VeraCrypt/src/Boot/Windows/BootSector.asm

;
; Derived from source code of TrueCrypt 7.1a, which is
; Copyright (c) 2008-2012 TrueCrypt Developers Association and which is governed
; by the TrueCrypt License 3.0.
;
; Modifications and additions to the original source code (contained in this file)
; and all other portions of this file are Copyright (c) 2013-2016 IDRIX
; and are governed by the Apache License 2.0 the full text of which is
; contained in the file License.txt included in VeraCrypt binary and source
; code distribution packages.
;

.MODEL tiny
.386
_TEXT SEGMENT USE16

INCLUDE BootDefs.i

ORG 7C00h	; Standard boot sector offset

start:
	; BIOS executes boot sector from 0:7C00 or 7C0:0000 (default CD boot loader address).
	; Far jump to the next instruction sets IP to the standard offset 7C00.
	db 0EAh				; jmp 0:main
	dw main, 0

loader_name_msg:
	db ' VeraCrypt Boot Loader', 13, 10, 0

main:
	cli
	xor ax, ax
	mov ds, ax
	mov ss, ax
	mov sp, 7C00h
	sti

	; Display boot loader name
	test byte ptr [start + TC_BOOT_SECTOR_USER_CONFIG_OFFSET], TC_BOOT_USER_CFG_FLAG_SILENT_MODE
	jnz skip_loader_name_msg

	lea si, loader_name_msg
	call print
skip_loader_name_msg:

	; Determine boot loader segment
	mov ax, TC_BOOT_LOADER_SEGMENT

	; Check available memory
	cmp word ptr [ds:413h], TC_BOOT_LOADER_SEGMENT / 1024 * 16 + TC_BOOT_MEMORY_REQUIRED
	jge memory_ok

	mov ax, TC_BOOT_LOADER_SEGMENT_LOW

	cmp word ptr [ds:413h], TC_BOOT_LOADER_SEGMENT_LOW / 1024 * 16 + TC_BOOT_MEMORY_REQUIRED
	jge memory_ok

	; Insufficient memory
	mov ax, TC_BOOT_LOADER_LOWMEM_SEGMENT

memory_ok:
	mov es, ax

	; Clear BSS section
	xor al, al
	mov di, TC_COM_EXECUTABLE_OFFSET
	mov cx, TC_BOOT_MEMORY_REQUIRED * 1024 - TC_COM_EXECUTABLE_OFFSET - 1
	cld
	rep stosb

	mov ax, es
	sub ax, TC_BOOT_LOADER_DECOMPRESSOR_MEMORY_SIZE / 16	; Decompressor segment
	mov es, ax

	; Load decompressor
	mov cl, TC_BOOT_LOADER_DECOMPRESSOR_START_SECTOR
retry_backup:
	mov al, TC_BOOT_LOADER_DECOMPRESSOR_SECTOR_COUNT
	mov bx, TC_COM_EXECUTABLE_OFFSET
	call read_sectors

	; Decompressor checksum
	xor ebx, ebx
	mov si, TC_COM_EXECUTABLE_OFFSET
	mov cx, TC_BOOT_LOADER_DECOMPRESSOR_SECTOR_COUNT * TC_LB_SIZE
	call checksum
	push ebx

	; Load compressed boot loader
	mov bx, TC_BOOT_LOADER_COMPRESSED_BUFFER_OFFSET
	mov cl, TC_BOOT_LOADER_START_SECTOR
	mov al, TC_MAX_BOOT_LOADER_SECTOR_COUNT

	test backup_loader_used, 1
	jz non_backup
	mov al, TC_BOOT_LOADER_BACKUP_SECTOR_COUNT - TC_BOOT_LOADER_DECOMPRESSOR_SECTOR_COUNT
	mov cl, TC_BOOT_LOADER_START_SECTOR + TC_BOOT_LOADER_BACKUP_SECTOR_COUNT

non_backup:
	call read_sectors

	; Boot loader checksum
	pop ebx
	mov si, TC_BOOT_LOADER_COMPRESSED_BUFFER_OFFSET
	mov cx, word ptr [start + TC_BOOT_SECTOR_LOADER_LENGTH_OFFSET]
	call checksum

