User Mode vs Kernel Mode: Privilege Rings and the Boundary

Objective: Understand the architectural separation between user mode (Ring 3) and kernel mode (Ring 0) on Windows — how Intel hardware enforces it, how the Windows OS layers process isolation and the system call dispatch path on top, and why this boundary is the central battleground for rootkits, EDR, and modern kernel hardening.

1. Why Rings Exist — The Hardware Contract

Intel x86/x64 CPUs define four hardware privilege levels — called rings — numbered 0 (most privileged) through 3 (least privileged). The currently executing privilege level is encoded in the low two bits of the CS segment register and is referred to as the Current Privilege Level (CPL). Every memory access, every instruction fetch, and every attempt at a privileged instruction is checked against this value by the CPU itself, before any OS code runs.

Windows collapses Intel’s four rings into two:

Rings 1 and 2 exist in hardware but are unused by Windows — using only Ring 0 and Ring 3 maps cleanly to the “supervisor vs. user” model and is portable to architectures (ARM64, older RISC) that don’t expose intermediate levels. The instant Ring 3 code attempts to execute LGDT, LIDT, RDMSR, WRMSR, HLT, CLI, STI, or any I/O instruction outside its IOPB, the CPU raises a General Protection Fault (#GP) and the kernel terminates the offending thread.

This single hardware guarantee — CPL is checked by silicon, not software — is what makes the user/kernel boundary trustworthy in the first place.

2. User Mode: The Sandboxed World

When Windows launches an application, it creates a process with its own private virtual address space, its own handle table, and a security token. On x64, the user-mode half of the address space spans 0x0000000000000000 – 0x00007FFFFFFFFFFF (128 TB). Anything above that canonical boundary is kernel territory and is unmapped from user mode page tables (especially under KVA Shadow).

User-mode code can:

• Allocate memory in its own VA via VirtualAlloc.
• Open handles to kernel objects through documented APIs.
• Spawn threads and processes via the Win32 subsystem (csrss.exe).

User-mode code cannot:

• Read or write another process’s memory without explicit handle access.
• Touch kernel VA, modify page tables, or read MSRs directly.
• Service interrupts, install drivers, or hook the IDT/GDT.

Every meaningful operation that touches hardware, files, networking, or kernel objects must therefore traverse the user/kernel boundary through a system call.

3. Kernel Mode: The Shared Kingdom

In contrast to user mode’s per-process isolation, all kernel-mode code shares a single virtual address space. ntoskrnl.exe, the HAL, file system drivers, network stack drivers, and every third-party driver loaded on the system all coexist in the same address space, on the same privilege level, with no memory protection between them.

A buggy driver writing to the wrong pointer can corrupt another driver’s structures or the kernel’s own state. A crash in any kernel component triggers a bug check (BSOD) because, unlike user mode, there is no isolation boundary to contain the damage. This is also exactly why attackers want Ring 0: once executing in kernel mode, malicious code has the same authority over the OS as the OS itself.

4. Crossing the Boundary — The SYSCALL Path

Every Win32 API that touches the kernel eventually reaches an ntdll.dll stub. On x64 those stubs all have the same shape:

; ntdll!NtReadFile (representative)
mov   r10, rcx              ; preserve arg1 (RCX is clobbered by SYSCALL)
mov   eax, 0x06             ; syscall number (build-specific; illustrative)
syscall                     ; user -> kernel transition
ret

The SYSCALL instruction is the choreography of the boundary crossing. The CPU performs all of the following atomically:

From here, control is in Windows. The kernel-side dispatch chain is:

The key takeaway for defenders: every user-mode action eventually appears in EAX as a syscall number — and EDR products that hook only in user space (in ntdll) can be bypassed by re-implementing this exact stub in attacker code (direct/indirect syscalls).

5. The SSDT — Routing Calls Inside the Kernel

The System Service Descriptor Table (SSDT) is the array of function pointers that turns EAX into a kernel routine address.

Patching SSDT entries to redirect kernel calls was the classic 32-bit rootkit technique (and the canonical kernel hook for early HIPS products). On x64, PatchGuard (KPP) periodically verifies the SSDT and several other critical structures; modification triggers Bug Check 0x109 — CRITICAL_STRUCTURE_CORRUPTION.

6. Key Kernel Structures at the Boundary

The kernel maintains per-CPU and per-thread state that defenders inspect to understand mode transitions.

// Conceptual layout — verify offsets against your build's symbols.
typedef struct _KPCR {
    // ...
    struct _KPRCB Prcb;        // at +0x180 on x64; embedded
} KPCR, *PKPCR;

typedef struct _KPRCB {
    // ...
    struct _KTHREAD *CurrentThread;   // GS:[0x188] in kernel mode
} KPRCB, *PKPRCB;

typedef struct _KTHREAD {
    // ...
    UCHAR           PreviousMode;     // 0 = KernelMode, 1 = UserMode
    PKTRAP_FRAME    TrapFrame;        // saved register state from SYSCALL
} KTHREAD, *PKTHREAD;

PreviousMode is one of the most consequential bytes in the system: kernel routines branch on it to decide whether to probe and capture caller-supplied pointers (user mode) or trust them directly (kernel mode). Bugs in that check have been the root cause of multiple Windows LPE CVEs.

