操作系统-特权级与内核安全示例
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Q：通常情况下选择子中的RPL与对应描述符中的DPL相同，那么是否可以取缔RPL？
RPL是保证内核数据安全的关键要素之一；在内核代码中有决定性作用，绝对不能取缔
A.获取操作系统的内核
该实验主要通过调用门调用将内核数据进行打印
具体的实现代码如下
%include "inc.asm"
org 0x9000
jmp ENTRY_SEGMENT
[section .gdt]
; GDT definition
; 段基址， 段界限， 段属性
GDT_ENTRY : Descriptor 0, 0, 0
CODE32_DESC : Descriptor 0, Code32SegLen - 1, DA_C + DA_32 + DA_DPL3
VIDEO_DESC : Descriptor 0xB8000, 0x07FFF, DA_DRWA + DA_32 + DA_DPL3
DATA32_KERNEL_DESC : Descriptor 0, Data32KernelSegLen - 1, DA_DRW + DA_32 + DA_DPL0
DATA32_USER_DESC : Descriptor 0, Data32UserSegLen - 1, DA_DRW + DA_32 + DA_DPL3
STACK32_KERNEL_DESC : Descriptor 0, TopOfKernelStack32, DA_DRW + DA_32 + DA_DPL0
STACK32_USER_DESC : Descriptor 0, TopOfUserStack32, DA_DRW + DA_32 + DA_DPL3
TSS_DESC : Descriptor 0, TSSLen - 1, DA_386TSS + DA_DPL0
FUNCTION_DESC : Descriptor 0, FunctionSegLen - 1, DA_C + DA_32 + DA_DPL0;内核函数段
; Call Gate
; 选择子, 偏移, 参数个数, 属性
FUNC_GETKERNELDATA_DESC : Gate FunctionSelector, GetKernelData, 0, DA_386CGate + DA_DPL3
; GDT end
GdtLen equ $ - GDT_ENTRY
GdtPtr:
dw GdtLen - 1
dd 0
; GDT Selector
Code32Selector equ (0x0001 << 3) + SA_TIG + SA_RPL3
VideoSelector equ (0x0002 << 3) + SA_TIG + SA_RPL3
KernelData32Selector equ (0x0003 << 3) + SA_TIG + SA_RPL0
UserData32Selector equ (0x0004 << 3) + SA_TIG + SA_RPL3
KernelStack32Selector equ (0x0005 << 3) + SA_TIG + SA_RPL0
UserStack32Selector equ (0x0006 << 3) + SA_TIG + SA_RPL3
TSSSelector equ (0x0007 << 3) + SA_TIG + SA_RPL0
FunctionSelector equ (0x0008 << 3) + SA_TIG + SA_RPL0
; Gate Selector
GetKernelDataSelector equ (0x0009 << 3) + SA_TIG + SA_RPL3
; end of [section .gdt]
TopOfStack16 equ 0x7c00
[section .s16]
[bits 16]
ENTRY_SEGMENT:
mov ax, cs
mov ds, ax
mov es, ax
mov ss, ax
mov sp, TopOfStack16
; initialize GDT for 32 bits code segment
mov esi, CODE32_SEGMENT
mov edi, CODE32_DESC
call InitDescItem
mov esi, DATA32_KERNEL_SEGMENT
mov edi, DATA32_KERNEL_DESC
call InitDescItem
mov esi, DATA32_USER_SEGMENT
mov edi, DATA32_USER_DESC
call InitDescItem
mov esi, STACK32_KERNEL_SEGMENT
mov edi, STACK32_KERNEL_DESC
call InitDescItem
mov esi, STACK32_USER_SEGMENT
mov edi, STACK32_USER_DESC
call InitDescItem
mov esi, FUNCTION_SEGMENT
mov edi, FUNCTION_DESC
call InitDescItem
mov esi, TSS_SEGMENT
mov edi, TSS_DESC
call InitDescItem
; initialize GDT pointer struct
mov eax, 0
mov ax, ds
shl eax, 4
