api: elevate only when needed for system operations

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

api/adapter.c

@@ -597,10 +597,16 @@ WintunGetAdapter(
     _In_z_count_c_(MAX_ADAPTER_NAME) const WCHAR *Name,
     _Out_ WINTUN_ADAPTER **Adapter)
 {
+    if (!ElevateToSystem())
+        return LOG(WINTUN_LOG_ERR, L"Failed to impersonate SYSTEM user"), ERROR_ACCESS_DENIED;
+
     DWORD Result;
     HANDLE Mutex = NamespaceTakeMutex(Pool);
     if (!Mutex)
-        return ERROR_INVALID_HANDLE;
+    {
+        Result = ERROR_INVALID_HANDLE;
+        goto cleanupToken;
+    }
 
     HDEVINFO DevInfo = SetupDiGetClassDevsExW(&GUID_DEVCLASS_NET, NULL, NULL, DIGCF_PRESENT, NULL, NULL, NULL);
     if (DevInfo == INVALID_HANDLE_VALUE)
@@ -675,6 +681,8 @@ cleanupDevInfo:
     SetupDiDestroyDeviceInfoList(DevInfo);
 cleanupMutex:
     NamespaceReleaseMutex(Mutex);
+cleanupToken:
+    RevertToSelf();
     return Result;
 }
 
@@ -1562,15 +1570,25 @@ WintunCreateAdapter(
     _Out_ WINTUN_ADAPTER **Adapter,
     _Inout_ BOOL *RebootRequired)
 {
-#ifdef MAYBE_WOW64
+    if (!ElevateToSystem())
-    if (NativeMachine != IMAGE_FILE_PROCESS)
+        return LOG(WINTUN_LOG_ERR, L"Failed to impersonate SYSTEM user"), ERROR_ACCESS_DENIED;
-        return CreateAdapterNatively(Pool, Name, RequestedGUID, Adapter, RebootRequired);
-#endif
 
     DWORD Result = ERROR_SUCCESS;
+#ifdef MAYBE_WOW64
+    if (NativeMachine != IMAGE_FILE_PROCESS)
+    {
+        Result = CreateAdapterNatively(Pool, Name, RequestedGUID, Adapter, RebootRequired);
+        RevertToSelf();
+        return Result;
+    }
+#endif
+
     WCHAR RandomTempSubDirectory[MAX_PATH];
     if ((Result = CreateTemporaryDirectory(RandomTempSubDirectory)) != ERROR_SUCCESS)
-        return LOG(WINTUN_LOG_ERR, L"Failed to create temporary folder"), Result;
+    {
+        LOG(WINTUN_LOG_ERR, L"Failed to create temporary folder");
+        goto cleanupToken;
+    }
 
     WCHAR CatPath[MAX_PATH] = { 0 };
     WCHAR SysPath[MAX_PATH] = { 0 };
@@ -1619,6 +1637,8 @@ cleanupDelete:
     DeleteFileW(InfPath);
 cleanupDirectory:
     RemoveDirectoryW(RandomTempSubDirectory);
+cleanupToken:
+    RevertToSelf();
     return Result;
 }
 
@@ -1663,20 +1683,31 @@ cleanupArgv:
 WINTUN_STATUS WINAPI
 WintunDeleteAdapter(_In_ const WINTUN_ADAPTER *Adapter, _Inout_ BOOL *RebootRequired)
 {
+    if (!ElevateToSystem())
+        return LOG(WINTUN_LOG_ERR, L"Failed to impersonate SYSTEM user"), ERROR_ACCESS_DENIED;
+
+    DWORD Result;
 #ifdef MAYBE_WOW64
     if (NativeMachine != IMAGE_FILE_PROCESS)
-        return DeleteAdapterNatively(Adapter, RebootRequired);
+    {
+        Result = DeleteAdapterNatively(Adapter, RebootRequired);
+        RevertToSelf();
+        return Result;
+    }
 #endif
 