Inspect any of these live in WinDbg on a kernel debug target:

0: kd> rdmsr 0xC0000082          ; IA32_LSTAR -> KiSystemCall64
0: kd> dg cs                     ; show CS selector + CPL
0: kd> dt nt!_KPCR @$pcr
0: kd> dt nt!_KTHREAD @$thread PreviousMode TrapFrame
0: kd> dt nt!_KTRAP_FRAME @$thread->TrapFrame
0: kd> dps KeServiceDescriptorTable L4

7. Hardening the Boundary

Microsoft has spent two decades hardening the user/kernel boundary in layers. Each mechanism closes a class of attacks against Ring 0.

Together these turn Ring 0 from “anything goes once you’re in” into a far more constrained environment — and explain why modern attackers gravitate toward Bring Your Own Vulnerable Driver (BYOVD) as their cleanest path to kernel code execution.

8. Common Attacker Techniques

The boundary is a target precisely because Ring 0 sits underneath every defensive product. Attackers care about three categories of abuse:

9. Defensive Strategies & Detection

The fact that all roads cross the boundary is a defender’s leverage: even attackers using direct syscalls leave telemetry at driver load, privilege use, and kernel object access layers.

Sysmon coverage

Windows audit policies

High-value ETW providers

• Microsoft-Windows-Kernel-Process — process/thread/image events at the kernel boundary.
• Microsoft-Windows-Kernel-File / Microsoft-Windows-Kernel-Registry — kernel-side file and registry ops, useful for catching driver-stage persistence.
• Microsoft-Windows-Threat-Intelligence (ETWTI) — emits high-fidelity events for ReadProcessMemory, WriteProcessMemory, MapViewOfSection, QueueUserApc. Consumption requires a PPL or kernel consumer; verify provider availability on your build with logman query providers.

Sigma — BYOVD pattern

title: Suspicious Kernel Driver Load - BYOVD Pattern
logsource:
  product: windows
  category: driver_load
detection:
  selection:
    EventID: 6
    Signed: 'false'
  filter_legit_path:
    ImageLoaded|startswith:
      - 'C:\Windows\System32\drivers\'
      - 'C:\Windows\SysWOW64\drivers\'
  condition: selection and not filter_legit_path
fields:
  - ImageLoaded
  - Hashes
  - Signature
  - SignatureStatus
level: high

Sigma — SeLoadDriverPrivilege exercised by non-system principal

title: SeLoadDriverPrivilege Use by Non-System Account
logsource:
  product: windows
  service: security
detection:
  selection:
    EventID: 4673
    PrivilegeList|contains: 'SeLoadDriverPrivilege'
  filter_machine_accounts:
    SubjectUserName|endswith: '$'
  condition: selection and not filter_machine_accounts
level: medium

Hardening checklist

• Enable HVCI / Memory Integrity (HKLM\SYSTEM\CurrentControlSet\Control\DeviceGuard).
• Enable Secure Boot in UEFI.
• Apply the Microsoft Vulnerable Driver Blocklist (HVCI-enforced).
• Verify Meltdown mitigations / KVA Shadow via Get-SpeculationControlSettings.
• Alert on bcdedit /set testsigning on and on driver loads where Signed=false or hashes match loldrivers.io.
• Enable Kernel DMA Protection for laptops with Thunderbolt/USB4.
• Limit SeLoadDriverPrivilege assignment and monitor every use via Event 4673.

10. Tools for Boundary Analysis

11. MITRE ATT&CK Mapping

Summary

• The user/kernel boundary is enforced by silicon — CPL in CS — not by software, which is what makes it trustworthy.
• Windows uses only Ring 0 and Ring 3; user mode runs in a per-process private VA, kernel mode runs in a single shared VA where any bug is a BSOD.
• Every user→kernel transition flows through SYSCALL → IA32_LSTAR → KiSystemCall64 → SSDT dispatch, leaving EAX and KTHREAD.PreviousMode as the canonical fingerprints.
• Modern hardening — PatchGuard, DSE, HVCI, KVA Shadow, and the vulnerable driver blocklist — has pushed attackers toward BYOVD and direct syscalls.
• Defenders watch the boundary through Sysmon EID 6, Security EID 4673 (SeLoadDriverPrivilege), ETWTI, and kernel-callback EDR telemetry — every Ring 0 attack eventually touches one of them.

Related Tutorials

• System Calls and SSDT: How User Mode Reaches the Kernel
• Fibers: User-Mode Cooperative Threads
• Access Tokens and Privileges: The Kernel’s Security Context
• HAL and Ntoskrnl: The Kernel Core Components
• SIDs and Security Descriptors: Identity in Windows Security

References

• User Mode and Kernel Mode – Windows Drivers | Microsoft Learn
• Kernel-Mode Driver Architecture Design Guide – Windows Drivers | Microsoft Learn
• Exploitation for Privilege Escalation, Technique T1068 – Enterprise | MITRE ATT&CK®
• Privilege Escalation, Tactic TA0004 – Enterprise | MITRE ATT&CK®
• CPU Rings, Privilege, and Protection | Many But Finite (Gustavo Duarte)