add eax, GDT_ENTRY
mov dword [GdtPtr + 2], eax
; 1. load GDT
lgdt [GdtPtr]
; 2. close interrupt
cli
; 3. open A20
in al, 0x92
or al, 00000010b
out 0x92, al
; 4. enter protect mode
mov eax, cr0
or eax, 0x01
mov cr0, eax
; 5. load TSS
mov ax, TSSSelector
ltr ax
; 6. jump to 32 bits code
;jmp word Code32Selector : 0
push UserStack32Selector
push TopOfUserStack32
push Code32Selector
push 0
retf
; esi --> code segment label
; edi --> descriptor label
InitDescItem:
push eax
mov eax, 0
mov ax, cs
shl eax, 4
add eax, esi
mov word [edi + 2], ax
shr eax, 16
mov byte [edi + 4], al
mov byte [edi + 7], ah
pop eax
ret
[section .kdat]
[bits 32]
DATA32_KERNEL_SEGMENT:
KDAT db "Kernel Data", 0
KDAT_LEN equ $ - KDAT
KDAT_OFFSET equ KDAT - $$
Data32KernelSegLen equ $ - DATA32_KERNEL_SEGMENT
[section .udat]
[bits 32]
DATA32_USER_SEGMENT:
UDAT times 16 db 0
UDAT_LEN equ $ - UDAT
UDAT_OFFSET equ UDAT - $$
Data32UserSegLen equ $ - DATA32_USER_SEGMENT
[section .tss]
[bits 32]
TSS_SEGMENT:
dd 0
dd TopOfKernelStack32 ; 0
dd KernelStack32Selector ;
dd 0 ; 1
dd 0 ;
dd 0 ; 2
dd 0 ;
times 4 * 18 dd 0
dw 0
dw $ - TSS_SEGMENT + 2
db 0xFF
TSSLen equ $ - TSS_SEGMENT
[section .s32]
[bits 32]
CODE32_SEGMENT:
mov ax, VideoSelector
mov gs, ax
mov ax, UserData32Selector
mov es, ax
mov di, UDAT_OFFSET
call GetKernelDataSelector : 0
mov ax, UserData32Selector ; eip ==> 0x17
mov ds, ax
mov ebp, UDAT_OFFSET
mov bx, 0x0C
mov dh, 12
mov dl, 33
call PrintString
jmp $
; ds:ebp --> string address
; bx --> attribute
; dx --> dh : row, dl : col
PrintString:
push ebp
push eax
push edi
push cx
push dx
print:
mov cl, [ds:ebp]
cmp cl, 0
je end
mov eax, 80
mul dh
add al, dl
shl eax, 1
mov edi, eax
mov ah, bl
mov al, cl
mov [gs:edi], ax
inc ebp
inc dl
jmp print
end:
pop dx
pop cx
pop edi
pop eax
pop ebp
ret
Code32SegLen equ $ - CODE32_SEGMENT
[section .func]
[bits 32]
FUNCTION_SEGMENT:
; es:di --> data buffer
GetKernelDataFunc:
mov ax, KernelData32Selector
mov ds, ax
mov si, KDAT_OFFSET
mov cx, KDAT_LEN
call KMemCpy
retf
; ds:si --> source
; es:di --> destination
; cx --> length
KMemCpy:
cmp si, di
ja btoe
add si, cx
add di, cx
dec si
dec di
jmp etob
btoe:
cmp cx, 0
jz done
mov al, [ds:si]
mov byte [es:di], al
inc si
inc di
dec cx
jmp btoe
etob:
cmp cx, 0
jz done
mov al, [ds:si]
mov byte [es:di], al
dec si
dec di
dec cx
jmp etob
done:
ret
GetKernelData equ GetKernelDataFunc - $$
