Indirect Syscall Dev

Indirect Syscall Dispatch — Windows x64

Invoke an NT API by executing the syscall instruction from inside an unmodified module (ntdll), without calling the hooked Nt* / Zw* stub prologue. The result: a call stack that ends in ntdll!<some Nt function>+0x12 instead of yourloader!your_trampoline, and no inline-hook byte-pattern detection on your dispatch path.

This skill is the dispatch companion to stack-spoofing. It assumes you understand ntdll stub layout and the syscall ABI at the level of windows-internals/references/syscalls.md. It focuses on implementing the dispatcher correctly.

When to activate

• Implementing or reviewing indirect syscall dispatchers in C / C++ / Rust / Go
• Selecting between Hell's / Halo's / Tartarus' / FreshyCalls / RecycledGate on a specific EDR hook pattern
• Debugging SSN drift after a Patch Tuesday (STATUS_INVALID_PARAMETER from an NT API that used to work)
• Diagnosing syscall;ret gadget discovery failures on stripped ntdll builds
• Composing an indirect dispatcher with a stack spoof trampoline (Draugr / SilentMoonwalk)
• Writing a multi-arg wrapper (NtCreateThreadEx has 11, NtQuerySystemInformationEx has 6)
• Porting a dispatcher between languages — preserving the SSN-table and gadget-cache contracts

If the question is "what does mov r10, rcx mean" → wrong skill, read windows-internals/references/syscalls.md. If the question is "how do I build a dispatcher that still works when ntdll is inline-hooked at offset 0" → right skill.

The five SSN resolution strategies

2026 resolver expansion (recommended additions)

Keep the five classic strategies above as baseline literacy, then model modern implementations as a resolver stack:

For day-to-day development, default to RecycledGate-style resolution + indirect ntdll gadget invocation and keep disk/HWBP as explicit fallback routes.

Default choice on modern Windows (build ≥ 19041): Recycled Gate / DWhisper. It is hook-immune by design (never reads stub bytes), runs once at init with O(n log n) cost, and lookup is O(1) after. The other strategies retain educational value but none offer a production advantage.

Why the "read the stub" strategies lose on modern EDRs

Hell's Gate reads byte +4 of the Nt* stub expecting B8 <ssn:4>. Modern inline hooks replace the stub with E9 <jmp> at byte 0, making byte +4 garbage. Halo's/Tartarus walk to neighbor stubs hoping they are unhooked — this fails on EDRs that hook all Nt* stubs (Defender for Endpoint on Server 2022 does this by default). FreshyCalls sorts stub addresses — fine in theory, but the sort order assumption breaks on Kaspersky-style hooks that relocate stubs.

RecycledGate sidesteps all of this: it enumerates ntdll's export table (a read-only data structure in .edata, not modified by stub hooks), extracts only Zw* exports, sorts them by address, and derives SSN from position in the sorted list. This is a structural property of how Windows ntdll is built, not of what the stubs look like.

The dispatch side — indirect vs direct

Direct syscall: your own code emits syscall — call stack shows your module as source. Trivially flagged.

Indirect syscall: your code calls syscall;ret gadget inside ntdll. From inside a CALL, the callstack at syscall time is:

ntdll!NtSomeCleanStub+0x12    (syscall;ret gadget from another Nt* stub)
  ← your trampoline
    ← your high-level caller

From an EDR's perspective: "syscall from ntdll" is normal. "syscall from nonstandard address" is anomalous.

Where is the gadget? Every non-hooked Nt* stub ends with 0F 05 C3 (syscall; ret). Pick any clean Nt* stub + 18 bytes = syscall;ret gadget address. RecycledGate's GetRecyCall enumerates Nt/Zw exports, validates the 3-byte pattern at func_addr + 18, and returns the address of the syscall instruction.

Gotcha: if the target Nt* stub is itself hooked, the 3-byte pattern at +18 is gone. Scan all Nt/Zw exports, not just one — collect all unhooked syscall;ret sites and pick one at random (resilience) or by policy (prefer gadgets from different stubs per call, to vary your callstack signature).

Decision matrix

Target EDR profile:
│
├── No hook on ntdll (EDR disabled, or using only ETW-TI kernel-side)
│   └── Direct syscall from a CALL inside ntdll is fine. Use RecycledGate
│       for consistency; SSNs resolve correctly from unhooked stubs.
│
├── IAT hook only (legacy AV, Defender early builds)
│   └── Hell's Gate works. But use RecycledGate anyway — future-proof.
│
├── Inline hook at stub byte 0 (modern Defender, SentinelOne, CrowdStrike)
│   └── Hell's fails. Use RecycledGate or Tartarus'. Halo's if you need
│       minimal code size (~40 LOC including neighbor walk).
│
├── Inline hook + kernel callback mirror (Defender for Endpoint, Server 2022)
│   └── Indirect syscall alone does NOT evade — callback fires regardless.
│       Combine with: SilentMoonwalk DESYNC stack spoof + sleep obfuscation.
│
└── Nirvana instrumentation callback (research-grade / some HIDS)
    └── Every syscall trap is intercepted kernel-side regardless of userland
        technique. Indirect dispatch is irrelevant here; use process-hollow
        or module-stomp and get out of the user-mode game.

