This piece of source code shows you how to override both RtlUserFiberStart and BaseFiberStart for fibers on the Windows operating system. This was tested on Windows 11 and all the way down to Windows Vista. Does not support ARM64 because I don't own such a device.

fibers.cpp

#include <cstdio>
#define UNICODE
#define NOMINMAX
#define WIN32_NO_STATUS
#define WIN32_LEAN_AND_MEAN
#define _WIN32_WINNT 0x0A00
#include <windows.h>
#include <windowsx.h>
#undef WIN32_NO_STATUS
#include <ntstatus.h>
#include <winnt.h>
#include <winternl.h>

#define NtCurrentProcess() ((HANDLE)(LONG_PTR) - 1)

#if defined(_WIN64)
    #define NtGetFiberData() reinterpret_cast<FIBER*>(__readgsqword(0x20)) // NtTib.FiberData
#else
    #define NtGetFiberData() reinterpret_cast<FIBER*>(__readfsdword(0x10)) // NtTib.FiberData
#endif

using FiberInitRoutine_t = VOID (WINAPI*)();

struct FIBER
{
    PVOID UserData;
    PEXCEPTION_REGISTRATION_RECORD ExceptionList;
    PVOID StackBase;
    PVOID StackLimit;
    PVOID DeallocationStack;
    CONTEXT FiberContext;
    // ...
};

VOID(NTAPI* pRtlExitUserThread)(NTSTATUS ExitStatus) = nullptr;
NTSTATUS(NTAPI* pNtTerminateProcess)(HANDLE ProcessHandle, NTSTATUS ExitStatus) = nullptr;
VOID(NTAPI* pRtlUserFiberStart)() = nullptr;

VOID WINAPI CustomBaseFiberStart()
{
    std::puts(__FUNCTION__);

    auto const fiberData = NtGetFiberData();
#if defined(_WIN64)
    auto const fiberRoutine = reinterpret_cast<LPFIBER_START_ROUTINE>(fiberData->FiberContext.Rcx);
    auto const fiberParam = reinterpret_cast<LPVOID>(fiberData->FiberContext.Rdx);
#else
    auto const fiberRoutine = reinterpret_cast<LPFIBER_START_ROUTINE>(fiberData->FiberContext.Eax);
    auto const fiberParam = reinterpret_cast<LPVOID>(fiberData->FiberContext.Ebx);
#endif
    fiberRoutine(fiberParam);

    pRtlExitUserThread(0);
    pNtTerminateProcess(NtCurrentProcess(), 0);
}

DECLSPEC_NORETURN VOID WINAPI CustomRtlUserFiberStart()
{
    std::puts(__FUNCTION__);

    auto const fiberData = NtGetFiberData();
#if defined(_WIN64)
    auto const fiberBaseRoutine = reinterpret_cast<VOID(WINAPI*)()>(fiberData->FiberContext.Rax);
#else
    auto const fiberBaseRoutine = reinterpret_cast<VOID(WINAPI*)()>(fiberData->FiberContext.Ecx);
#endif
    fiberBaseRoutine();

    pRtlExitUserThread(STATUS_UNSUCCESSFUL);
    __debugbreak();
}

LPVOID g_mainFiber = nullptr;
LPVOID g_workerFiber = nullptr;

VOID WINAPI FiberWorker(LPVOID parameter)
{
    std::puts(__FUNCTION__);

    if (parameter != reinterpret_cast<LPVOID>(0xDEADBEEFull))
    {
        std::fputs("Parameter for FiberWorker does not equal 0xDEADBEEF", stderr);
    }

    SwitchToFiber(g_mainFiber);
    DebugBreak();
}

int main()
{
    if (HMODULE const ntdll = GetModuleHandleW(L"ntdll.dll"); ntdll != nullptr)
    {
        pRtlUserFiberStart = reinterpret_cast<decltype(pRtlUserFiberStart)>(GetProcAddress(ntdll, "RtlUserFiberStart"));
        pRtlExitUserThread = reinterpret_cast<decltype(pRtlExitUserThread)>(GetProcAddress(ntdll, "RtlExitUserThread"));
        pNtTerminateProcess = reinterpret_cast<decltype(pNtTerminateProcess)>(GetProcAddress(ntdll, "NtTerminateProcess"));
    }

    if (!pRtlUserFiberStart)
    {
        std::fputs("INFO: RtlUserFiberStart not found in ntdll.dll", stderr);
    }
    if (!pRtlExitUserThread)
    {
        std::fputs("ERROR: RtlExitUserThread not found in ntdll.dll", stderr);
        return 1;
    }
    if (!pNtTerminateProcess)
    {
        std::fputs("ERROR: NtTerminateProcess not found in ntdll.dll", stderr);
        return 2;
    }

    std::puts("[+] Found all required functions in ntdll.dll");

    g_mainFiber = ConvertThreadToFiber(nullptr);

    if (!g_mainFiber)
    {
        std::fprintf(stderr, "ERROR: ConvertThreadToFiber failed with error code %#x", GetLastError());
        return 3;
    }

    std::puts("[+] Converted current thread to a fiber");

    g_workerFiber = CreateFiber(0, FiberWorker, reinterpret_cast<LPVOID>(0xDEADBEEFull));

    if (!g_workerFiber)
    {
        std::fprintf(stderr, "ERROR: CreateFiber failed with error code %#x", GetLastError());
        return 4;
    }

    std::puts("[+] Created a fiber");

    auto fiberData = static_cast<FIBER*>(g_workerFiber);

#if defined(_WIN64)
    auto pBaseFiberStart = reinterpret_cast<FiberInitRoutine_t*>(&fiberData->FiberContext.Rax);
    auto pFiberStart = reinterpret_cast<FiberInitRoutine_t**>(&fiberData->FiberContext.Rsp);
#else
    auto pBaseFiberStart = reinterpret_cast<FiberInitRoutine_t*>(&fiberData->FiberContext.Ecx);
    auto pFiberStart = reinterpret_cast<FiberInitRoutine_t**>(&fiberData->FiberContext.Esp);
#endif

    if (pRtlUserFiberStart)
    {
        std::puts("[+] Overwriting RtlUserFiberStart and BaseFiberStart");

        // Windows 10 moved fibers code to kernelbase.dll, and introduced ntdll!RtlUserFiberStart.
        *pBaseFiberStart = CustomBaseFiberStart;
        **pFiberStart = CustomRtlUserFiberStart;
    }
    else
    {
        std::puts("[+] Overwriting BaseFiberStart");

        **pFiberStart = CustomBaseFiberStart;
    }

    std::puts("[+] Switching to fiber");
    SwitchToFiber(g_workerFiber);
    std::puts("[+] Returned from fiber");

    std::puts("[+] Deleting fiber");
    DeleteFiber(g_workerFiber);

    std::puts("[+] Done");
    return 0;
}