FunctionSegLen equ $ - FUNCTION_SEGMENT
[section .kgs]
[bits 32]
STACK32_KERNEL_SEGMENT:
times 256 db 0
Stack32KernelSegLen equ $ - STACK32_KERNEL_SEGMENT
TopOfKernelStack32 equ Stack32KernelSegLen - 1
[section .ugs]
[bits 32]
STACK32_USER_SEGMENT:
times 256 db 0c
Stack32UserSegLen equ $ - STACK32_USER_SEGMENT
TopOfUserStack32 equ Stack32UserSegLen - 1实现结果
从结果中可以看到内核数据被用户进行了拷贝，导致内核数据不安全
B.对上述出现的错误进行改进
初步的解决方法-提出一个检查函数
1.获取段寄存器中RPL的值
2.判断RPL的值是否为SA_RPL0；true则为检查通过，可以继续访问数据，如果为false,特权级较低，出发异常
具体实现是在[section .func] [bits 32]中定义一个检查函数，并在拷贝之前将其进行调用，对RPL的值进行判断
代码如下
[section .func]
[bits 32]
FUNCTION_SEGMENT:
; es:di --> data buffer
GetKernelDataFunc:
mov cx, [esp + 4]
and cx, 0x0003
mov ax, es
and ax, 0xFFFC
or ax, cx
mov es, ax
mov ax, KernelData32Selector
mov ds, ax
mov si, KDAT_OFFSET
mov cx, KDAT_LEN
call KMemCpy
retf
; ds:si --> source
; es:di --> destination
; cx --> length
KMemCpy:
mov ax, es
call CheckRPL;进行调用
cmp si, di
ja btoe
add si, cx
add di, cx
dec si
dec di
jmp etob
btoe:
cmp cx, 0
jz done
mov al, [ds:si]
mov byte [es:di], al
inc si
inc di
dec cx
jmp btoe
etob:
cmp cx, 0
jz done
mov al, [ds:si]
mov byte [es:di], al
dec si
dec di
dec cx
jmp etob
done:
ret
; ax --> selector value
;检查函数
CheckRPL:
and ax, 0x0003;进行与操作
cmp ax, SA_RPL0;进行比较操作
jz valid;进行判断跳转
mov ax, 0
mov fs, ax
mov byte [fs:0], 0;触发异常代码运行结果
从运行结果可以看出，代码没有对内核数据进行拷贝，同时检查函数起到作用， 对RPL的值进行判断，并触发异常进行打印。但是对用户的选择子RPL的值进行修改伪造，将其改为0，又可以将内核数据进行拷贝。
C.用户程序可以通过伪造的选择子中的RPL的值，从而绕开安全检查的机制，在这里需要提出新的解决方案
1.在栈中获取函数远调用前CS寄存器的值(请求者)
2.从之前CS寄存器的值获取RPL
3.用RPL更新到数据缓冲区对应的段寄存器中
4.实验检查函数CheckRPL对段寄存器进行安全检查
代码
%include "inc.asm"
org 0x9000
jmp ENTRY_SEGMENT
[section .gdt]
; GDT definition
; 段基址， 段界限， 段属性
GDT_ENTRY : Descriptor 0, 0, 0
CODE32_DESC : Descriptor 0, Code32SegLen - 1, DA_C + DA_32 + DA_DPL3
VIDEO_DESC : Descriptor 0xB8000, 0x07FFF, DA_DRWA + DA_32 + DA_DPL3
DATA32_KERNEL_DESC : Descriptor 0, Data32KernelSegLen - 1, DA_DRW + DA_32 + DA_DPL0
DATA32_USER_DESC : Descriptor 0, Data32UserSegLen - 1, DA_DRW + DA_32 + DA_DPL3
STACK32_KERNEL_DESC : Descriptor 0, TopOfKernelStack32, DA_DRW + DA_32 + DA_DPL0
STACK32_USER_DESC : Descriptor 0, TopOfUserStack32, DA_DRW + DA_32 + DA_DPL3
TSS_DESC : Descriptor 0, TSSLen - 1, DA_386TSS + DA_DPL0
FUNCTION_DESC : Descriptor 0, FunctionSegLen - 1, DA_C + DA_32 + DA_DPL0;内核函数段
; Call Gate
; 选择子, 偏移, 参数个数, 属性
FUNC_GETKERNELDATA_DESC : Gate FunctionSelector, GetKernelData, 0, DA_386CGate + DA_DPL3
