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
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
|
#include "BaseLibInternals.h"
;------------------------------------------------------------------------------
;
; Copyright (c) 2006 - 2018, Intel Corporation. All rights reserved.<BR>
; SPDX-License-Identifier: BSD-2-Clause-Patent
;
; Module Name:
;
; Thunk.asm
;
; Abstract:
;
; Real mode thunk
;
;------------------------------------------------------------------------------
global ASM_PFX(m16Size)
global ASM_PFX(mThunk16Attr)
global ASM_PFX(m16Gdt)
global ASM_PFX(m16GdtrBase)
global ASM_PFX(mTransition)
global ASM_PFX(m16Start)
struc IA32_REGS
._EDI: resd 1
._ESI: resd 1
._EBP: resd 1
._ESP: resd 1
._EBX: resd 1
._EDX: resd 1
._ECX: resd 1
._EAX: resd 1
._DS: resw 1
._ES: resw 1
._FS: resw 1
._GS: resw 1
._EFLAGS: resq 1
._EIP: resd 1
._CS: resw 1
._SS: resw 1
.size:
endstruc
SECTION .data
;
; These are global constant to convey information to C code.
;
ASM_PFX(m16Size) DW ASM_PFX(InternalAsmThunk16) - ASM_PFX(m16Start)
ASM_PFX(mThunk16Attr) DW _BackFromUserCode.ThunkAttrEnd - 4 - ASM_PFX(m16Start)
ASM_PFX(m16Gdt) DW _NullSeg - ASM_PFX(m16Start)
ASM_PFX(m16GdtrBase) DW _16GdtrBase - ASM_PFX(m16Start)
ASM_PFX(mTransition) DW _EntryPoint - ASM_PFX(m16Start)
SECTION .text
ASM_PFX(m16Start):
SavedGdt:
dw 0
dq 0
;------------------------------------------------------------------------------
; _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:
;
; 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.
;
BITS 16
push ss
push cs
;
; Note: We can't use o32 on the next instruction because of a bug
; in NASM 2.09.04 through 2.10rc1.
;
call dword .Base ; push eip
.Base:
push dword 0 ; reserved high order 32 bits of EFlags
pushfd
cli ; disable interrupts
push gs
push fs
push es
push ds
pushad
mov edx, strict dword 0
.ThunkAttrEnd:
test dl, THUNK_ATTRIBUTE_DISABLE_A20_MASK_INT_15
jz .1
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:
xor eax, eax
mov ax, ss
lea ebp, [esp + IA32_REGS.size]
mov [bp - IA32_REGS.size + IA32_REGS._ESP], ebp
mov ebx, [bp - IA32_REGS.size + IA32_REGS._EIP]
shl eax, 4 ; shl eax, 4
add ebp, eax ; add ebp, eax
mov eax, cs
shl eax, 4
lea eax, [eax + ebx + (.X64JmpEnd - .Base)]
mov [cs:bx + (.X64JmpEnd - 6 - .Base)], eax
mov eax, strict dword 0
.SavedCr4End:
mov cr4, eax
o32 lgdt [cs:bx + (SavedGdt - .Base)]
mov ecx, 0c0000080h
rdmsr
or ah, 1
wrmsr
mov eax, strict dword 0
.SavedCr0End:
mov cr0, eax
jmp 0:strict dword 0
.X64JmpEnd:
BITS 64
nop
mov rsp, strict qword 0
.SavedSpEnd:
nop
ret
_EntryPoint:
DD _ToUserCode - ASM_PFX(m16Start)
DW CODE16
_16Gdtr:
DW GDT_SIZE - 1
_16GdtrBase:
DQ 0
_16Idtr:
DW (1 << 10) - 1
DD 0
;------------------------------------------------------------------------------
; _ToUserCode() takes control in real mode before passing control to user code.
; It will be shadowed to somewhere in memory below 1MB.