     HDEVINFO DevInfo;
     SP_DEVINFO_DATA DevInfoData;
-    DWORD Result = GetDevInfoData(&Adapter->CfgInstanceID, &DevInfo, &DevInfoData);
+    Result = GetDevInfoData(&Adapter->CfgInstanceID, &DevInfo, &DevInfoData);
     if (Result == ERROR_FILE_NOT_FOUND)
-        return ERROR_SUCCESS;
+    {
-    if (Result != ERROR_SUCCESS)
+        Result = ERROR_SUCCESS;
+        goto cleanupToken;
+    }
+    else if (Result != ERROR_SUCCESS)
     {
         LOG(WINTUN_LOG_ERR, L"Failed to get device info data");
-        return Result;
+        goto cleanupToken;
     }
     SetQuietInstall(DevInfo, &DevInfoData);
     SP_REMOVEDEVICE_PARAMS RemoveDeviceParams = { .ClassInstallHeader = { .cbSize = sizeof(SP_CLASSINSTALL_HEADER),
@@ -1689,6 +1720,8 @@ WintunDeleteAdapter(_In_ const WINTUN_ADAPTER *Adapter, _Inout_ BOOL *RebootRequ
     else
         Result = LOG_LAST_ERROR(L"Unable to remove existing adapter");
     SetupDiDestroyDeviceInfoList(DevInfo);
+cleanupToken:
+    RevertToSelf();
     return Result;
 }
 

api/api.vcxproj

@@ -195,6 +195,7 @@
     <ClInclude Include="api.h" />
     <ClInclude Include="adapter.h" />
     <ClInclude Include="atomic.h" />
+    <ClInclude Include="elevate.h" />
     <ClInclude Include="logger.h" />
     <ClInclude Include="namespace.h" />
     <ClInclude Include="nci.h" />
@@ -206,6 +207,7 @@
   <ItemGroup>
     <ClCompile Include="api.c" />
     <ClCompile Include="adapter.c" />
+    <ClCompile Include="elevate.c" />
     <ClCompile Include="logger.c" />
     <ClCompile Include="namespace.c" />
     <ClCompile Include="nci.c" />

api/api.vcxproj.filters

@@ -55,6 +55,9 @@
     <ClInclude Include="atomic.h">
       <Filter>Header Files</Filter>
     </ClInclude>
+    <ClInclude Include="elevate.h">
+      <Filter>Header Files</Filter>
+    </ClInclude>
   </ItemGroup>
   <ItemGroup>
     <ClCompile Include="api.c">
@@ -87,5 +90,8 @@
     <ClCompile Include="session.c">
       <Filter>Source Files</Filter>
     </ClCompile>
+    <ClCompile Include="elevate.c">
+      <Filter>Source Files</Filter>
+    </ClCompile>
   </ItemGroup>
 </Project>

