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TUCoPS :: VMWare :: tb10747.htm

VMware Products - GPF Denial of Service

VMware Products - GPF Denial of Service
VMware Products - GPF Denial of Service

                  VMWARE PRODUCTS 	=09
Rub=E9n Santamarta  
Affected products:
All VMware products with a hypervisor are affected
 	+ VMware WorkStation 5.5.3 and earlier
 	+ VMware Player 	1.0.3 and earlier
 	+ VMware Server 	1.0.2 and earlier
 	+ VMware ACE 		1.0.2 and earlier


Vmware Virtualization products are affected by a design flaw which can
lead to a local DoS vulnerability within the Guest OS. The flaw is due
to improper handling of GPF  in Windows Guest Operating Systems (x86) .


x86 processors running Protected Mode generate a General Protection
exception (#GP) on any attempt to load an invalid segment selector in
certain segment registers like FS,DS,GS or ES.

Using documented Win32  API any user-mode thread can modify its  own
context in such a way that the Kernel will try to load a specially
crafted invalid segment selector, pointing to the LDT, in FS (pop fs)
just before returning to user-mode.

On a physical,meaning opposite to virtual, machine this attempt triggers
a General Protection Fault  which is handled internally by KiTrap0D.

KiTrap0D looks for this type of invalid instructions  and sanitizes the
wrong selector.

Let's see:
Windows XP SP2
   +0x000 DbgEbp           	: Uint4B
   +0x004 DbgEip           	: Uint4B
   +0x008 DbgArgMark       	: Uint4B
   +0x00c DbgArgPointer    	: Uint4B
   +0x010 TempSegCs        	: Uint4B
   +0x014 TempEsp          	: Uint4B
   +0x018 Dr0              	: Uint4B
   +0x01c Dr1              	: Uint4B
   +0x020 Dr2              	: Uint4B
   +0x024 Dr3              	: Uint4B
   +0x028 Dr6              	: Uint4B
   +0x02c Dr7             	 : Uint4B
   +0x030 SegGs            	: Uint4B    	=09
   +0x034 SegEs            	: Uint4B		=09
   +0x038 SegDs            	: Uint4B		=09
   +0x03c Edx              	: Uint4B
   +0x040 Ecx              	: Uint4B
   +0x044 Eax              	: Uint4B
   +0x048 PreviousPreviousMode : Uint4B
   +0x04c ExceptionList    : Ptr32 _EXCEPTION_REGISTRATION_RECORD
   +0x050 SegFs            	: Uint4B		=09
   +0x054 Edi              	: Uint4B
   +0x058 Esi              	: Uint4B
   +0x05c Ebx              	: Uint4B
   +0x060 Ebp              	: Uint4B
   +0x064 ErrCode          	: Uint4B
   +0x068 Eip              	: Uint4B
   +0x06c SegCs            	: Uint4B
   +0x070 EFlags           	: Uint4B
   +0x074 HardwareEsp      	: Uint4B
   +0x078 HardwareSegSs    : Uint4B
   +0x07c V86Es            	: Uint4B
   +0x080 V86Ds            	: Uint4B
   +0x084 V86Fs            	: Uint4B
   +0x088 V86Gs            	: Uint4B

.text:0040A116        mov     eax, [ebp+68h]  	; Eip faulting
.text:0040A119        mov     eax, [eax]		; opcode
.text:0040A11B        mov     edx, [ebp+60h]  	; TrapFrame
.text:0040A11E        add     edx, 38h	 	; TrapFrame.SegDs=09
.text:0040A121        cmp     al, 1Fh		; Opcode 1F = POP DS
.text:0040A123        jz      loc_40A224
.text:0040A129        add     edx, 0FFFFFFFCh	; TrapFrame.SegES
.text:0040A12C        cmp     al, 7		; Opcode 07 = POP ES
.text:0040A12E        jz      loc_40A224
.text:0040A134        add     edx, 1Ch		; TrapFrame.SegFS
.text:0040A137        cmp     ax, 0A10Fh	; Opcode 0FA1 = POP FS
.text:0040A13B        jz      loc_40A224
.text:0040A141        add     edx, 0FFFFFFE0h	; TrapFrame.SegGS
.text:0040A144        cmp     ax, 0A90Fh	; Opcode 0FA9 = POP GS
.text:0040A148        jz      loc_40A224
.text:0040A224 loc_40A224:
.text:0040A224        xor     eax, eax		; Sanitizing invalid selector
.text:0040A226        mov     [edx], eax  ;TrapFrame.Seg(D,E,F,G)S=0

However, Vmware assumes a non-null and initialized LDT. Hence, Vmware
prioritizes a #PF deprecating the GPF, so a wrong segment selector
pointing to the LDT is generating a Page Fault instead of the proper
GPF.  Please,note that  Windows allocates  LDT(s) from non-paged  memory.

