作者：[email protected]知道创宇404实验室
日期：2023年5月5日

本文将介绍 Windows 的 PPL 安全机制，以及在实验环境下如何绕过该机制，从而实现对 PPL 的进程进行动态调试。

本文实验环境：

Windows 10 专业版 22H2
Visual Studio 2019

对于安全研究来说，PPL机制最直观的感受就是即便使用管理员权限也无法 attach 这个进程进行调试：

通过官网文档(https://learn.microsoft.com/en-us/windows/win32/procthread/zwqueryinformationprocess)可以了解到 PS_PROTECTION 的结构如下：

typedef struct _PS_PROTECTION {
    union {
        UCHAR Level;
        struct {
            UCHAR Type   : 3;
            UCHAR Audit  : 1;                  // Reserved
            UCHAR Signer : 4;
        };
    };
} PS_PROTECTION, *PPS_PROTECTION;

前 3 位包含进程保护的类型：

typedef enum _PS_PROTECTED_TYPE {
    PsProtectedTypeNone = 0,
    PsProtectedTypeProtectedLight = 1,
    PsProtectedTypeProtected = 2
} PS_PROTECTED_TYPE, *PPS_PROTECTED_TYPE;

后 4 位包含进程保护的签名者标识：

typedef enum _PS_PROTECTED_SIGNER {
    PsProtectedSignerNone = 0,
    PsProtectedSignerAuthenticode,
    PsProtectedSignerCodeGen,
    PsProtectedSignerAntimalware,
    PsProtectedSignerLsa,
    PsProtectedSignerWindows,
    PsProtectedSignerWinTcb,
    PsProtectedSignerWinSystem,
    PsProtectedSignerApp,
    PsProtectedSignerMax
} PS_PROTECTED_SIGNER, *PPS_PROTECTED_SIGNER;

通过 WinDBG 进行本地内核调试，查看上图进程 smss.exe(412) 的内核对象 EPROCESS 可以查看 PPL=0x61，如下：

PPL 机制在内核函数 NtOpenProcess 进行实现，当我们访问进程时最终都会调用该函数；NtOpenProcess 位于 ntoskrnl.exe 内，结合符号表逆向如下：

经过一系列的调用，最终进入到 PPL 检查的关键逻辑 RtlTestProtectedAccess，其调用栈如下：

RtlTestProtectedAccess 的判断逻辑如下：

其中 Protection.Signer 经过 RtlProtectedAccess 转换的权限如下：

PsProtectedSignerNone         0 => 0x0
PsProtectedSignerAuthenticode 1 => 0x2
PsProtectedSignerCodeGen      2 => 0x4
PsProtectedSignerAntimalware  3 => 0x108
PsProtectedSignerLsa          4 => 0x110
PsProtectedSignerWindows      5 => 0x13e
PsProtectedSignerWinTcb       6 => 0x17e
PsProtectedSignerWinSystem    7 => 0x1fe
PsProtectedSignerApp          8 => 0x0

实际 NtOpenProcess 中还有诸多条件影响 PPL 的检查，不过本文我们主要关注核心判断逻辑 RtlTestProtectedAccess 就可以了。

但是实际场景下双机调试可能受环境限制，同时双机调试也不如用户模式下方便，下面我们看看通过本地调试的方法来绕过 PPL 机制。

配置好本地内核调试环境后，使用管理员权限启动 WinDBG，覆写 smss.exe(412) 进程的 Protection = 0x00 命令如下：

# 获取 smss.exe 进程的 EPROCESS 地址
lkd > !process 0 0 smss.exe
# 从 EPROCESS 获取 Protection 的偏移和值
lkd > dt nt!_eprocess ffffc40b2c45e080 Protection
lkd > db ffffc40b2c45e080+0x87a l1
# 将 Protection 值修改为 0x00
lkd > eb ffffc40b2c45e080+0x87a 0x00

执行如下：

随后我们再以管理员权限启动 WinDBG，attach 到目标进程上，可以成功进行调试：

根据本地内核调试去除 PPL 标识的思路，我们可以编写驱动程序如下，使用 ZwQuerySystemInformation() 遍历进程，使用 PsLookupProcessByProcessId() 获取进程的 EPROCESS，随后按 Protection 的偏移将其内存值覆写为 0x00：

