api: rundll32: repair token spawning semantics

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

api/adapter.c

@@ -593,8 +593,7 @@ WINTUN_STATUS WINAPI
 WintunGetAdapter(_In_z_ const WCHAR *Pool, _In_z_ const WCHAR *Name, _Out_ WINTUN_ADAPTER **Adapter)
 {
     if (!ElevateToSystem())
-        return LOG(WINTUN_LOG_ERR, L"Failed to impersonate SYSTEM user"), ERROR_ACCESS_DENIED;
+        return LOG_LAST_ERROR(L"Failed to impersonate SYSTEM user");
-
     DWORD Result;
     HANDLE Mutex = NamespaceTakePoolMutex(Pool);
     if (!Mutex)
@@ -1681,7 +1680,7 @@ WintunCreateAdapter(
     _Out_opt_ BOOL *RebootRequired)
 {
     if (!ElevateToSystem())
-        return LOG(WINTUN_LOG_ERR, L"Failed to impersonate SYSTEM user"), ERROR_ACCESS_DENIED;
+        return LOG_LAST_ERROR(L"Failed to impersonate SYSTEM user");
     BOOL DummyRebootRequired;
     if (!RebootRequired)
         RebootRequired = &DummyRebootRequired;
@@ -1699,7 +1698,7 @@ WINTUN_STATUS WINAPI
 WintunDeleteAdapter(_In_ const WINTUN_ADAPTER *Adapter, _In_ BOOL ForceCloseSessions, _Out_opt_ BOOL *RebootRequired)
 {
     if (!ElevateToSystem())
-        return LOG(WINTUN_LOG_ERR, L"Failed to impersonate SYSTEM user"), ERROR_ACCESS_DENIED;
+        return LOG_LAST_ERROR(L"Failed to impersonate SYSTEM user");
 
     BOOL DummyRebootRequired;
     if (!RebootRequired)
@@ -1790,7 +1789,7 @@ WINTUN_STATUS WINAPI
 WintunDeleteDriver(void)
 {
     if (!ElevateToSystem())
-        return LOG(WINTUN_LOG_ERR, L"Failed to impersonate SYSTEM user"), ERROR_ACCESS_DENIED;
+        return LOG_LAST_ERROR(L"Failed to impersonate SYSTEM user");
 
     DWORD Result = ERROR_SUCCESS;
 

api/elevate.c

@@ -102,3 +102,59 @@ cleanup:
     SetLastError(LastError);
     return FALSE;
 }
+
+HANDLE
+GetPrimarySystemTokenFromThread(void)
+{
+    HANDLE CurrentThreadToken, DuplicatedToken;
+    BOOL Ret;
+    DWORD LastError;
+    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);
+    if (!Ret)
+        return NULL;
+    Ret = OpenThreadToken(
+        GetCurrentThread(), TOKEN_QUERY | TOKEN_ADJUST_PRIVILEGES | TOKEN_DUPLICATE, FALSE, &CurrentThreadToken);
+    if (!Ret)
+        return NULL;
+    Ret = GetTokenInformation(CurrentThreadToken, TokenUser, &TokenUserBuffer, sizeof(TokenUserBuffer), &RequiredBytes);
+    LastError = GetLastError();
+    if (!Ret)
+        goto cleanup;
+    LastError = ERROR_ACCESS_DENIED;
+    if (!EqualSid(TokenUserBuffer.MaybeLocalSystem.User.Sid, LocalSystemSid))
+        goto cleanup;
+    Ret = LookupPrivilegeValueW(NULL, SE_ASSIGNPRIMARYTOKEN_NAME, &Privileges.Privileges[0].Luid);
+    LastError = GetLastError();
+    if (!Ret)
+        goto cleanup;
+    Ret = AdjustTokenPrivileges(CurrentThreadToken, FALSE, &Privileges, sizeof(Privileges), NULL, NULL);
+    LastError = GetLastError();
+    if (!Ret)
+        goto cleanup;
+    Ret = DuplicateTokenEx(
+        CurrentThreadToken,
+        TOKEN_QUERY | TOKEN_DUPLICATE | TOKEN_ASSIGN_PRIMARY,
+        NULL,
+        SecurityImpersonation,
+        TokenPrimary,
+        &DuplicatedToken);
+    LastError = GetLastError();
+    if (!Ret)
+        goto cleanup;
+    CloseHandle(CurrentThreadToken);
+    return DuplicatedToken;
+
+cleanup:
+    CloseHandle(CurrentThreadToken);
+    SetLastError(LastError);
+    return FALSE;
+}

api/elevate.h

@@ -9,3 +9,6 @@
 
 BOOL
 ElevateToSystem(void);
+
+HANDLE
+GetPrimarySystemTokenFromThread(void);

api/rundll32.h

@@ -157,8 +157,8 @@ ExecuteRunDll32(
                                                 .Response = Response,
                                                 .ResponseCapacity = ResponseCapacity };
     HANDLE ThreadStdout = NULL, ThreadStderr = NULL;
-    if ((ThreadStdout = CreateThread(&SecurityAttributes, 0, ProcessStdout, &ProcessStdoutState, 0, NULL)) == NULL ||
+    if ((ThreadStdout = CreateThread(NULL, 0, ProcessStdout, &ProcessStdoutState, 0, NULL)) == NULL ||
-        (ThreadStderr = CreateThread(&SecurityAttributes, 0, ProcessStderr, StreamRStderr, 0, NULL)) == NULL)
+        (ThreadStderr = CreateThread(NULL, 0, ProcessStderr, StreamRStderr, 0, NULL)) == NULL)
     {
         Result = LOG_LAST_ERROR(L"Failed to spawn readers");
         goto cleanupThreads;
@@ -169,14 +169,22 @@ ExecuteRunDll32(
                         .hStdOutput = StreamWStdout,
                         .hStdError = StreamWStderr };
     PROCESS_INFORMATION pi;
-    if (!CreateProcessW(RunDll32Path, CommandLine, NULL, NULL, TRUE, 0, NULL, NULL, &si, &pi))
+    HANDLE ProcessToken = GetPrimarySystemTokenFromThread();
+    if (!ProcessToken)
     {
-        Result = LOG_LAST_ERROR(L"Creating process failed");
+        Result = LOG_LAST_ERROR(L"Failed to get primary system token from thread");
         goto cleanupThreads;
     }
+    if (!CreateProcessAsUserW(ProcessToken, RunDll32Path, CommandLine, NULL, NULL, TRUE, 0, NULL, NULL, &si, &pi))
+    {
+        Result = LOG_LAST_ERROR(L"Failed to create process");
+        goto cleanupToken;
+    }
     WaitForSingleObject(pi.hProcess, INFINITE);
     CloseHandle(pi.hProcess);
     CloseHandle(pi.hThread);
+cleanupToken:
+    CloseHandle(ProcessToken);
 cleanupThreads:
     if (ThreadStderr)
     {