api/elevate.c

@@ -0,0 +1,101 @@
+/* SPDX-License-Identifier: GPL-2.0
+ *
+ * Copyright (C) 2018-2020 WireGuard LLC. All Rights Reserved.
+ */
+
+#include "pch.h"
+
+BOOL
+ElevateToSystem(void)
+{
+    HANDLE CurrentProcessToken, ThreadToken, ProcessSnapshot, WinlogonProcess, WinlogonToken, DuplicatedToken;
+    PROCESSENTRY32W ProcessEntry = { .dwSize = sizeof(PROCESSENTRY32W) };
+    BOOL Ret;
+    DWORD LastError = ERROR_SUCCESS;
+    TOKEN_PRIVILEGES Privileges = { .PrivilegeCount = 1, .Privileges = { { .Attributes = SE_PRIVILEGE_ENABLED } } };
+    CHAR LocalSystemSid[0x400];
+    DWORD RequiredBytes = sizeof(LocalSystemSid);
+    struct
+    {
+        TOKEN_USER MaybeLocalSystem;
+        CHAR LargeEnoughForLocalSystem[0x400];
+    } TokenUserBuffer;
+
+    Ret = CreateWellKnownSid(WinLocalSystemSid, NULL, &LocalSystemSid, &RequiredBytes);
+    LastError = GetLastError();
+    if (!Ret)
+        goto cleanup;
+    Ret = OpenProcessToken(GetCurrentProcess(), TOKEN_QUERY, &CurrentProcessToken);
+    LastError = GetLastError();
+    if (!Ret)
+        goto cleanup;
+    Ret =
+        GetTokenInformation(CurrentProcessToken, TokenUser, &TokenUserBuffer, sizeof(TokenUserBuffer), &RequiredBytes);
+    LastError = GetLastError();
+    CloseHandle(CurrentProcessToken);
+    if (!Ret)
+        goto cleanup;
+    if (EqualSid(TokenUserBuffer.MaybeLocalSystem.User.Sid, LocalSystemSid))
+        return TRUE;
+    Ret = LookupPrivilegeValueW(NULL, SE_DEBUG_NAME, &Privileges.Privileges[0].Luid);
+    LastError = GetLastError();
+    if (!Ret)
+        goto cleanup;
+    ProcessSnapshot = CreateToolhelp32Snapshot(TH32CS_SNAPPROCESS, 0);
+    LastError = GetLastError();
+    if (ProcessSnapshot == INVALID_HANDLE_VALUE)
+        goto cleanup;
+    for (Ret = Process32FirstW(ProcessSnapshot, &ProcessEntry); Ret;
+         Ret = Process32NextW(ProcessSnapshot, &ProcessEntry))
+    {
+        if (_wcsicmp(ProcessEntry.szExeFile, L"winlogon.exe"))
+            continue;
+        RevertToSelf();
+        Ret = ImpersonateSelf(SecurityImpersonation);
+        LastError = GetLastError();
+        if (!Ret)
+            continue;
+        Ret = OpenThreadToken(GetCurrentThread(), TOKEN_ADJUST_PRIVILEGES, FALSE, &ThreadToken);
+        LastError = GetLastError();
+        if (!Ret)
+            continue;
+        Ret = AdjustTokenPrivileges(ThreadToken, FALSE, &Privileges, sizeof(Privileges), NULL, NULL);
+        LastError = GetLastError();
+        CloseHandle(ThreadToken);
+        if (!Ret)
+            continue;
+
+        WinlogonProcess = OpenProcess(PROCESS_QUERY_INFORMATION, FALSE, ProcessEntry.th32ProcessID);
+        LastError = GetLastError();
+        if (!WinlogonProcess)
+            continue;
+        Ret = OpenProcessToken(WinlogonProcess, TOKEN_IMPERSONATE | TOKEN_DUPLICATE, &WinlogonToken);
+        LastError = GetLastError();
+        CloseHandle(WinlogonProcess);
+        if (!Ret)
+            continue;
+        Ret = DuplicateToken(WinlogonToken, SecurityImpersonation, &DuplicatedToken);
+        LastError = GetLastError();
+        CloseHandle(WinlogonToken);
+        if (!Ret)
+            continue;
+        if (!GetTokenInformation(DuplicatedToken, TokenUser, &TokenUserBuffer, sizeof(TokenUserBuffer), &RequiredBytes))
+            goto next;
+        if (SetLastError(ERROR_ACCESS_DENIED), !EqualSid(TokenUserBuffer.MaybeLocalSystem.User.Sid, LocalSystemSid))
+            goto next;
+        if (!SetThreadToken(NULL, DuplicatedToken))
+            goto next;
+        CloseHandle(DuplicatedToken);
+        CloseHandle(ProcessSnapshot);
+        SetLastError(ERROR_SUCCESS);
+        return TRUE;
+    next:
+        LastError = GetLastError();
+        CloseHandle(DuplicatedToken);
+    }
+    RevertToSelf();
+    CloseHandle(ProcessSnapshot);
+cleanup:
+    SetLastError(LastError);
+    return FALSE;
+}

api/elevate.h

@@ -0,0 +1,9 @@
+/* SPDX-License-Identifier: GPL-2.0
+ *
+ * Copyright (C) 2018-2020 WireGuard LLC. All Rights Reserved.
+ */
+
+#pragma once
+
+BOOL
+ElevateToSystem(void);

api/pch.h

@@ -8,6 +8,7 @@
 #include "adapter.h"
 #include "atomic.h"
 #include "api.h"
+#include "elevate.h"
 #include "logger.h"
 #include "namespace.h"
 #include "nci.h"

api/rundll32.c

@@ -56,101 +56,6 @@ ConsoleLogger(_In_ WINTUN_LOGGER_LEVEL Level, _In_ const WCHAR *LogLine)
     return TRUE;
 }
 
