Tutorial :Implementation details for SystemPropertiesAdvanced.exe (specifically SYSDM)


I'm investigating how C:\Windows\System32\SystemPropertiesAdvanced.exe on Windows 2008 interacts with explorer.exe when updating system environment variables. When updating environment variables this way, explorer.exe will dynamically update its environment block to pick up the changes. I'm aware of multiple ways, to remotely inject environment variables, but was curious as to the implementation SystemPropertiesAdvanced.exe uses.

When I press the OK button on the SystemPropertiesAdvanced dialog after updating an environment variable, a new thread is created. Debugging this thread I've come to the following lines of interest:

WINSTA!WinStationBSMWorkerThread:  75b292bc 8bff            mov     edi,edi  75b292be 55              push    ebp  75b292bf 8bec            mov     ebp,esp  75b292c1 51              push    ecx  75b292c2 8365fc00        and     dword ptr [ebp-4],0  75b292c6 53              push    ebx  75b292c7 56              push    esi  75b292c8 8b7508          mov     esi,dword ptr [ebp+8]  75b292cb 837e4000        cmp     dword ptr [esi+40h],0  75b292cf 57              push    edi  75b292d0 0f95c0          setne   al  75b292d3 50              push    eax  75b292d4 ff763c          push    dword ptr [esi+3Ch]  75b292d7 8d4624          lea     eax,[esi+24h]  75b292da ff7638          push    dword ptr [esi+38h]  75b292dd ff7628          push    dword ptr [esi+28h]  75b292e0 50              push    eax  75b292e1 ff7620          push    dword ptr [esi+20h]  ds:0023:00404468={SYSDM!szUserEnv (6b3ec434)}  75b292e4 8d4614          lea     eax,[esi+14h]  75b292e7 ff761c          push    dword ptr [esi+1Ch]  75b292ea ff7618          push    dword ptr [esi+18h]  75b292ed 50              push    eax  75b292ee ff7610          push    dword ptr [esi+10h]  75b292f1 ff760c          push    dword ptr [esi+0Ch]  75b292f4 ff7608          push    dword ptr [esi+8]  75b292f7 ff7604          push    dword ptr [esi+4]  75b292fa ff36            push    dword ptr [esi]  75b292fc e821fdffff      call    WINSTA!WinStationBroadcastSystemMessageWorker (75b29022)  75b29301 8d7e44          lea     edi,[esi+44h]  75b29304 57              push    edi  75b29305 894508          mov     dword ptr [ebp+8],eax  75b29308 ff15d810b275    call    dword ptr [WINSTA!_imp__EnterCriticalSection (75b210d8)]  75b2930e 33db            xor     ebx,ebx  75b29310 43              inc     ebx  75b29311 837e3000        cmp     dword ptr [esi+30h],0  75b29315 0f85663d0000    jne     WINSTA!WinStationBSMWorkerThread+0x5b (75b2d081)  75b2931b 57              push    edi  75b2931c 895e2c          mov     dword ptr [esi+2Ch],ebx  75b2931f ff15d410b275    call    dword ptr [WINSTA!_imp__LeaveCriticalSection (75b210d4)]  75b29325 837dfc00        cmp     dword ptr [ebp-4],0  75b29329 0f85683d0000    jne     WINSTA!WinStationBSMWorkerThread+0x7c (75b2d097)  75b2932f 8b4508          mov     eax,dword ptr [ebp+8]  75b29332 5f              pop     edi  75b29333 5e              pop     esi  75b29334 5b              pop     ebx  75b29335 c9              leave  75b29336 c20400          ret     4  

I believe that the call to WINSTA!WinStationBroadcastSystemMessageWorker is some how letting explorer.exe know that it should pick up these new environment variables. It's interesting that the reference at 75b292e1 is being identified as SYSDM!szUserEnv by the debugger.

Perhaps not your standard inquiry, but maybe there are other reverse engineers out there. :) [at 100rep I'll set the "tags" properly]



Pradeep Prem Kamal explains how to use WM_SETTINGCHANGE and SendMessageTimeout to update all top-level processes in his blog post here. Another example (in Delphi) can be found here.

My guess is that WINSTA!WinStationBroadcastSystemMessageWorker wraps updating the user environment variables (SYSDM!szUserEnv ?) and makes the SendMessageTimeout call.

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