X86-assembly/Instructions/sldt

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sldt

Description

  • The sgdt and sldt instruction technique for VMware detection is commonly known as No Pill.
  • On VMware, the LDT location on the host will be non zero.
     ┌──────────┬──────────┬──────────┬──────────┐
  ┌─ │   0xDD   │   0xCC   │   0xBB   │   0xAA   │ (hex)
  │  │ 11011101 │ 11001100 │ 10111011 │ 10101010 │ (bin)
SLDT ├──────────┼──────────┼──────────┼──────────┤
  │  │   0xDD   │   0xCC   │   0x000x00   │ (hex)
  └► │ 11011101 │ 11001100 │ 0000000000000000 │ (bin)
     └──────────┴──────────┴──────────┴──────────┘
                           ¦   low-order bytes   ¦
                           ¦ = 0 on native host  ¦
                           ¦ != 0 on VMware      ¦

SLDT test

C source

// ...SNIP...

unsigned long
get_ldtr_base (void)
{
	unsigned char   ldtr[5] = "\xef\xbe\xad\xde";
	unsigned long   ldt			= 0;
 
	_asm sldt ldtr
	ldt = *((unsigned long *)&ldtr[0]);
 
	return (ldt);
}

// ...SNIP...

void
test2 (void)
{
	unsigned int	ldt_base	= 0;
 
	ldt_base = get_ldtr_base ();
 
	printf ("\n[+] Test 2: LDT\n");
	printf ("LDT base: 0x%x\n", ldt_base);
 
	if (ldt_base == 0xdead0000) {
		printf ("Result  : Native OS\n\n");
		return;
	}
 
	else {
		printf ("Result  : VMware detected\n\n");
		return;
	}
}

Assembly

.text:00401040 get_ldtr_base   proc near
.text:00401040
.text:00401040 var_10          = dword ptr -10h
.text:00401040 var_C           = byte ptr -0Ch
.text:00401040 var_8           = dword ptr -8
.text:00401040 var_4           = dword ptr -4
.text:00401040
.text:00401040                 push    ebp
.text:00401041                 mov     ebp, esp
.text:00401043                 sub     esp, 10h
.text:00401046                 mov     eax, ___security_cookie
.text:0040104B                 xor     eax, ebp
.text:0040104D                 mov     [ebp+var_8], eax
.text:00401050                 mov     eax, dword_40C000
.text:00401055                 mov     [ebp+var_10], eax
.text:00401058                 mov     cl, byte_40C004
.text:0040105E                 mov     [ebp+var_C], cl
.text:00401061                 mov     [ebp+var_4], 0
.text:00401068                 sldt    word ptr [ebp+var_10]
.text:0040106C                 mov     edx, [ebp+var_10]
.text:0040106F                 mov     [ebp+var_4], edx
.text:00401072                 mov     eax, [ebp+var_4]
.text:00401075                 mov     ecx, [ebp+var_8]
.text:00401078                 xor     ecx, ebp
.text:0040107A                 call    @[email protected] ; __security_check_cookie(x)
.text:0040107F                 mov     esp, ebp
.text:00401081                 pop     ebp
.text:00401082                 retn
.text:00401082 get_ldtr_base   endp

[SNIP]

.text:00401140 sldt_test       proc near
.text:00401140
.text:00401140 var_4           = dword ptr -4
.text:00401140
.text:00401140                 push    ebp
.text:00401141                 mov     ebp, esp
.text:00401143                 push    ecx
.text:00401144                 mov     [ebp+var_4], 0
.text:0040114B                 call    get_ldtr_base
.text:00401150                 mov     [ebp+var_4], eax
.text:00401153                 push    offset aTest2Ldt ; "\n[+] Test 2: LDT\n"
.text:00401158                 call    _printf
.text:0040115D                 add     esp, 4
.text:00401160                 mov     eax, [ebp+var_4]
.text:00401163                 push    eax
.text:00401164                 push    offset aLdtBase0xX ; "LDT base: 0x%x\n"
.text:00401169                 call    _printf
.text:0040116E                 add     esp, 8
.text:00401171                 cmp     [ebp+var_4], 0DEAD0000h
.text:00401178                 jnz     short loc_40118B
.text:0040117A                 push    offset aResultNative_0 ; "Result  : Native OS\n\n"
.text:0040117F                 call    _printf
.text:00401184                 add     esp, 4
.text:00401187                 jmp     short loc_401198
.text:00401189 ; ---------------------------------------------------------------------------
.text:00401189                 jmp     short loc_401198
.text:0040118B ; ---------------------------------------------------------------------------
.text:0040118B
.text:0040118B loc_40118B:
.text:0040118B                 push    offset aResultVmware_0 ; "Result  : VMware detected\n\n"
.text:00401190                 call    _printf
.text:00401195                 add     esp, 4
.text:00401198
.text:00401198 loc_401198:
.text:00401198                 mov     esp, ebp
.text:0040119A                 pop     ebp
.text:0040119B                 retn
.text:0040119B sldt_test       endp

Thwart the No Pill technique

The sldt method can be subverted in VMware by disabling acceleration. To do that, change the following setting:

Vmware-disable-acceleration.png

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Keywords: sldt no-pill