheck;如果atomic=0,则pipe_iov_copy_to_user -> copy_to_user -> access_ok.

(3).如果atomic为1,pipe_iov_copy_to_user拷贝出现错误,会进入redo的逻辑,将再次调用pipe_iov_copy_to_user函数进行拷贝,且将atomic置为0.但是pipe_iov_copy_to_user的第三个参数chars并没有更新,还是会拷贝total_len大小的数据

static int

pipe_iov_copy_to_user(struct iovec *iov, const void *from, unsigned long len,

int atomic)

{

unsigned long copy;

while (len > 0)

{

while (!iov->iov_len)

iov++;

copy = min_t(unsigned long, len, iov->iov_len);

if (atomic)

{

if (__copy_to_user_inatomic(iov->iov_base, from, copy))

//(4)

return -EFAULT;

}

else

{

if (copy_to_user(iov->iov_base, from, copy))

//(4)

return -EFAULT;

}

from += copy;

len -= copy;

iov->iov_base += copy;

//每次对iov->iov_len进行更新

iov->iov_len -= copy;

}

return 0;

}

4. 如果copy到某种情况出错返回,已经copy成功的iov->len会被减去但总长度total_len并不会同步减去.也就是说如果total_len是0x100,第一次消耗掉了x;再次进入redo逻辑后还是0x100,然而实际已经被消耗掉了x.

0x3 具体探究

假设有一个iov结构,total_len为0x40,len为0x20.

iov[0]: iov_base = 0xdead0000 iov_len = 0x10

iov[1]: iov_base = 0xdead1000 iov_len = 0x10

iov[2]: iov_base = 0xdead2000 iov_len = 0x10

iov[3]: iov_base = 0xdead3000 iov_len = 0x10

如果iov[1].iov_base的地址被设置成不可写入.那么第一次pipe_iov_copy_to_user()会返回失败.而iov->iov_base += copy,iov->iov_len -= copy.

iov[0]: iov_base = 0xdead0010 iov_len = 0

iov[1]: iov_base = 0xdead1000 iov_len = 0x10

iov[2]: iov_base = 0xdead2000 iov_len = 0x10

iov[3]: iov_base = 0xdead3000 iov_len = 0x10

现在,redo的逻辑发生在0xdead0010,它以某种方式被设置成可写,并且len仍未0x20.那么iov[1]和iov[2]都将被用掉.

iov[0]: iov_base = 0xdead0010 iov_len = 0

iov[1]: iov_base = 0xdead1010 iov_len = 0

iov[2]: iov_base = 0xdead2010 iov_len = 0

iov[3]: iov_base = 0xdead3000 iov_len = 0x10

在注释(5)中,根据total_len -= chars;那么total_len的大小就被设置为0x20(0x40 -0x20).如果total_len变为了0x20,可我们iov[3]的大小只有0x10.这就会导致pipe_iov_copy_to_user()函数有可能读取到一个未知的iov[4].具体来查看下代码

static int iov_fault_in_pages_write(struct iovec *iov, unsigned long len)

{

//(6)

while (!iov->iov_len)

iov++;

while (len > 0) {

unsigned long this_len;

this_len = min_t(unsigned long, len, iov->iov_len);

if (fault_in_pages_writeable(iov->iov_base, this_len))

break;

len -= this_len;

iov++;

}

return len;

}

static inline int fault_in_pages_writeable(char __user *uaddr, int size)

{

int ret;

if (unlikely(size == 0))

return 0;

* Writing zeroes into userspace here is OK, because we know that if

* the zero gets there, we'll be overwriting it.

ret = __put_user(0, uaddr);

if (ret == 0) {

char __user *end = uaddr + size - 1;

* If the page was already mapped, this will get a cache miss

* for sure, so try to avoid doing it.

if (((unsigned long)uaddr & PAGE_MASK) !=

((unsigned long)end & PAGE_MASK))

ret = __put_user(0, end);

}

return ret;

}

在iov_fault_in_pages_write()函数中的注释(6),也就意味着iov[0],iov[1],iov[2]都会被跳过,iov[3]被用掉.之后len -= this_len;len被设置为0x10.iov的指针将指向一块未知的内存区域.iov[4].iov_base将被__put_user使用.

0x4 如何利用

核心的思路就是想办法触发redo的逻辑,之后精心构造一个readv()调用.把payload结构定义在已经被校验过的iov数组后,让它成为__put_user()等函数调用的目标地址.如果我们再以某种方式让构造的slab结构在iov数组后包含一个函数指针,让它指向要写的内核地址.

1.第一次循环要保证pipe_iov_copy_to_user()函数失败,这样会进入redo逻辑

2.第二次要保证pipe_iov_copy_to_user()成功,但是不能在这里overrun,否则会走向copy_to_user,要校验地址,所以还是无法写内核地址

3.当iov->len走完之后,total_len还有剩余,所以第三次循环的时候,atomic=1.可以overrun触发

4.第一次要保证失败,也就是说需要把iov_base的地址设置成不可写,第二次要成功,就要保证iov_base的地址有效.所以这里可以通过创建竞争关系的线程,调用mmap/munmap等函数来实现.

0x5 POC

影响所有Nexus手机的漏洞,浅析CVE-2015-1805