archiveJan 3, 2021

TEB (Thread Environment Block)

Windows TEB layout, how NtCurrentTeb and FS:[0x18]/0x30]/0] resolve TEB, PEB, and SEH, and how the FS segment selector indexes the descriptor table.

TEB (Thread Environment Block)

The TEB holds per-thread state for a process. Each thread gets its own TEB, and the exact layout shifts a bit across Windows versions.

typedef struct _TEB {
  PVOID Reserved1[12];
  PPEB  ProcessEnvironmentBlock;
  PVOID Reserved2[399];
  BYTE  Reserved3[1952];
  PVOID TlsSlots[64];
  BYTE  Reserved4[8];
  PVOID Reserved5[26];
  PVOID ReservedForOle;
  PVOID Reserved6[4];
  PVOID TlsExpansionSlots;
} TEB, *PTEB;

0:001> dt _TEB
ntdll!_TEB
   +0x000 NtTib            : _NT_TIB
   +0x01c EnvironmentPointer : Ptr32 Void
   +0x020 ClientId         : _CLIENT_ID
   +0x028 ActiveRpcHandle  : Ptr32 Void
   +0x02c ThreadLocalStoragePointer : Ptr32 Void
   +0x030 ProcessEnvironmentBlock : Ptr32 _PEB
   +0x034 LastErrorValue   : Uint4B
   +0x038 CountOfOwnedCriticalSections : Uint4B
   +0x03c CsrClientThread  : Ptr32 Void
   +0x040 Win32ThreadInfo  : Ptr32 Void
   +0x044 User32Reserved   : [26] Uint4B
   +0x0ac UserReserved     : [5] Uint4B
   +0x0c0 WOW32Reserved    : Ptr32 Void
   +0x0c4 CurrentLocale    : Uint4B
   +0x0c8 FpSoftwareStatusRegister : Uint4B
   +0x0cc ReservedForDebuggerInstrumentation : [16] Ptr32 Void
   +0x10c SystemReserved1  : [26] Ptr32 Void
   +0x174 PlaceholderCompatibilityMode : Char
   +0x175 PlaceholderHydrationAlwaysExplicit : UChar
   +0x176 PlaceholderReserved : [10] Char
   +0x180 ProxiedProcessId : Uint4B
   +0x184 _ActivationStack : _ACTIVATION_CONTEXT_STACK
   +0x19c WorkingOnBehalfTicket : [8] UChar
   +0x1a4 ExceptionCode    : Int4B
   +0x1a8 ActivationContextStackPointer : Ptr32 _ACTIVATION_CONTEXT_STACK
   +0x1ac InstrumentationCallbackSp : Uint4B
   +0x1b0 InstrumentationCallbackPreviousPc : Uint4B
   +0x1b4 InstrumentationCallbackPreviousSp : Uint4B
   +0x1b8 InstrumentationCallbackDisabled : UChar
   +0x1b9 SpareBytes       : [23] UChar
   +0x1d0 TxFsContext      : Uint4B
   +0x1d4 GdiTebBatch      : _GDI_TEB_BATCH
   +0x6b4 RealClientId     : _CLIENT_ID
   +0x6bc GdiCachedProcessHandle : Ptr32 Void
   +0x6c0 GdiClientPID     : Uint4B
   +0x6c4 GdiClientTID     : Uint4B
   +0x6c8 GdiThreadLocalInfo : Ptr32 Void
   +0x6cc Win32ClientInfo  : [62] Uint4B
   +0x7c4 glDispatchTable  : [233] Ptr32 Void
   +0xb68 glReserved1      : [29] Uint4B
   +0xbdc glReserved2      : Ptr32 Void
   +0xbe0 glSectionInfo    : Ptr32 Void
   +0xbe4 glSection        : Ptr32 Void
   +0xbe8 glTable          : Ptr32 Void
   +0xbec glCurrentRC      : Ptr32 Void
   +0xbf0 glContext        : Ptr32 Void
   +0xbf4 LastStatusValue  : Uint4B
   +0xbf8 StaticUnicodeString : _UNICODE_STRING
   +0xc00 StaticUnicodeBuffer : [261] Wchar
   +0xe0c DeallocationStack : Ptr32 Void
   +0xe10 TlsSlots         : [64] Ptr32 Void
   +0xf10 TlsLinks         : _LIST_ENTRY
   +0xf18 Vdm              : Ptr32 Void
   +0xf1c ReservedForNtRpc : Ptr32 Void
   +0xf20 DbgSsReserved    : [2] Ptr32 Void
   +0xf28 HardErrorMode    : Uint4B
   +0xf2c Instrumentation  : [9] Ptr32 Void
   +0xf50 ActivityId       : _GUID
   +0xf60 SubProcessTag    : Ptr32 Void
   +0xf64 PerflibData      : Ptr32 Void
   +0xf68 EtwTraceData     : Ptr32 Void
   +0xf6c WinSockData      : Ptr32 Void
   +0xf70 GdiBatchCount    : Uint4B
   +0xf74 CurrentIdealProcessor : _PROCESSOR_NUMBER
   +0xf74 IdealProcessorValue : Uint4B
   +0xf74 ReservedPad0     : UChar
   +0xf75 ReservedPad1     : UChar
   +0xf76 ReservedPad2     : UChar
   +0xf77 IdealProcessor   : UChar
   +0xf78 GuaranteedStackBytes : Uint4B
   +0xf7c ReservedForPerf  : Ptr32 Void
   +0xf80 ReservedForOle   : Ptr32 Void
   +0xf84 WaitingOnLoaderLock : Uint4B
   +0xf88 SavedPriorityState : Ptr32 Void
   +0xf8c ReservedForCodeCoverage : Uint4B
   +0xf90 ThreadPoolData   : Ptr32 Void
   +0xf94 TlsExpansionSlots : Ptr32 Ptr32 Void
   +0xf98 MuiGeneration    : Uint4B
   +0xf9c IsImpersonating  : Uint4B
   +0xfa0 NlsCache         : Ptr32 Void
   +0xfa4 pShimData        : Ptr32 Void
   +0xfa8 HeapData         : Uint4B
   +0xfac CurrentTransactionHandle : Ptr32 Void
   +0xfb0 ActiveFrame      : Ptr32 _TEB_ACTIVE_FRAME
   +0xfb4 FlsData          : Ptr32 Void
   +0xfb8 PreferredLanguages : Ptr32 Void
   +0xfbc UserPrefLanguages : Ptr32 Void
   +0xfc0 MergedPrefLanguages : Ptr32 Void
   +0xfc4 MuiImpersonation : Uint4B
   +0xfc8 CrossTebFlags    : Uint2B
   +0xfc8 SpareCrossTebBits : Pos 0, 16 Bits
   +0xfca SameTebFlags     : Uint2B
   +0xfca SafeThunkCall    : Pos 0, 1 Bit
   +0xfca InDebugPrint     : Pos 1, 1 Bit
   +0xfca HasFiberData     : Pos 2, 1 Bit
   +0xfca SkipThreadAttach : Pos 3, 1 Bit
   +0xfca WerInShipAssertCode : Pos 4, 1 Bit
   +0xfca RanProcessInit   : Pos 5, 1 Bit
   +0xfca ClonedThread     : Pos 6, 1 Bit
   +0xfca SuppressDebugMsg : Pos 7, 1 Bit
   +0xfca DisableUserStackWalk : Pos 8, 1 Bit
   +0xfca RtlExceptionAttached : Pos 9, 1 Bit
   +0xfca InitialThread    : Pos 10, 1 Bit
   +0xfca SessionAware     : Pos 11, 1 Bit
   +0xfca LoadOwner        : Pos 12, 1 Bit
   +0xfca LoaderWorker     : Pos 13, 1 Bit
   +0xfca SkipLoaderInit   : Pos 14, 1 Bit
   +0xfca SpareSameTebBits : Pos 15, 1 Bit
   +0xfcc TxnScopeEnterCallback : Ptr32 Void
   +0xfd0 TxnScopeExitCallback : Ptr32 Void
   +0xfd4 TxnScopeContext  : Ptr32 Void
   +0xfd8 LockCount        : Uint4B
   +0xfdc WowTebOffset     : Int4B
   +0xfe0 ResourceRetValue : Ptr32 Void
   +0xfe4 ReservedForWdf   : Ptr32 Void
   +0xfe8 ReservedForCrt   : Uint8B
   +0xff0 EffectiveContainerId : _GUID

