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Caching Access Checks

When an application performs an access check by calling the AuthzAccessCheck function, the results of that access check can be cached. When the pAuthzHandle parameter of the AuthzAccessCheck function is not NULL, the function performs a separate access check, with a requested ACCESS_MASK of MAXIMUM_ALLOWED, and caches the results of that check. A handle to the results of that check can then be passed as the AuthzHandle parameter to the AuthzCachedAccessCheck function. This allows faster access checking for a given client and security descriptors.

Only the static portion of an access check can be cached. Any callback access control entries (ACEs) or ACEs that contain the PRINCIPAL_SELF SID must be evaluated for each access check.

Attribute variables must be in the form of an expression when used with logical operators; otherwise, they are evaluated as unknown.

Example

The following example checks access against a cached result from a previous access check. The previous access check was performed in the example in Checking Access with Authz API.


BOOL CheckCachedAccess(AUTHZ_CLIENT_CONTEXT_HANDLE hClientContext)
{
    #define MY_MAX 4096


    PSECURITY_DESCRIPTOR                pSecurityDescriptor = NULL;
    ULONG                                cbSecurityDescriptorSize = 0;
    AUTHZ_ACCESS_REQUEST                Request;
    CHAR                                ReplyBuffer[MY_MAX];
    CHAR                                CachedReplyBuffer[MY_MAX];
    PAUTHZ_ACCESS_REPLY                    pReply = (PAUTHZ_ACCESS_REPLY)ReplyBuffer;
    PAUTHZ_ACCESS_REPLY                    pCachedReply = (PAUTHZ_ACCESS_REPLY)CachedReplyBuffer;
    DWORD                                AuthzError =0;
    AUTHZ_ACCESS_CHECK_RESULTS_HANDLE    hCached;

    //Allocate memory for the access request structure.
    RtlZeroMemory(&Request, sizeof(AUTHZ_ACCESS_REQUEST));

    //Set up the access request structure.
    Request.DesiredAccess = READ_CONTROL;
    
    //Allocate memory for the initial access reply structure.
    RtlZeroMemory(ReplyBuffer, MY_MAX);

    //Set up the access reply structure.
    pReply->ResultListLength = 1;
    pReply->Error = (PDWORD) ((PCHAR) pReply + sizeof(AUTHZ_ACCESS_REPLY));
    pReply->GrantedAccessMask = (PACCESS_MASK) (pReply->Error + pReply->ResultListLength);
    pReply->SaclEvaluationResults = NULL;

    //Allocate memory for the cached access reply structure.
    RtlZeroMemory(ReplyBuffer, MY_MAX);

    //Set up the cached access reply structure.
    pCachedReply->ResultListLength = 1;
    pCachedReply->Error = (PDWORD) ((PCHAR) pCachedReply + sizeof(AUTHZ_ACCESS_REPLY));
    pCachedReply->GrantedAccessMask = (PACCESS_MASK) (pCachedReply->Error + pCachedReply->ResultListLength);
    pCachedReply->SaclEvaluationResults = NULL;

    //Create security descriptor.
    if(!ConvertStringSecurityDescriptorToSecurityDescriptor(
        L"O:LAG:BAD:(A;;RC;;;BA)",
        SDDL_REVISION_1,
        &pSecurityDescriptor,
        NULL))
    {
        printf_s("ConvertStringSecurityDescriptorToSecurityDescriptor failed with %d\n", GetLastError()); 
        return FALSE;
    }

    //Call AuthzAccessCheck and cache results.
    if(!AuthzAccessCheck(
        0,
        hClientContext,
        &Request,
        NULL,
        pSecurityDescriptor,
        NULL,
        0,
        pReply,
        &hCached))
    {
        printf_s("AuthzAccessCheck failed with %d\n", GetLastError());
        
    LocalFree(pSecurityDescriptor);
        
        return FALSE;
    }

    //Call AuthzCachedAccessCheck with the cached result from the previous call.
    if(!AuthzCachedAccessCheck(
        0,
        hCached,
        &Request,
        NULL,
        pCachedReply))
    {
        printf_s("AuthzCachedAccessCheck failed with %d\n", GetLastError());
        
        LocalFree(pSecurityDescriptor);
    AuthzFreeHandle(hCached);
    
        return FALSE;
    }

    //Print results.
    if(*pCachedReply->GrantedAccessMask & READ_CONTROL)
    {
        printf_s("Access granted.\n");
    }
    else
    {
        printf_s("Access denied.\n");
    }


  LocalFree(pSecurityDescriptor);
  AuthzFreeHandle(hCached);
    return TRUE;

}


Related topics

Adding SIDs to a Client Context
Checking Access with Authz API
Initializing a Client Context
Querying a Client Context

 

 

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