System Service Descriptor Table - SSDT

System Service Dispatch Table or SSDT, simply is an array of addresses to kernel routines for 32 bit operating systems or an array of relative offsets to the same routines for 64 bit operating systems.
SSDT is the first member of the Service Descriptor Table kernel memory structure as shown below:
typedef struct tagSERVICE_DESCRIPTOR_TABLE {
SYSTEM_SERVICE_TABLE nt; //effectively a pointer to Service Dispatch Table (SSDT) itself
SYSTEM_SERVICE_TABLE sst3; //pointer to a memory address that contains how many routines are defined in the table
SSDTs used to be hooked by AVs as well as rootkits that wanted to hide files, registry keys, network connections, etc. Microsoft introduced PatchGuard for x64 systems to fight SSDT modifications by BSOD'ing the system.

When a program in user space calls a function, say CreateFile, eventually code execution is transfered to ntdll!NtCreateFile and via a syscall to the kernel routine nt!NtCreateFile.
Syscall is merely an index in the System Service Dispatch Table (SSDT) which contains an array of pointers for 32 bit OS'es (or relative offsets to the Service Dispatch Table for 64 bit OSes) to all critical system APIs like ZwCreateFile, ZwOpenFile and so on..
Below is a simplified diagram that shows how offsets in SSDT KiServiceTable are converted to absolute addresses of corresponding kernel routines:
Effectively, syscalls and SSDT (KiServiceTable) work togeher as a bridge between userland API calls and their corresponding kernel routines, allowing the kernel to know which routine should be executed for a given syscall that originated in the user space.

In WinDBG, we can check the Service Descriptor Table structure KeServiceDescriptorTable as shown below. Note that the first member is recognized as KiServiceTable - this is a pointer to the SSDT itself - the dispatch table (or simply an array) containing all those pointers/offsets:
0: kd> dps nt!keservicedescriptortable L4
fffff801`9210b880 fffff801`9203b470 nt!KiServiceTable
fffff801`9210b888 00000000`00000000
fffff801`9210b890 00000000`000001ce
fffff801`9210b898 fffff801`9203bbac nt!KiArgumentTable
Let's try and print out a couple of values from the SSDT:
0: kd> dd /c1 KiServiceTable L2
fffff801`9203b470 fd9007c4
fffff801`9203b474 fcb485c0
As mentioned earlier, on x64 which is what I'm running in my lab, SSDT contains relative offsets to kernel routines. In order to get the absolute address for a given offset, the following formula needs to be applied:
RoutineAbsoluteAddress=KiServiceTableAddress+(routineOffset>>>4)RoutineAbsoluteAddress = KiServiceTableAddress + (routineOffset >>> 4)
Using the above formula and the first offset fd9007c4 we got from the KiServiceTable, we can work out that this offset is pointing to nt!NtAccessCheck:
0: kd> u KiServiceTable + (0xfd9007c4 >>> 4)
fffff801`91dcb4ec 4c8bdc mov r11,rsp
fffff801`91dcb4ef 4883ec68 sub rsp,68h
fffff801`91dcb4f3 488b8424a8000000 mov rax,qword ptr [rsp+0A8h]
fffff801`91dcb4fb 4533d2 xor r10d,r10d
We can confirm it if we try to disassemble the nt!NtAccessCheck - routine addresses (fffff801`91dcb4ec) and first instructions (mov r11, rsp) of the above and below commands match:
0: kd> u nt!NtAccessCheck L1
fffff801`91dcb4ec 4c8bdc mov r11,rsp
If we refer back to the original drawing on how SSDT offsets are converted to absolute addresses, we can redraw it with specific values for syscall 0x1:

As a simple exercise, given a known syscall number, we can try to work out what kernel routine will be called once that syscall is issued. Let's load the debugging symbols for ntdll module:
.reload /f ntdll.dll
lm ntdll
Let's now find the syscall for ntdll!NtCreateFile:
0: kd> u ntdll!ntcreatefile L2
...we can see the syscall is 0x55:
Offsets in the KiServiceTable are 4 bytes in size, so we can work out the offset for syscall 0x55 by looking into the value the KiServiceTable holds at position 0x55:
0: kd> dd /c1 kiservicetable+4*0x55 L1
fffff801`9203b5c4 01fa3007
We see from the above that the offset for NtCreateFile is 01fa3007. Using the formula discussed previously for working out the absolute routine address, we confirm that we're looking at the nt!tCreateFile kernel routine that will be called once ntdll!NtCreateFile issues the 0x55 syscall:
0: kd> u kiservicetable + (01fa3007>>>4) L1
fffff801`92235770 4881ec88000000 sub rsp,88h
Let's redraw the earlier diagram once more for the syscall 0x55 for ntdll!NtCreateFile:

As another exercise, we could loop through all items in the service dispatch table and print absolute addresses for all routines defined in the dispatch table:
.foreach /ps 1 /pS 1 ( offset {dd /c 1 nt!KiServiceTable L poi(keservicedescriptortable+0x10) }){ dp kiservicetable + ( offset >>> 4 ) L1 }
Nice, but not very human readable. We can update the loop a bit and print out the API names associated with those absolute addresses:
0: kd> .foreach /ps 1 /pS 1 ( offset {dd /c 1 nt!KiServiceTable L poi(nt!KeServiceDescriptorTable+10)}){ r $t0 = ( offset >>> 4) + nt!KiServiceTable; .printf "%p - %y\n", $t0, $t0 }
fffff80191dcb4ec - nt!NtAccessCheck (fffff801`91dcb4ec)
fffff80191cefccc - nt!NtWorkerFactoryWorkerReady (fffff801`91cefccc)
fffff8019218df1c - nt!NtAcceptConnectPort (fffff801`9218df1c)
fffff801923f8848 - nt!NtMapUserPhysicalPagesScatter (fffff801`923f8848)
fffff801921afc10 - nt!NtWaitForSingleObject (fffff801`921afc10)
fffff80191e54010 - nt!NtCallbackReturn (fffff801`91e54010)
fffff8019213cf60 - nt!NtReadFile (fffff801`9213cf60)
fffff801921b2e80 - nt!NtDeviceIoControlFile (fffff801`921b2e80)
fffff80192212dc0 - nt!NtWriteFile (fffff801`92212dc0)
.....cut for brewity.....

The Quest for the SSDTs
.printf (WinDbg) - Windows drivers
.foreach (WinDbg) - Windows drivers
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On this page
What is SSDT
In Human Terms
Service Descriptor Table
Finding a Dispatch Routine for a Given Userland Syscall
Finding Address of All SSDT Routines