Bug in KiFastCallEntry

[Hartmut Birr]

Hi,

it seems we have a little bug in KiFastCallEntry. It is initialized a
trap frame on the stack. It is a trap frame without the v86 segment
registers. ESP points to TSS->ESP0 - sizeof(KTRAP_FRAME) + 16. 16 for
the missing v86 segment register. ESP is compared with
KTHREAD->InitialStack - 0x29c. 0x29c is equal to sizeof(FX_SAVE_AREA) +
sizeof(KTRAP_FRAME). If the compare operation fails, it is called
BadStack which results in a v86 invalid opcode exception. To bypass this
bug, there is added a value of  0x10 to TSS->ESP0 in
Ke386InitThreadWithContext. A friend of me has looked to WinXp. He has
found a piece of code which is an exact copy of BadStack and he has
found a piece of code which looks like this:

_KiFastCallEntry:
...
    /* Use the thread's stack */
    mov ebp, [esi+KTHREAD_INITIAL_STACK]
...
    /* Make space for us on the stack */
    sub ebp, 0x29C

    /* Write the previous mode */
    mov byte ptr [esi+KTHREAD_PREVIOUS_MODE], UserMode

    /* Sanity check */
    cmp ebp, esp
    jnz BadStack
...
  
- Hartmut