#include <ntifs.h>
#include <wdf.h>
#define EPROCESS_PROTECTION_OFFSET 0x87A        // windows10 professional 22H2
DRIVER_INITIALIZE DriverEntry;
typedef enum _SYSTEM_INFORMATION_CLASS {
    SystemProcessInformation = 5,
    // ...
} SYSTEM_INFORMATION_CLASS;
typedef struct _SYSTEM_PROCESS_INFORMATION {
    ULONG NextEntryOffset;
    ULONG NumberOfThreads;
    BYTE Reserved1[48];
    PVOID Reserved2[3];
    HANDLE UniqueProcessId;
    PVOID Reserved3;
    ULONG HandleCount;
    BYTE Reserved4[4];
    PVOID Reserved5[11];
    SIZE_T PeakPagefileUsage;
    SIZE_T PrivatePageCount;
    LARGE_INTEGER Reserved6[6];
} SYSTEM_PROCESS_INFORMATION, *PSYSTEM_PROCESS_INFORMATION;
NTSTATUS NTAPI ZwQuerySystemInformation(
    _In_      SYSTEM_INFORMATION_CLASS SystemInformationClass,
    _Inout_   PVOID                    SystemInformation,
    _In_      ULONG                    SystemInformationLength,
    _Out_opt_ PULONG                   ReturnLength
);
NTKERNELAPI UCHAR* PsGetProcessImageFileName(__in PEPROCESS Process);
VOID OnUnload(_In_ PDRIVER_OBJECT DriverObject)
{
    UNREFERENCED_PARAMETER(DriverObject);
    KdPrintEx((DPFLTR_IHVDRIVER_ID, DPFLTR_INFO_LEVEL, "remove_ppl: unload driver\n"));
}
NTSTATUS DriverEntry(_In_ PDRIVER_OBJECT DriverObject, _In_ PUNICODE_STRING RegistryPath) {
    ULONG BufferSize = 0;
    NTSTATUS Status = STATUS_SUCCESS;
    PVOID Buffer = NULL;
    PSYSTEM_PROCESS_INFORMATION pInfo = NULL;
    UNREFERENCED_PARAMETER(DriverObject);
    UNREFERENCED_PARAMETER(RegistryPath);
    KdPrintEx((DPFLTR_IHVDRIVER_ID, DPFLTR_INFO_LEVEL, "remove_ppl: driver entry\n"));
    // register unload function
    DriverObject->DriverUnload = OnUnload;
    // get size of SYSTEM_PROCESS_INFORMATION
    Status = ZwQuerySystemInformation(SystemProcessInformation, NULL, 0, &BufferSize);
    if (Status != STATUS_INFO_LENGTH_MISMATCH) {
        KdPrintEx((DPFLTR_IHVDRIVER_ID, DPFLTR_INFO_LEVEL, "remove_ppl: ZwQuerySystemInformation get size failed status=0x%x\n", Status));
        goto _LABEL_EXIT;
    }
    // alloc memory and get SYSTEM_PROCESS_INFORMATION
    Buffer = ExAllocatePoolWithTag(PagedPool, BufferSize, '1gaT');
    if (Buffer == NULL) {
        KdPrintEx((DPFLTR_IHVDRIVER_ID, DPFLTR_INFO_LEVEL, "remove_ppl: ExAllocatePoolWithTag failed\n"));
        goto _LABEL_EXIT;
    }
    Status = ZwQuerySystemInformation(SystemProcessInformation, Buffer, BufferSize, &BufferSize);
    if (Status != STATUS_SUCCESS) {
        KdPrintEx((DPFLTR_IHVDRIVER_ID, DPFLTR_INFO_LEVEL, "remove_ppl: ZwQuerySystemInformation get info failed status=0x%x\n", Status));
        goto _LABEL_EXIT;
    }
    // traverse all processes and rewrite "Protection" to 0x00
    pInfo = (PSYSTEM_PROCESS_INFORMATION)Buffer;
    do {
        PEPROCESS Process = NULL;
        Status = PsLookupProcessByProcessId(pInfo->UniqueProcessId, &Process);
        if (NT_SUCCESS(Status)) {
            BYTE* Protection = (BYTE*)Process + EPROCESS_PROTECTION_OFFSET;
            if (*Protection != 0) {
                KdPrintEx((DPFLTR_IHVDRIVER_ID, DPFLTR_INFO_LEVEL, "remove_ppl: rewrite %s[%d] Protection=0x%x to 0x00\n",
                    PsGetProcessImageFileName(Process), pInfo->UniqueProcessId, *Protection));
                *Protection = 0x00;
            }
        }
        else {
            KdPrintEx((DPFLTR_IHVDRIVER_ID, DPFLTR_INFO_LEVEL, "remove_ppl: PsLookupProcessByProcessId [%d] failed status=0x%x\n",
                pInfo->UniqueProcessId, Status));
        }
        pInfo = (PSYSTEM_PROCESS_INFORMATION)((PUCHAR)pInfo + pInfo->NextEntryOffset);
    } while (pInfo->NextEntryOffset);
_LABEL_EXIT:
    if (Buffer != NULL) {
        ExFreePoolWithTag(Buffer, '1gaT');
    }
    return STATUS_SUCCESS;
}

成功编译后，将驱动程序注册为服务来启动运行(需设置主机为测试模式)：

# 注册驱动程序为服务
sc.exe create remove_ppl type= kernel start= demand binPath= [src]remove_ppl.sys
# 查看服务信息
sc.exe queryex remove_ppl
# 启动驱动程序/服务
sc.exe start remove_ppl

运行驱动程序，并使用 Process Explorer 查看，所有进程的 PPL 标识都被去除了：

除了以上实验代码外，也可以参考更加完善的 PPL 控制工具：

• https://github.com/Mattiwatti/PPLKiller

• https://github.com/itm4n/PPLcontrol

• https://learn.microsoft.com/en-us/windows/win32/services/protecting-anti-malware-services-

• https://learn.microsoft.com/en-us/windows/win32/procthread/zwqueryinformationprocess

• https://learn.microsoft.com/en-us/windows/win32/procthread/process-security-and-access-rights

• https://download.microsoft.com/download/a/f/7/af7777e5-7dcd-4800-8a0a-b18336565f5b/process_vista.doc

• https://www.crowdstrike.com/blog/evolution-protected-processes-part-1-pass-hash-mitigations-windows-81/

• https://www.crowdstrike.com/blog/evolution-protected-processes-part-2-exploitjailbreak-mitigations-unkillable-processes-and/

• https://www.cnblogs.com/H4ck3R-XiX/p/15872255.html

• https://www.cnblogs.com/revercc/p/16961961.html

• https://itm4n.github.io/debugging-protected-processes/

• https://paper.seebug.org/1892/

• https://github.com/Mattiwatti/PPLKiller

• https://github.com/itm4n/PPLcontrol