The final result is that the VMM is delivering a PF instead of a GPF to
the Guest OS. When the Guest's Kernel receives that PF, it will bugcheck
due to a physical memory error.


Under certain circumstances this flaw cannot be reproduced. On the other
hand, analyzing  this behaviour a workaround comes to light.


This flaw can be mitigated initializing the LDTR register with the
KGDT_LDT selector. This task cannot be directly performed  from
user-mode  since =93lldt=94 is a privileged instruction. However,
elaborating the correct path using native API we can do it.

1.NtSetLdtEntries -> 2. PsSetLdtEntries -> 3.KiLoadLdtr
       Ring3/Ring0		Ring0		Ring0

Let's see:

Windows XP SP2

NtSetLdtEntries(ULONG Selector1,
			LDT_ENTRY LdtEntry1,
			ULONG Selector2,
			LDT_ENTRY LdtEntry2)

PAGE:00555E57 Selector1       = dword ptr  8
PAGE:00555E57 LdtEntry1       = LDT_ENTRY ptr  0Ch
PAGE:00555E57 Selector2       = dword ptr  14h
PAGE:00555E57 LdtEntry2       = LDT_ENTRY ptr  18h
PAGE:00555E57                 mov     edi, edi
PAGE:00555E59                 push    ebp
PAGE:00555E5A                 mov     ebp, esp
PAGE:00555E5C                 pop     ebp
PAGE:00555E5D                 jmp     _PsSetLdtEntries@24 ;
PAGE:00555E5D _NtSetLdtEntries@24 endp
PsSetLdtEntries(int,int,PVOID P,int,int,int)
.text:0047170E ; __stdcall Ke386SetLdtProcess(x, x, x)
.text:0047170E _Ke386SetLdtProcess@12 proc near        ; CODE XREF:
.text:0047170E                                      ;
PsSetLdtEntries(x,x,x,x,x,x)+14C#p ...
.text:0047170E var_8           = dword ptr -8

.text:00471780                 call    _KiLoadLdtr@0   ; KiLoadLdtr()

.text:00441892 ; __stdcall KiLoadLdtr()
.text:00441892 _KiLoadLdtr@0   proc near               ; CODE XREF:
.text:00441892                push    esi
.text:00441893                push    edi
.text:00441894                mov     eax, large fs:124h		;
.text:0044189A                 mov     eax, [eax+44h]		; CurrentProcess
.text:0044189D                 lea     esi, [eax+20h]		;
.text:004418A0                 xor     dx, dx
.text:004418A3                 cmp     word ptr [esi], 0
.text:004418A7                 jz      short loc_4418B9
.text:004418A9                 mov     edi, large fs:3Ch	; PCR.GDT
.text:004418B0                 add     edi, 48h	; GDT+KGDT_LDT
.text:004418B3                 movsd	; Copy descriptor
.text:004418B4                 movsd
.text:004418B5                 mov     dx, 48h			; KGDT_LDT
.text:004418B9 loc_4418B9:                             ; CODE XREF:
.text:004418B9                 lldt    dx				; Initializing ldtr
.text:004418BC                 pop     edi
.text:004418BD                 pop     esi
.text:004418BE                 call    _KiFlushDescriptors@0 ;
.text:004418C3                 retn
.text:004418C3 _KiLoadLdtr@0   endp

Disclosure Timeline
10.01.2007 =96 Initial vendor notification.
13.01.2007 =96 Vendor response. Vulnerability confirmed.
07.05.2007 =96 Coordinated Disclosure.

This update is specially recommended for  malware research labs or
Honeynets since  malware can easily exploit this flaw in order to avoid
virtualized enviroments.
No exploits are released. Ethical security companies can contact for
requesting samples : 

Special thanks to Mario Ballano for providing additional exploit code.


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