0:001> dt _TEB
ntdll!_TEB
   +0x000 NtTib            : _NT_TIB
   +0x01c EnvironmentPointer : Ptr32 Void
   +0x020 ClientId         : _CLIENT_ID
   +0x028 ActiveRpcHandle  : Ptr32 Void
   +0x02c ThreadLocalStoragePointer : Ptr32 Void
   +0x030 ProcessEnvironmentBlock : Ptr32 _PEB
	 ...

ProcessEnvironmentBlock member

At offset 0x30, ProcessEnvironmentBlock is a pointer to the process PEB. There is one PEB per process.

NtTib member

The first TEB member is _NT_TIB (NT Thread Information Block). It stores info for the currently running thread.

typedef struct _NT_TIB {
    struct _EXCEPTION_REGISTRATION_RECORD *ExceptionList;
    PVOID StackBase;
    PVOID StackLimit;
    PVOID SubSystemTib;
#if defined(_MSC_EXTENSIONS)
    union {
        PVOID FiberData;
        DWORD Version;
    };
#else
    PVOID FiberData;
#endif
    PVOID ArbitraryUserPointer;
    struct _NT_TIB *Self;
} NT_TIB;
typedef NT_TIB *PNT_TIB;

ExceptionList points at a linked list of _EXCEPTION_REGISTRATION_RECORD nodes used by SEH (Structured Exception Handling). Self is a self-pointer to the _NT_TIB, which is also the TEB base because _NT_TIB sits at offset zero of the TEB.

Reading the TEB from user mode

Ntdll.NtCurrentTeb()

Ntdll.NtCurrentTeb() returns the current thread's TEB address.

The implementation is tiny: it returns the value at FS:[18]. In the session above that resolved to 00281018, and the memory window showed 00281000 at that location — the TEB base. That base matches the segment memory base selected by the FS register.

FS segment register

On IA-32, FS points at the current thread's TEB. Virtual address space is 4 GB, so a full pointer is 32 bits, while FS itself is only 16 bits. FS does not store the TEB address directly; it stores an index into the Segment Descriptor Table. That table lives in kernel memory, and GDTR holds its address.

Because segment registers hold table indexes, they are also called segment selectors. The segment memory base selected by FS is where the TEB sits.

FS:[0x18] = TEB base address

FS:[0x18] = TEB.NtTib.Self = address of TIB = address of TEB = FS:0 = 0x2810100
 
FS:0 means the start address of the segment memory described by the
descriptor that FS indexes.

FS:[0x30] = PEB base address

FS:[0x30] = TEB.ProcessEnvironmentBlock = address of PEB

FS:[0x0] = SEH base address

FS:[0] = TEB.NtTib.ExceptionList = address of SEH