	; Verify checksum
	cmp ebx, dword ptr [start + TC_BOOT_SECTOR_LOADER_CHECKSUM_OFFSET]
	je checksum_ok

	; Checksum incorrect - try using backup if available
	test backup_loader_used, 1
	jnz loader_damaged

	mov backup_loader_used, 1
	mov cl, TC_BOOT_LOADER_DECOMPRESSOR_START_SECTOR + TC_BOOT_LOADER_BACKUP_SECTOR_COUNT

	test TC_BOOT_CFG_FLAG_BACKUP_LOADER_AVAILABLE, byte ptr [start + TC_BOOT_SECTOR_CONFIG_OFFSET]
	jnz retry_backup

loader_damaged:
	lea si, loader_damaged_msg
	call print
	lea si, loader_name_msg
	call print
	jmp $
checksum_ok:

	; Set up decompressor segment
	mov ax, es
	mov ds, ax
	cli
	mov ss, ax
	mov sp, TC_BOOT_LOADER_DECOMPRESSOR_MEMORY_SIZE
	sti

	push dx

	; Decompress boot loader
	mov cx, word ptr [start + TC_BOOT_SECTOR_LOADER_LENGTH_OFFSET]
	sub cx, TC_GZIP_HEADER_SIZE
	push cx																		; Compressed data size
	push TC_BOOT_LOADER_COMPRESSED_BUFFER_OFFSET + TC_GZIP_HEADER_SIZE			; Compressed data
	push TC_MAX_BOOT_LOADER_DECOMPRESSED_SIZE									; Output buffer size
	push TC_BOOT_LOADER_DECOMPRESSOR_MEMORY_SIZE + TC_COM_EXECUTABLE_OFFSET		; Output buffer

	push cs
	push decompressor_ret
	push es
	push TC_COM_EXECUTABLE_OFFSET
	retf
decompressor_ret:

	add sp, 8
	pop dx

	; Restore boot sector segment
	push cs
	pop ds

	; Check decompression result
	test ax, ax
	jz decompression_ok

	lea si, loader_damaged_msg
	call print
	jmp $
decompression_ok:

	; DH = boot sector flags
	mov dh, byte ptr [start + TC_BOOT_SECTOR_CONFIG_OFFSET]

	; Set up boot loader segment
	mov ax, es
	add ax, TC_BOOT_LOADER_DECOMPRESSOR_MEMORY_SIZE / 16
	mov es, ax
	mov ds, ax
	cli
	mov ss, ax
	mov sp, TC_BOOT_LOADER_STACK_TOP
	sti

	; Execute boot loader
	push es
	push TC_COM_EXECUTABLE_OFFSET
	retf

	; Print string
print:
	xor bx, bx
	mov ah, 0eh
	cld

@@:	lodsb
	test al, al
	jz print_end

	int 10h
	jmp @B

print_end:
	ret

	; Read sectors of the first cylinder
read_sectors:
	mov ch, 0           ; Cylinder
	mov dh, 0           ; Head
						; DL = drive number passed from BIOS
	mov ah, 2
	int 13h
	jnc read_ok

	lea si, disk_error_msg
	call print
read_ok:
	ret

	; Calculate checksum
checksum:
	push ds
	push es
	pop ds
	xor eax, eax
	cld

@@:	lodsb
	add ebx, eax
	rol ebx, 1
	loop @B

	pop ds
	ret

backup_loader_used		db 0

disk_error_msg			db 'Disk error', 13, 10, 7, 0
loader_damaged_msg		db 7, 'Loader damaged! Repair with Rescue Disk', 0

ORG 7C00h + 510
	dw 0AA55h			; Boot sector signature

_TEXT ENDS
END start