; GDT end
GdtLen equ $ - GDT_ENTRY
GdtPtr:
dw GdtLen - 1
dd 0
; GDT Selector
Code32Selector equ (0x0001 << 3) + SA_TIG + SA_RPL3
VideoSelector equ (0x0002 << 3) + SA_TIG + SA_RPL3
KernelData32Selector equ (0x0003 << 3) + SA_TIG + SA_RPL0
UserData32Selector equ (0x0004 << 3) + SA_TIG + SA_RPL3
KernelStack32Selector equ (0x0005 << 3) + SA_TIG + SA_RPL0
UserStack32Selector equ (0x0006 << 3) + SA_TIG + SA_RPL3
TSSSelector equ (0x0007 << 3) + SA_TIG + SA_RPL0
FunctionSelector equ (0x0008 << 3) + SA_TIG + SA_RPL0
; Gate Selector
GetKernelDataSelector equ (0x0009 << 3) + SA_TIG + SA_RPL3
; end of [section .gdt]
TopOfStack16 equ 0x7c00
[section .s16]
[bits 16]
ENTRY_SEGMENT:
mov ax, cs
mov ds, ax
mov es, ax
mov ss, ax
mov sp, TopOfStack16
; initialize GDT for 32 bits code segment
mov esi, CODE32_SEGMENT
mov edi, CODE32_DESC
call InitDescItem
mov esi, DATA32_KERNEL_SEGMENT
mov edi, DATA32_KERNEL_DESC
call InitDescItem
mov esi, DATA32_USER_SEGMENT
mov edi, DATA32_USER_DESC
call InitDescItem
mov esi, STACK32_KERNEL_SEGMENT
mov edi, STACK32_KERNEL_DESC
call InitDescItem
mov esi, STACK32_USER_SEGMENT
mov edi, STACK32_USER_DESC
call InitDescItem
mov esi, FUNCTION_SEGMENT
mov edi, FUNCTION_DESC
call InitDescItem
mov esi, TSS_SEGMENT
mov edi, TSS_DESC
call InitDescItem
; initialize GDT pointer struct
mov eax, 0
mov ax, ds
shl eax, 4
add eax, GDT_ENTRY
mov dword [GdtPtr + 2], eax
; 1. load GDT
lgdt [GdtPtr]
; 2. close interrupt
cli
; 3. open A20
in al, 0x92
or al, 00000010b
out 0x92, al
; 4. enter protect mode
mov eax, cr0
or eax, 0x01
mov cr0, eax
; 5. load TSS
mov ax, TSSSelector
ltr ax
; 6. jump to 32 bits code
;jmp word Code32Selector : 0
push UserStack32Selector
push TopOfUserStack32
push Code32Selector
push 0
retf
; esi --> code segment label
; edi --> descriptor label
InitDescItem:
push eax
mov eax, 0
mov ax, cs
shl eax, 4
add eax, esi
mov word [edi + 2], ax
shr eax, 16
mov byte [edi + 4], al
mov byte [edi + 7], ah
pop eax
ret
[section .kdat]
[bits 32]
DATA32_KERNEL_SEGMENT:
KDAT db "Kernel Data", 0
KDAT_LEN equ $ - KDAT
KDAT_OFFSET equ KDAT - $$
Data32KernelSegLen equ $ - DATA32_KERNEL_SEGMENT
[section .udat]
[bits 32]
DATA32_USER_SEGMENT:
UDAT times 16 db 0
UDAT_LEN equ $ - UDAT
UDAT_OFFSET equ UDAT - $$
Data32UserSegLen equ $ - DATA32_USER_SEGMENT
[section .tss]
[bits 32]
TSS_SEGMENT:
dd 0
