1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
|
#include "BaseLibInternals.h"
;------------------------------------------------------------------------------
;
; Copyright (c) 2006 - 2008, Intel Corporation
; All rights reserved. This program and the accompanying materials
; are licensed and made available under the terms and conditions of the BSD License
; which accompanies this distribution. The full text of the license may be found at
; http://opensource.org/licenses/bsd-license.php
;
; THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
; WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
;
; Module Name:
;
; Thunk.asm
;
; Abstract:
;
; Real mode thunk
;
;------------------------------------------------------------------------------
.686p
.model flat,C
EXTERNDEF C m16Start:BYTE
EXTERNDEF C m16Size:WORD
EXTERNDEF C mThunk16Attr:WORD
EXTERNDEF C m16Gdt:WORD
EXTERNDEF C m16GdtrBase:WORD
EXTERNDEF C mTransition:WORD
;
; Here is the layout of the real mode stack. _ToUserCode() is responsible for
; loading all these registers from real mode stack.
;
IA32_REGS STRUC 4t
_EDI DD ?
_ESI DD ?
_EBP DD ?
_ESP DD ?
_EBX DD ?
_EDX DD ?
_ECX DD ?
_EAX DD ?
_DS DW ?
_ES DW ?
_FS DW ?
_GS DW ?
_EFLAGS DD ?
_EIP DD ?
_CS DW ?
_SS DW ?
IA32_REGS ENDS
.const
;
; These are global constant to convey information to C code.
;
m16Size DW InternalAsmThunk16 - m16Start
mThunk16Attr DW _ThunkAttr - m16Start
m16Gdt DW _NullSegDesc - m16Start
m16GdtrBase DW _16GdtrBase - m16Start
mTransition DW _EntryPoint - m16Start
.code
m16Start LABEL BYTE
SavedGdt LABEL FWORD
DW ?
DD ?
;------------------------------------------------------------------------------
; _BackFromUserCode() takes control in real mode after 'retf' has been executed
; by user code. It will be shadowed to somewhere in memory below 1MB.
;------------------------------------------------------------------------------
_BackFromUserCode PROC
;
; The order of saved registers on the stack matches the order they appears
; in IA32_REGS structure. This facilitates wrapper function to extract them
; into that structure.
;
push ss
push cs
DB 66h
call @Base ; push eip
@Base:
pushf ; pushfd actually
cli ; disable interrupts
push gs
push fs
push es
push ds
pushaw ; pushad actually
DB 66h, 0bah ; mov edx, imm32
_ThunkAttr DD ?
test dl, THUNK_ATTRIBUTE_DISABLE_A20_MASK_INT_15
jz @1
mov eax, 15cd2401h ; mov ax, 2401h & int 15h
cli ; disable interrupts
jnc @2
@1:
test dl, THUNK_ATTRIBUTE_DISABLE_A20_MASK_KBD_CTRL
jz @2
in al, 92h
or al, 2
out 92h, al ; deactivate A20M#
@2:
mov eax, ss
DB 67h
lea bp, [esp + sizeof (IA32_REGS)]
;
; esi's in the following 2 instructions are indeed bp in 16-bit code. Fact
; is "esi" in 32-bit addressing mode has the same encoding of "bp" in 16-
; bit addressing mode.
;
mov word ptr (IA32_REGS ptr [esi - sizeof (IA32_REGS)])._ESP, bp
mov ebx, (IA32_REGS ptr [esi - sizeof (IA32_REGS)])._EIP
shl ax, 4 ; shl eax, 4
add bp, ax ; add ebp, eax
DB 66h, 0b8h ; mov eax, imm32
SavedCr4 DD ?
mov cr4, eax
DB 66h
lgdt fword ptr cs:[edi + (SavedGdt - @Base)]
DB 66h, 0b8h ; mov eax, imm32
SavedCr0 DD ?
mov cr0, eax
DB 0b8h ; mov ax, imm16
SavedSs DW ?
mov ss, eax
DB 66h, 0bch ; mov esp, imm32
SavedEsp DD ?