;------------------------------------------------------------------------------
_ToUserCode:
BITS 16
mov ss, dx ; set new segment selectors
mov ds, dx
mov es, dx
mov fs, dx
mov gs, dx
mov ecx, 0c0000080h
mov cr0, eax ; real mode starts at next instruction
rdmsr
and ah, ~1
wrmsr
mov cr4, ebp
mov ss, si ; set up 16-bit stack segment
mov esp, ebx ; set up 16-bit stack pointer
call dword .Base ; push eip
.Base:
pop ebp ; ebp <- address of .Base
push word [dword esp + IA32_REGS.size + 2]
lea ax, [bp + (.RealMode - .Base)]
push ax
retf ; execution begins at next instruction
.RealMode:
o32 lidt [cs:bp + (_16Idtr - .Base)]
popad
pop ds
pop es
pop fs
pop gs
popfd
lea esp, [esp + 4] ; skip high order 32 bits of EFlags
o32 retf ; transfer control to user code
ALIGN 8
CODE16 equ _16Code - $
DATA16 equ _16Data - $
DATA32 equ _32Data - $
_NullSeg DQ 0
_16Code:
DW -1
DW 0
DB 0
DB 9bh
DB 8fh ; 16-bit segment, 4GB limit
DB 0
_16Data:
DW -1
DW 0
DB 0
DB 93h
DB 8fh ; 16-bit segment, 4GB limit
DB 0
_32Data:
DW -1
DW 0
DB 0
DB 93h
DB 0cfh ; 16-bit segment, 4GB limit
DB 0
GDT_SIZE equ $ - _NullSeg
;------------------------------------------------------------------------------
; IA32_REGISTER_SET *
; EFIAPI
; InternalAsmThunk16 (
; IN IA32_REGISTER_SET *RegisterSet,
; IN OUT VOID *Transition
; );
;------------------------------------------------------------------------------
global ASM_PFX(InternalAsmThunk16)
ASM_PFX(InternalAsmThunk16):
BITS 64
push rbp
push rbx
push rsi
push rdi
mov ebx, ds
push rbx ; Save ds segment register on the stack
mov ebx, es
push rbx ; Save es segment register on the stack
mov ebx, ss
push rbx ; Save ss segment register on the stack
push fs
push gs
mov rsi, rcx
movzx r8d, word [rsi + IA32_REGS._SS]
mov edi, [rsi + IA32_REGS._ESP]
lea rdi, [edi - (IA32_REGS.size + 4)]
imul eax, r8d, 16 ; eax <- r8d(stack segment) * 16
mov ebx, edi ; ebx <- stack for 16-bit code
push IA32_REGS.size / 4
add edi, eax ; edi <- linear address of 16-bit stack
pop rcx
rep movsd ; copy RegSet
lea ecx, [rdx + (_BackFromUserCode.SavedCr4End - ASM_PFX(m16Start))]
mov eax, edx ; eax <- transition code address
and edx, 0fh
shl eax, 12 ; segment address in high order 16 bits
lea ax, [rdx + (_BackFromUserCode - ASM_PFX(m16Start))] ; offset address
stosd ; [edi] <- return address of user code
sgdt [rsp + 60h] ; save GDT stack in argument space
movzx r10, word [rsp + 60h] ; r10 <- GDT limit
lea r11, [rcx + (ASM_PFX(InternalAsmThunk16) - _BackFromUserCode.SavedCr4End) + 0xf]
and r11, ~0xf ; r11 <- 16-byte aligned shadowed GDT table in real mode buffer
mov [rcx + (SavedGdt - _BackFromUserCode.SavedCr4End)], r10w ; save the limit of shadowed GDT table
mov [rcx + (SavedGdt - _BackFromUserCode.SavedCr4End) + 2], r11 ; save the base address of shadowed GDT table
mov rsi, [rsp + 62h] ; rsi <- the original GDT base address
xchg rcx, r10 ; save rcx to r10 and initialize rcx to be the limit of GDT table
inc rcx ; rcx <- the size of memory to copy
xchg rdi, r11 ; save rdi to r11 and initialize rdi to the base address of shadowed GDT table
rep movsb ; perform memory copy to shadow GDT table
mov rcx, r10 ; restore the orignal rcx before memory copy
mov rdi, r11 ; restore the original rdi before memory copy
sidt [rsp + 50h] ; save IDT stack in argument space
mov rax, cr0
mov [rcx + (_BackFromUserCode.SavedCr0End - 4 - _BackFromUserCode.SavedCr4End)], eax
and eax, 7ffffffeh ; clear PE, PG bits
mov rbp, cr4
mov [rcx - 4], ebp ; save CR4 in _BackFromUserCode.SavedCr4End - 4
and ebp, ~30h ; clear PAE, PSE bits
mov esi, r8d ; esi <- 16-bit stack segment
push DATA32
pop rdx ; rdx <- 32-bit data segment selector
lgdt [rcx + (_16Gdtr - _BackFromUserCode.SavedCr4End)]
mov ss, edx
pushfq
lea edx, [rdx + DATA16 - DATA32]
lea r8, [REL .RetFromRealMode]
push r8
mov r8d, cs
mov [rcx + (_BackFromUserCode.X64JmpEnd - 2 - _BackFromUserCode.SavedCr4End)], r8w
mov [rcx + (_BackFromUserCode.SavedSpEnd - 8 - _BackFromUserCode.SavedCr4End)], rsp
jmp dword far [rcx + (_EntryPoint - _BackFromUserCode.SavedCr4End)]
.RetFromRealMode:
popfq
lgdt [rsp + 60h] ; restore protected mode GDTR
lidt [rsp + 50h] ; restore protected mode IDTR
lea eax, [rbp - IA32_REGS.size]
pop gs
pop fs
pop rbx
mov ss, ebx
pop rbx
mov es, ebx
pop rbx
mov ds, ebx
pop rdi
pop rsi
pop rbx
pop rbp
ret
|
