Windows Vista build 5000 (vbl core.040808-2000)

Windows Vista build 5000 (vbl_core.040808-2000) is a build of Windows Vista. On 23 January 2020, this build was listed on a thread by UX.Unleaked blog founder Grabberslasher to be released to the BetaArchive FTP, and was released in the third set of his 33 Longhorn/Vista builds on 26 January 2020, alongside build 4048, other compiles of this build, and build 5001.

This build by default has a severe bug where it appears to hang as soon as Windows switches to graphical mode, ending up with a solid blue or black screen, making this build uninstallable under normal conditions.

What is failing?
Module name:

Reason: An uninitialised variable which causes  to raise Access Violation exception (by reading null pointer) and failing without any notice.

When Windows loads, it only shows a blank blue or black background without any cursor or watermark. Keyboard interrupts are also stuck/unregistered. First checks show that kernel idle loop is running fine and processing interrupts. There are no exceptions or bugchecks logged into Windows Debugger (WinDbg), hence the next check is to set breakpoints on  and , which apparently is what   is calling and leaving the function it is in. According to the following stack, it is either being called at  (the preceding instruction), or the called function has called it: f75d6d24 nt!ExRaiseAccessViolation f75d6d48 win32k!EngFreeModule+0x5ce f75d6d50 win32k!EngFreeModule+0x4b5 f75d6da0 nt!KeReleaseInStackQueuedSpinLockFromDpcLevel+0xb14 f75d6da4 ntdll+0x18c3c f75d6da8 0x75a69626

Checking the code, it is calling the function located at : win32k!EngFreeModule+0x5b1: bf8a0581 e84a5effff     call    win32k!EngMultiByteToUnicodeN+0x4310 (bf8963d0) bf8a0586 893530079cbf   mov     dword ptr [win32k!HT_Get8BPPMaskPalette+0x30bb0 (bf9c0730)],esi bf8a058c 8935cc219cbf   mov     dword ptr [win32k!HT_Get8BPPMaskPalette+0x3264c (bf9c21cc)],esi bf8a0592 ff1500119abf   call    dword ptr [win32k!HT_Get8BPPMaskPalette+0x11580 (bf9a1100)] bf8a0598 50             push    eax bf8a0599 e8e280f8ff     call    win32k!EngMulDiv+0x29c0 (bf828680) bf8a059e 8bd8           mov     ebx,eax   <-- The return address in the stack, second frame (win32k!EngFreeModule+0x5ce) bf8a05a0 3bdf           cmp     ebx,edi

Inside that procedure, with a line-by-line tracing we found where it ends up calling ExRaiseAccessViolation in the following part: win32k!EngMulDiv+0x2b62: bf828822 3b0d9c219cbf   cmp     ecx,dword ptr [win32k!HT_Get8BPPMaskPalette+0x3261c (bf9c219c)] ds:0023:bf9c219c=00000000

win32k!EngMulDiv+0x2b68: bf828828 0f8721fcffff   ja      win32k!EngMulDiv+0x278f (bf82844f)      [br=1]

win32k!EngMulDiv+0x278f: bf82844f ff1580109abf   call    dword ptr [win32k!HT_Get8BPPMaskPalette+0x11500 (bf9a1080)] ds:0023:bf9a1080={nt!ExRaiseAccessViolation (809849aa)}

Now, comparing it to 5000.0.040809, we can notice the content of  is 7fff0000 instead of 00000000. It is now believed that we pinpointed the location of the failure, and with additional comparison against 040808's  we found where that address is being filled up:

How to fix
It will now adjust the content of that address and continue.
 * 1) Replace   with 040809's, as comparing it against 040808 turns up this only change and it will operate just as it was patched.
 * 2) By using WinDbg: once you break into the session, type: ba e1 bf9c73b3 "r eax=7fff0000;e bf9c219c 00 00 ff 7f;gc"