-static BOOL
-ElevateToSystem(void)
-{
-    HANDLE CurrentProcessToken, ThreadToken, ProcessSnapshot, WinlogonProcess, WinlogonToken, DuplicatedToken;
-    PROCESSENTRY32W ProcessEntry = { .dwSize = sizeof(PROCESSENTRY32W) };
-    BOOL Ret;
-    DWORD LastError = ERROR_SUCCESS;
-    TOKEN_PRIVILEGES Privileges = { .PrivilegeCount = 1, .Privileges = { { .Attributes = SE_PRIVILEGE_ENABLED } } };
-    CHAR LocalSystemSid[0x400];
-    DWORD RequiredBytes = sizeof(LocalSystemSid);
-    struct
-    {
-        TOKEN_USER MaybeLocalSystem;
-        CHAR LargeEnoughForLocalSystem[0x400];
-    } TokenUserBuffer;
-
-    Ret = CreateWellKnownSid(WinLocalSystemSid, NULL, &LocalSystemSid, &RequiredBytes);
-    LastError = GetLastError();
-    if (!Ret)
-        goto cleanup;
-    Ret = OpenProcessToken(GetCurrentProcess(), TOKEN_QUERY, &CurrentProcessToken);
-    LastError = GetLastError();
-    if (!Ret)
-        goto cleanup;
-    Ret =
-        GetTokenInformation(CurrentProcessToken, TokenUser, &TokenUserBuffer, sizeof(TokenUserBuffer), &RequiredBytes);
-    LastError = GetLastError();
-    CloseHandle(CurrentProcessToken);
-    if (!Ret)
-        goto cleanup;
-    if (EqualSid(TokenUserBuffer.MaybeLocalSystem.User.Sid, LocalSystemSid))
-        return TRUE;
-    Ret = LookupPrivilegeValueW(NULL, SE_DEBUG_NAME, &Privileges.Privileges[0].Luid);
-    LastError = GetLastError();
-    if (!Ret)
-        goto cleanup;
-    ProcessSnapshot = CreateToolhelp32Snapshot(TH32CS_SNAPPROCESS, 0);
-    LastError = GetLastError();
-    if (ProcessSnapshot == INVALID_HANDLE_VALUE)
-        goto cleanup;
-    for (Ret = Process32FirstW(ProcessSnapshot, &ProcessEntry); Ret;
-         Ret = Process32NextW(ProcessSnapshot, &ProcessEntry))
-    {
-        if (_wcsicmp(ProcessEntry.szExeFile, L"winlogon.exe"))
-            continue;
-        RevertToSelf();
-        Ret = ImpersonateSelf(SecurityImpersonation);
-        LastError = GetLastError();
-        if (!Ret)
-            continue;
-        Ret = OpenThreadToken(GetCurrentThread(), TOKEN_ADJUST_PRIVILEGES, FALSE, &ThreadToken);
-        LastError = GetLastError();
-        if (!Ret)
-            continue;
-        Ret = AdjustTokenPrivileges(ThreadToken, FALSE, &Privileges, sizeof(Privileges), NULL, NULL);
-        LastError = GetLastError();
-        CloseHandle(ThreadToken);
-        if (!Ret)
-            continue;
-
-        WinlogonProcess = OpenProcess(PROCESS_QUERY_INFORMATION, FALSE, ProcessEntry.th32ProcessID);
-        LastError = GetLastError();
-        if (!WinlogonProcess)
-            continue;
-        Ret = OpenProcessToken(WinlogonProcess, TOKEN_IMPERSONATE | TOKEN_DUPLICATE, &WinlogonToken);
-        LastError = GetLastError();
-        CloseHandle(WinlogonProcess);
-        if (!Ret)
-            continue;
-        Ret = DuplicateToken(WinlogonToken, SecurityImpersonation, &DuplicatedToken);
-        LastError = GetLastError();
-        CloseHandle(WinlogonToken);
-        if (!Ret)
-            continue;
-        if (!GetTokenInformation(DuplicatedToken, TokenUser, &TokenUserBuffer, sizeof(TokenUserBuffer), &RequiredBytes))
-            goto next;
-        if (SetLastError(ERROR_ACCESS_DENIED), !EqualSid(TokenUserBuffer.MaybeLocalSystem.User.Sid, LocalSystemSid))
-            goto next;
-        if (!SetThreadToken(NULL, DuplicatedToken))
-            goto next;
-        CloseHandle(DuplicatedToken);
-        CloseHandle(ProcessSnapshot);
-        SetLastError(ERROR_SUCCESS);
-        return TRUE;
-    next:
-        LastError = GetLastError();
-        CloseHandle(DuplicatedToken);
-    }
-    RevertToSelf();
-    CloseHandle(ProcessSnapshot);
-cleanup:
-    SetLastError(LastError);
-    return FALSE;
-}
-
 static int Argc;
 static WCHAR **Argv;
 