Additional practical branches (2026):

- If all in-memory stubs are aggressively tampered and your resolver confidence drops, switch to **KnownDlls/disk-backed clean ntdll parsing**.
- If opcode reads are untrustworthy and export-order assumptions are contested in your target profile, keep a **HWBP/VEH extraction** fallback for selected APIs only.

RecycledGate / DWhisper — the canonical algorithm

Init

1. PEB walk → find ntdll base (second entry in InMemoryOrderModuleList)
2. Parse PE headers → locate export directory
3. Iterate ExportDirectory.AddressOfNames
4. Filter: keep only names starting with "Zw"
5. Build list of (name_hash, export_rva)
6. Sort list by export_rva ascending
7. For each entry at index i: SSN[hash] = i

Why Zw, not Nt? Zw* and Nt* exports are aliases in ntdll (same function). Iterating Zw* guarantees one entry per syscall (no duplicates from the renames), and the Zw prefix is stable across builds.

Why sort by RVA? Microsoft builds ntdll with stub addresses monotonically assigned per-SSN at link time. The sorted-by-address order reconstructs the SSN table. Verified stable from Windows 7 through Windows 11 24H2.

Name hashing: SSNs are stored by hash, not by plaintext name. Use a seeded hash (FNV-1a with a random seed at compile time, or SHA-256 truncated to 64 bits) so the resulting table in your binary does not contain readable strings like "NtAllocateVirtualMemory". The seed is a compile-time constant; regenerate per build for per-sample uniqueness.

Lookup

hash = seeded_hash("NtAllocateVirtualMemory")
ssn = SSN_TABLE[hash]     // O(1) hashmap
gadget = find_syscall_ret_gadget()  // cached at init
return (ssn, gadget)

The dispatcher's contract

execute(ssn: u16, syscall_gadget: *const u8, args...) -> NTSTATUS

ASM trampoline minimum:

; RCX = ssn, RDX = gadget, R8/R9/[stack] = args...
; ABI-compliant Win64 entry: SSN in RCX, gadget in RDX, args shifted right by 2
mov  eax, ecx          ; SSN → EAX
mov  r11, rdx          ; gadget → R11
mov  rcx, r8           ; arg1 → RCX
mov  rdx, r9           ; arg2 → RDX
mov  r8,  [rsp+0x28]   ; arg3
mov  r9,  [rsp+0x30]   ; arg4
; args 5+: copy from [rsp+0x38..] to [rsp+0x28..] (shift-left by 0x10)
mov  r10, rcx          ; syscall ABI: kernel clobbers RCX, reads R10
call r11               ; CALL syscall;ret gadget
ret

Frame layout gotcha: your caller passes ssn and gadget as the first two args, which consumes RCX/RDX. The actual NT API args shift by two. Every arg-shuffling ASM trampoline has to compensate; the canonical approach is the one above.

Shadow space: the gadget is a legitimate Win64 function target, so you must reserve 0x20 bytes of shadow space before the CALL. Omitting this causes STATUS_ACCESS_VIOLATION inside the kernel's syscall handler on some builds (kernel's unwinder expects shadow).

Composition with stack spoofing

The spoofer gets (ssn, gadget, args) after the dispatcher has resolved SSN. The interface:

// plain indirect dispatch
nt_status_t indirect_execute(u16 ssn, void *gadget, ...args...);

// spoofed indirect dispatch
nt_status_t spoofed_execute(u16 ssn, void *gadget, SpoofContext *ctx, ...args...);

// composed
if (ctx != NULL)
    return spoofed_execute(ssn, gadget, ctx, args);
else
    return indirect_execute(ssn, gadget, args);

The spoofer does not care how SSN was resolved. The dispatcher does not care how the callstack was spoofed. Keep the two modules separate; compose at the top level.

See stack-spoofing/SKILL.md for the spoofer side.

Layered syscall interception

Some implants need to intercept specific NT APIs before they reach the gate (e.g., detect NtSuspendThread calls on GetCurrentThread() and abort). The pattern:

type LayeredSyscall struct {
    Syscall  // embedded; unoverridden calls delegate
}

// Override a specific syscall
func (s *LayeredSyscall) NtSuspendThread(h Handle) NTSTATUS {
    if h == currentThreadHandle() {
        return STATUS_INVALID_HANDLE  // pre-hook
    }
    return s.Syscall.NtSuspendThread(h)  // fall through
}

Use for:

• Blocking self-suspension primitives
• Logging a subset of NT calls during red team operations
• Replacing a specific NT API with a safer or stealthier variant

The Go idiom above uses struct embedding. In Rust: composition with trait delegation (manually implement the Syscall trait, forwarding each method except the ones you intercept). In C: function pointer table, with the wrapper calling the next layer unless it has its own override.