dd TopOfKernelStack32 ; 0
dd KernelStack32Selector ;
dd 0 ; 1
dd 0 ;
dd 0 ; 2
dd 0 ;
times 4 * 18 dd 0
dw 0
dw $ - TSS_SEGMENT + 2
db 0xFF
TSSLen equ $ - TSS_SEGMENT
[section .s32]
[bits 32]
CODE32_SEGMENT:
mov ax, VideoSelector
mov gs, ax
mov ax, UserData32Selector
mov es, ax
mov di, UDAT_OFFSET
call GetKernelDataSelector : 0
mov ax, UserData32Selector ; eip ==> 0x17
mov ds, ax
mov ebp, UDAT_OFFSET
mov bx, 0x0C
mov dh, 12
mov dl, 33
call PrintString
jmp $
; ds:ebp --> string address
; bx --> attribute
; dx --> dh : row, dl : col
PrintString:
push ebp
push eax
push edi
push cx
push dx
print:
mov cl, [ds:ebp]
cmp cl, 0
je end
mov eax, 80
mul dh
add al, dl
shl eax, 1
mov edi, eax
mov ah, bl
mov al, cl
mov [gs:edi], ax
inc ebp
inc dl
jmp print
end:
pop dx
pop cx
pop edi
pop eax
pop ebp
ret
Code32SegLen equ $ - CODE32_SEGMENT
[section .func]
[bits 32]
FUNCTION_SEGMENT:
; es:di --> data buffer
GetKernelDataFunc:
mov cx, [esp + 4]
and cx, 0x0003
mov ax, es
and ax, 0xFFFC
or ax, cx
mov es, ax
mov ax, KernelData32Selector
mov ds, ax
mov si, KDAT_OFFSET
mov cx, KDAT_LEN
call KMemCpy
retf
; ds:si --> source
; es:di --> destination
; cx --> length
KMemCpy:
mov ax, es
call CheckRPL;进行调用
cmp si, di
ja btoe
add si, cx
add di, cx
dec si
dec di
jmp etob
btoe:
cmp cx, 0
jz done
mov al, [ds:si]
mov byte [es:di], al
inc si
inc di
dec cx
jmp btoe
etob:
cmp cx, 0
jz done
mov al, [ds:si]
mov byte [es:di], al
dec si
dec di
dec cx
jmp etob
done:
ret
; ax --> selector value
;检查函数
CheckRPL:
and ax, 0x0003;进行与操作
cmp ax, SA_RPL0;进行比较操作
jz valid;进行判断跳转
mov ax, 0
mov fs, ax
mov byte [fs:0], 0;触发异常
valid:
ret
GetKernelData equ GetKernelDataFunc - $$
FunctionSegLen equ $ - FUNCTION_SEGMENT
[section .kgs]
[bits 32]
STACK32_KERNEL_SEGMENT:
times 256 db 0
Stack32KernelSegLen equ $ - STACK32_KERNEL_SEGMENT
TopOfKernelStack32 equ Stack32KernelSegLen - 1
[section .ugs]
[bits 32]
STACK32_USER_SEGMENT:
times 256 db 0
Stack32UserSegLen equ $ - STACK32_USER_SEGMENT
TopOfUserStack32 equ Stack32UserSegLen - 1首先通过反编译在call GetKernelDataSelector : 0跳转处设置断点，接下来对通用寄存器eip寄存器进行查看，由92a8到92af之间的差值为7，可以推断mov ax, UserData32Selector ; eip ==> 0x17，该处是返回地址。同时0x17会入栈，cs也会入栈，对寄存器进行查看，此时cs寄存器为0x0b，接下来对栈顶的6个字节进行查看，发现0x17与cs寄存器都入栈，再继续操作，触发异常，过程如图所示
小结
1.RPL是保证内核数据安全的关键要素之一
2.内核代码可通过追踪真实请求者特权级判断操作合法性
3.进行函数远调用时，真实请求者的选择子就会存储于栈中
4.通过提取真实特权级能够保证内核数据安全
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