@@ -159,13 +64,11 @@ Init(void)
 {
     WintunSetLogger(ConsoleLogger);
     Argv = CommandLineToArgvW(GetCommandLineW(), &Argc);
-    ElevateToSystem();
 }
 
 static void
 Done(void)
 {
-    RevertToSelf();
     LocalFree(Argv);
 }
 

api/session.c

@@ -78,13 +78,19 @@ WintunStartSession(_In_ const WINTUN_ADAPTER *Adapter, _In_ DWORD Capacity, _Out
         Result = LOG_LAST_ERROR(L"Failed to allocate ring memory");
         goto cleanupRings;
     }
+    if (!ElevateToSystem())
+    {
+        LOG(WINTUN_LOG_ERR, L"Failed to impersonate SYSTEM user");
+        Result = ERROR_ACCESS_DENIED;
+        goto cleanupAllocatedRegion;
+    }
     (*Session)->Descriptor.Send.RingSize = RingSize;
     (*Session)->Descriptor.Send.Ring = (TUN_RING *)AllocatedRegion;
     (*Session)->Descriptor.Send.TailMoved = CreateEventW(SecurityAttributes, FALSE, FALSE, NULL);
     if (!(*Session)->Descriptor.Send.TailMoved)
     {
         Result = LOG_LAST_ERROR(L"Failed to create send event");
-        goto cleanupAllocatedRegion;
+        goto cleanupToken;
     }
 
     (*Session)->Descriptor.Receive.RingSize = RingSize;
@@ -116,6 +122,7 @@ WintunStartSession(_In_ const WINTUN_ADAPTER *Adapter, _In_ DWORD Capacity, _Out
         Result = LOG_LAST_ERROR(L"Failed to perform ioctl");
         goto cleanupHandle;
     }
+    RevertToSelf();
     (*Session)->Capacity = Capacity;
     (void)InitializeCriticalSectionAndSpinCount(&(*Session)->Receive.Lock, LOCK_SPIN_COUNT);
     (void)InitializeCriticalSectionAndSpinCount(&(*Session)->Send.Lock, LOCK_SPIN_COUNT);
@@ -126,6 +133,8 @@ cleanupReceiveTailMoved:
     CloseHandle((*Session)->Descriptor.Receive.TailMoved);
 cleanupSendTailMoved:
     CloseHandle((*Session)->Descriptor.Send.TailMoved);
+cleanupToken:
+    RevertToSelf();
 cleanupAllocatedRegion:
     VirtualFree(AllocatedRegion, 0, MEM_RELEASE);
 cleanupRings:
@@ -137,7 +146,7 @@ cleanupRings:
 void WINAPI
 WintunEndSession(_In_ TUN_SESSION *Session)
 {
-    SetEvent(Session->Descriptor.Send.TailMoved); // wake the reader if it's sleeping
+    SetEvent(Session->Descriptor.Send.TailMoved); // Wake the reader if it's sleeping.
     DeleteCriticalSection(&Session->Send.Lock);
     DeleteCriticalSection(&Session->Receive.Lock);
     CloseHandle(Session->Handle);