The six implementation rules

R1. Resolve SSN at runtime, never hardcode. Hardcoded SSNs break on the first Patch Tuesday. RecycledGate init is O(n log n), runs once, caches everything. There is no reason to hardcode.

R2. Use a seeded hash for the SSN table keys. Plaintext "NtAllocateVirtualMemory" in .rdata is trivially grepable by forensics. Hash keys with a compile-time-constant seed so the table contents do not leak API names. Common pattern: FNV-1a over (seed || api_name_lowercased).

R3. Cache the syscall;ret gadget at init. Rescanning per-call wastes ~1ms on a typical ntdll; at high call rates this matters. More importantly: a scanner running at every dispatch is a behavioral pattern EDRs can detect. Scan once, cache the address, invalidate on suspected hook drift (rare).

R4. Validate the gadget before caching. If the stub you picked is itself hooked, +18 no longer reads 0F 05 C3. Validate with a 3-byte compare and fall through to the next export. GetRecyCall in RecycledGate does this; reimplementations often skip it and then fail mysteriously on specific Defender builds.

R5. Handle 6+ arg syscalls correctly. The trampoline's arg-shift-by-2 logic must extend through [rsp+0x28..0x38..0x40..]. Most published PoCs handle only 4 args (RCX/RDX/R8/R9) and silently break on NtCreateThreadEx(11 args). Test with NtQuerySystemInformationEx (6 args) or NtCreateThreadEx (11) before trusting.

R6. Emit minimal strings in release. "ntdll.dll", "Zw", "Nt" can all be obfuscated via compile-time XOR or a custom Xr/Xh macro. The RecycledGate reference codebase uses obfuscation.Xr(obfuscation.Mr, mNtdll) where mNtdll is a hex-encoded ciphertext. A loader binary that strings reveals no NT API names is substantially harder to triage.

R7. Implement explicit mode switches. Keep dispatch mode configurable (for example: 0=direct/IAT, 1=indirect-gadget, 2=clean Nt* function pointer) so you can test detections and stability without rewriting wrappers.

R8. Treat stack-shape as a first-class signal. Pair indirect syscalls with stack-spoof / return-address shaping when required, but keep spoofing decoupled from SSN resolution to avoid tight coupling bugs.

Agentic build-and-verify loop (dev-focused)

1. Implement one resolver mode at a time and keep wrappers unchanged.
2. Validate on a syscall panel (NtAllocateVirtualMemory, NtProtectVirtualMemory, NtCreateThreadEx, NtWaitForSingleObject).
3. Add structured debug markers for: selected mode, resolved SSN, selected gadget, NTSTATUS.
4. Stress under multiple Windows builds (or VM snapshots) before adding another evasion layer.
5. Only then compose with stack spoofing / sleep masking.

This loop reduces false attribution when a failure is actually ABI-shuffle or argument packing, not resolver logic.

Languages — what changes

C / C++ (mingw-w64)

• Flat asm file (.s) called from C is cleanest. Inline asm in C is tolerable for a single trampoline; becomes fragile for 4+ argcount variants.
• Emit 4 variants: indirect_syscall4, _6, _11, _18 for different arg counts. Compilers optimize stack shuffling better when the size is fixed.
• Link without CRT: -nostdlib -fno-ident -Wl,-e,<your_entry>.

Rust

• global_asm! for the trampoline; call via extern "win64".
• SSN table: phf::Map with compile-time-constant seeded hashes.
• Zero-alloc dispatch path: return NTSTATUS by value, no Result / heap. Call sites handle status checking.
• Avoid std entirely in the implant; the dispatcher is no_std + no_main.

Go

• Plan 9 ASM trampoline (asm_x64.s). Function signature:

  //go:nosplit
  //go:noescape
  func reCycall(ssn uint16, gadget uintptr, args ...uintptr) (errcode uint32)
  

• //go:nosplit is mandatory: prevents the Go runtime from inserting a stack-growth check that would corrupt the trampoline's register state.
• //go:noescape lets the compiler keep args in registers/on stack instead of boxing.
• Set LastError to 0 via TEB[gs:30h][68h] at trampoline entry if the NT status relies on no prior error leaking.

Full code sketches per language in references/lang-c-rust-go.md.

Resources

• references/lang-c-rust-go.md — Per-language dispatcher skeletons: C (mingw-w64 AT&T .s), Rust (global_asm!), Go (Plan 9), plus SSN table init patterns and obfuscation hooks
• references/strategies.md — In-depth on each SSN resolution strategy, detection footprint per strategy, migration between strategies, hook-pattern reverse engineering
• references/implementation-examples.md — Practical C/Rust implementation patterns distilled from real-world projects (mode switching, 4/6/11 arg dispatch, DESYNC composition)
• Start with references/strategies.md, then pair exactly one implementation path with the chosen strategy before debugging failures.