DB 66h
retf ; return to protected mode
_BackFromUserCode ENDP
_EntryPoint DD _ToUserCode - m16Start
DW 8h
_16Idtr FWORD (1 SHL 10) - 1
_16Gdtr LABEL FWORD
DW GdtEnd - _NullSegDesc - 1
_16GdtrBase DD _NullSegDesc
;------------------------------------------------------------------------------
; _ToUserCode() takes control in real mode before passing control to user code.
; It will be shadowed to somewhere in memory below 1MB.
;------------------------------------------------------------------------------
_ToUserCode PROC
mov edx, ss
mov ss, ecx ; set new segment selectors
mov ds, ecx
mov es, ecx
mov fs, ecx
mov gs, ecx
mov cr0, eax
mov cr4, ebp ; real mode starts at next instruction
mov ss, esi ; set up 16-bit stack segment
xchg sp, bx ; set up 16-bit stack pointer
DB 66h
call @Base ; push eip
@Base:
pop bp ; ebp <- address of @Base
DB 67h ; address size override
push [esp + sizeof (IA32_REGS) + 2]
lea eax, [esi + (@RealMode - @Base)]
push eax
retf
@RealMode:
mov cs:[esi + (SavedSs - @Base)], edx
mov cs:[esi + (SavedEsp - @Base)], bx
DB 66h
lidt fword ptr cs:[esi + (_16Idtr - @Base)]
popaw ; popad actually
pop ds
pop es
pop fs
pop gs
popf ; popfd
DB 66h ; Use 32-bit addressing for "retf" below
retf ; transfer control to user code
_ToUserCode ENDP
_NullSegDesc DQ 0
_16CsDesc LABEL QWORD
DW -1
DW 0
DB 0
DB 9bh
DB 8fh ; 16-bit segment, 4GB limit
DB 0
_16DsDesc LABEL QWORD
DW -1
DW 0
DB 0
DB 93h
DB 8fh ; 16-bit segment, 4GB limit
DB 0
GdtEnd LABEL QWORD
;------------------------------------------------------------------------------
; IA32_REGISTER_SET *
; EFIAPI
; InternalAsmThunk16 (
; IN IA32_REGISTER_SET *RegisterSet,
; IN OUT VOID *Transition
; );
;------------------------------------------------------------------------------
InternalAsmThunk16 PROC USES ebp ebx esi edi ds es fs gs
mov esi, [esp + 36] ; esi <- RegSet, the 1st parameter
movzx edx, (IA32_REGS ptr [esi])._SS
mov edi, (IA32_REGS ptr [esi])._ESP
add edi, - (sizeof (IA32_REGS) + 4) ; reserve stack space
mov ebx, edi ; ebx <- stack offset
imul eax, edx, 16 ; eax <- edx * 16
push sizeof (IA32_REGS) / 4
add edi, eax ; edi <- linear address of 16-bit stack
pop ecx
rep movsd ; copy RegSet
mov eax, [esp + 40] ; eax <- address of transition code
mov esi, edx ; esi <- 16-bit stack segment
lea edx, [eax + (SavedCr0 - m16Start)]
mov ecx, eax
and ecx, 0fh
shl eax, 12
lea ecx, [ecx + (_BackFromUserCode - m16Start)]
mov ax, cx
stosd ; [edi] <- return address of user code
sgdt fword ptr [edx + (SavedGdt - SavedCr0)]
sidt fword ptr [esp + 36] ; save IDT stack in argument space
mov eax, cr0
mov [edx], eax ; save CR0 in SavedCr0
and eax, 7ffffffeh ; clear PE, PG bits
mov ebp, cr4
mov [edx + (SavedCr4 - SavedCr0)], ebp
and ebp, 300h ; clear all but PCE and OSFXSR bits
push 10h
pop ecx ; ecx <- selector for data segments
lgdt fword ptr [edx + (_16Gdtr - SavedCr0)]
pushfd ; Save df/if indeed
call fword ptr [edx + (_EntryPoint - SavedCr0)]
popfd
lidt fword ptr [esp + 36] ; restore protected mode IDTR
lea eax, [ebp - sizeof (IA32_REGS)] ; eax <- the address of IA32_REGS
ret
InternalAsmThunk16 ENDP
END
|
