Mm locking is not safe at DISPATCH_LEVEL

How to reproduce:
1) Istall OS
2) Run in debug mode
3) Install guest additions using provided wizard
4) Reboot and set resolution 800x600x24bpp
5) Reboot again
6) Try to power off the guest

Eventually system displays zoomed part of the desktop (or complete garbage) and falls into debugger.

Backtrace is:

...
(ntoskrnl/po/poshtdwn.c:70) It's the final countdown...4
(ntoskrnl/po/power.c:82) No system power driver available
NTDDK: Pageable code called at IRQL > APC_LEVEL (2)
Assertion '0' failed at ARM³::CONTMEM line 251
Entered debugger on embedded INT3 at 0x0008:0x808e7d2e.
kdb:> bt
bt
Eip:
<ntoskrnl.exe:e7d2f (lib/rtl/i386/debug_asm.S:33 (DbgBreakPoint@0))>
Frames:
<ntoskrnl.exe:90855 (ARM³::CONTMEM:251 (MiFreeContiguousMemory@4))>
<ntoskrnl.exe:9099d (ARM³::CONTMEM:436 (MmFreeContiguousMemory@4))>
<VBoxVideo.sys:453c>
<VBoxVideo.sys:3c2d>
<VBoxVideo.sys:3ee1>
<VIDEOPRT.SYS:2540 (drivers/video/videoprt/dispatch.c:46 (IntVideoPortResetDisplayParameters@8))>
<ntoskrnl.exe:55e04 (ntoskrnl/inbv/inbv.c:200 (InbvAcquireDisplayOwnership@0))>
<ntoskrnl.exe:c7b5b (ntoskrnl/po/poshtdwn.c:36 (PopShutdownHandler@0))>
<ntoskrnl.exe:c7cf8 (ntoskrnl/po/poshtdwn.c:103 (PopShutdownSystem@4))>
<ntoskrnl.exe:c7f5e (ntoskrnl/po/poshtdwn.c:165 (PopGracefulShutdown@4))>
<ntoskrnl.exe:4eed4 (ntoskrnl/ex/work.c:162 (ExpWorkerThreadEntryPoint@4))>
<ntoskrnl.exe:d709e (ntoskrnl/ps/thread.c:156 (PspSys/thread.c:625 (PsCreateSystemThread@28))>
<00000000>

I am using:

• ReactOS 0.4-SVN (Build 20100501-r47068)
• VirtualBox 3.1.2 r56127
  Virtual Machine with:
• OS Type: Windows Other
• Base Memory: 256 MB
• Processor(s): 1
• Boot Order: CD/DVD-ROM, Hard Disk
• Vt-x/AMD-V: Enabled
• Nested Paging: Disabled
• Display:
• Video Memory: 64MB
• 3D Acceleration: Disabled
• 2D Video Acceleration Disabled
• Remote Display Server: Enabled
• Storage:
• IDE Primary Master: 5GB Virtual Hard Disk
• IDE Secondary Master: CDROM
• Audio: Disabled
• Network:
• Adapter 1: PCnet-FAST III (NAT)
• Serial Ports:
• Port 1:
• Port Number: COM1
• Port Mode: Raw File
• Port/File Path: R:\ReactOS-Debug-COM1.log.txt
• USB: Enabled with default settings