VB6: Single Instance Application in All User Sessions

Question

I have an application that should be a one-page application for all user sessions on a Windows PC. So far, my research has focused on using a mutex for this, but I have a problem in which I am not sure if this is really a problem. This is really the best question I think.

Here's the code first:

Private Const AppVer = "Global\UNIQUENAME" ' This is not what i am using but the name is unique

Public Sub Main()

    Dim mutexValue As Long

    mutexValue = CreateMutex(ByVal 0&, 1, AppVer)
    If (Err.LastDllError = ERROR_ALREADY_EXISTS) Then
        SaveTitle$ = App.Title
        App.Title = "... duplicate instance."
        MsgBox "A duplicate instance of this program exists."
        CloseHandle mutexValue
        Exit Sub
    End If
    ' Else keep on truckin'

Now, based on this article, I believe that I understand that by passing a NULL pointer to the CreateMutex function, as I am above, I basically assign any security descriptor to the current user.

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Answer 1

VB6 . VB6, , .

:

, , . API- Mutex, . GetSecurityDescriptor - , , Mutex.

Answer 2

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Option Explicit

Private Const STANDARD_RIGHTS_REQUIRED              As Long = &HF0000
Private Const SYNCHRONIZE                           As Long = &H100000
Private Const MUTANT_QUERY_STATE                    As Long = &H1
Private Const MUTANT_ALL_ACCESS                     As Long = (STANDARD_RIGHTS_REQUIRED Or SYNCHRONIZE Or MUTANT_QUERY_STATE)
Private Const SECURITY_DESCRIPTOR_REVISION          As Long = 1
Private Const DACL_SECURITY_INFORMATION             As Long = 4

Private Declare Function CreateMutex Lib "kernel32" Alias "CreateMutexA" (lpMutexAttributes As Any, ByVal bInitialOwner As Long, ByVal lpName As String) As Long
Private Declare Function OpenMutex Lib "kernel32" Alias "OpenMutexA" (ByVal dwDesiredAccess As Long, ByVal bInheritHandle As Long, ByVal lpName As String) As Long
Private Declare Function CloseHandle Lib "kernel32" (ByVal hObject As Long) As Long
Private Declare Function InitializeSecurityDescriptor Lib "advapi32.dll" (pSecurityDescriptor As Any, ByVal dwRevision As Long) As Long
Private Declare Function SetSecurityDescriptorDacl Lib "advapi32.dll" (pSecurityDescriptor As Any, ByVal bDaclPresent As Long, pDacl As Any, ByVal bDaclDefaulted As Long) As Long
Private Declare Function SetKernelObjectSecurity Lib "advapi32.dll" (ByVal Handle As Long, ByVal SecurityInformation As Long, pSecurityDescriptor As SECURITY_DESCRIPTOR) As Long

Private Type SECURITY_DESCRIPTOR
    Revision            As Byte
    Sbz1                As Byte
    Control             As Long
    Owner               As Long
    Group               As Long
    pSacl               As Long
    pDacl               As Long
End Type

Private Const MUTEX_NAME            As String = "Global\20b70e57-1c2e-4de9-99e5-20f3961e6812"

Private m_hCurrentMutex         As Long

Private Sub Form_Load()
    Dim hMutex          As Long
    Dim uSec            As SECURITY_DESCRIPTOR

    hMutex = OpenMutex(MUTANT_ALL_ACCESS, 0, MUTEX_NAME)
    If hMutex <> 0 Then
        Call CloseHandle(hMutex)
        MsgBox "Already running", vbExclamation
        Unload Me
        Exit Sub
    End If
    m_hCurrentMutex = CreateMutex(ByVal 0&, 1, MUTEX_NAME)
    Call InitializeSecurityDescriptor(uSec, SECURITY_DESCRIPTOR_REVISION)
    Call SetSecurityDescriptorDacl(uSec, 1, ByVal 0, 0)
    Call SetKernelObjectSecurity(m_hCurrentMutex, DACL_SECURITY_INFORMATION, uSec)
End Sub

Private Sub Form_Unload(Cancel As Integer)
    If m_hCurrentMutex <> 0 Then
        Call CloseHandle(m_hCurrentMutex)
        m_hCurrentMutex = 0
    End If
End Sub

Answer 3

, . , ERROR_ACCESS_DENIED, - , , ERROR_ALREADY_EXISTS ( .) .

Do you think setting the right security descriptor is really the right way to do this. MSDN says granting MUTEX_ALL_ACCESS privileges increases the risk that the user will be an administrator, and I think you need MUTEX_ALL_ACCESS. But in my experience, it works great for non-admins.

Your question intrigued me quickly enough. This means that I have the source code, and here:

int wmain(int argc, wchar_t* argv[])
{
    ACL *existing_dacl = NULL;
    ACL *new_dacl = NULL;
    PSECURITY_DESCRIPTOR security_descriptor = NULL;

    bool owner = false;
    HANDLE mutex = CreateMutex(NULL,FALSE,L"Global\\blah");
    if(mutex == NULL)
        wprintf(L"CreateMutex failed: 0x%08x\r\n",GetLastError());
    if(GetLastError() == ERROR_ALREADY_EXISTS)
        wprintf(L"Got handle to existing mutex\r\n");
    else
    {
        wprintf(L"Created new mutex\r\n");
        owner = true;
    }

    if(owner)
    {
        //  Get the DACL on the mutex
        HRESULT hr = GetSecurityInfo(mutex,SE_KERNEL_OBJECT,
                                    DACL_SECURITY_INFORMATION,NULL,NULL,
                                    &existing_dacl,NULL,
                                    &security_descriptor);
        if(hr != S_OK)
            wprintf(L"GetSecurityInfo failed: 0x%08x\r\n",hr);

        //  Add an ACE to the ACL
        EXPLICIT_ACCESSW ace;
        memset(&ace,0,sizeof(ace));
        ace.grfAccessPermissions = MUTEX_ALL_ACCESS;
        ace.grfAccessMode = GRANT_ACCESS;
        ace.grfInheritance = NO_INHERITANCE;
        ace.Trustee.pMultipleTrustee = NULL;
        ace.Trustee.MultipleTrusteeOperation = NO_MULTIPLE_TRUSTEE;
        ace.Trustee.TrusteeForm = TRUSTEE_IS_NAME;
        ace.Trustee.TrusteeType = TRUSTEE_IS_WELL_KNOWN_GROUP;
        ace.Trustee.ptstrName = L"EVERYONE";
        hr = SetEntriesInAcl(1,&ace,existing_dacl,&new_dacl);
        if(hr != S_OK)
            wprintf(L"SetEntriesInAcl failed: 0x%08x\r\n",hr);

        //  Set the modified DACL on the mutex
        hr = SetSecurityInfo(mutex,SE_KERNEL_OBJECT,
                            DACL_SECURITY_INFORMATION,NULL,NULL,new_dacl,NULL);
        if(hr != S_OK)
            wprintf(L"SetSecurityInfo failed: 0x%08x\r\n",hr);
        else
            wprintf(L"Changed ACL\r\n");

        LocalFree(existing_dacl);
        LocalFree(new_dacl);
        LocalFree(security_descriptor);
    }

    wprintf(L"Press any key...");
    _getch();
    CloseHandle(mutex);
    return 0;
}