Transcript Venturing into protected-mode

Venturing into protected-mode
Exploiting the x86 segment-registers’
hidden segment-descriptor cache to
briefly enter and leave protected-mode
The system-segment registers
• In protected-mode the CPU needs quick
access to some important data-structures,
such as:
– Memory-Segment Descriptors
– Interrupt-Gate Descriptors
– Call-Gate Descriptors
– Task-State Descriptors
– Page-Directory and Page-Table Descriptors
• Special CPU registers locate these items
GDT and IDT
• The two most vital system registers for
protected-mode execution are:
– GDTR (Global Descriptor Table Register)
– IDTR (Interrupt Descriptor Table Register)
• Each of these is 48-bits wide and contains
the base-address and segment-limit for an
array of descriptors (the GDT and the IDT)
• Special instructions allow access to these
registers: SGDT/LGDT and SIDT/LIDT
48-bit Register-Format
47
16 15
0
Segment Base-Address
Segment
Limit
32 bits
16 bits
System Relationships
GDTR
IDTR
Global Descriptor
Table
descriptor
descriptor
descriptor
descriptor
descriptor
descriptor
descriptor
Interrupt Descriptor
Table
descriptor
descriptor
descriptor
descriptor
descriptor
descriptor
descriptor
descriptor
descriptor
descriptor
descriptor
descriptor
descriptor
LDT and TSS
• For protected-mode multitasking, the CPU
needs to access two other data-structures:
– The current Local Descriptor Table (LDT)
– The current Task-State Segment (TSS)
• Again, special registers tell the CPU where
to find these data-structures in memory
(assuming protected-mode is enabled)
• And special instructions afford access to
them: SLDT/LLDT and STR/LTR
Indirection
• Registers LDTR and TR are like segmentregisters: they have a visible part (16-bits)
and a “hidden” descroptor-cache part
• The programmer-visible portion of these
two registers holds a “segment-selector”
(i.e., an array-index into the GDT array)
• The hidden portion is updated from the
GDT whenever these register get loaded
System Relationships
TR
LDTR
GDTR
Global Descriptor
Table
descriptor
descriptor
descriptor
descriptor
descriptor
descriptor
descriptor
descriptor
descriptor
descriptor
descriptor
descriptor
Task
State
Segment
Local Descriptor
Table
descriptor
descriptor
descriptor
descriptor
descriptor
descriptor
Reading LDTR and TR
• The LDTR and TR registers are not able to
be accessed while executing in real-mode
• An “Undefined Opcode” exception (INT-6)
will be generated if SLDT or STR opcodes
are encountered in a “real-mode” program
• So to obtain the values in these registers,
any bootsector program must temporarily
enable protected-mode
Control Register 0
• Register CR0 is the 32-bit version of the
MSW register (Machine Status Word)
• It contains the PE-bit (Protection Enabled)
– when PE=0 the CPU is in real-mode
– when PE=1 the CPU is in protected-mode
PCN
G DW
A
M
W
P
NE T EMP
E T SMP E
Machine Status Word
Using the LMSW instruction
• You can use the LMSW instruction to turn
on the PE-bit (enables protected-mode)
• But you cannot use LMSW to turn off PE
(i.e., PE was a “sticky bit” in the 80286)
• The Intel 80386 processor introduced a
new name and enlarged size for the MSW
• Special version of the ‘MOV’ instruction
can either enable or disable the PE-bit
How to enter protected-mode
; This instruction-sequence turns on PE-bit
mov eax, cr0
bts
eax, #0
mov cr0, eax
; Warning: you need to do this with interrupts
; temporarily disabled, since the real-mode
; Interrupt Vector Table won’t work any more
How to leave protected-mode
; This instruction-sequence turns off PE-bit
mov eax, cr0
btr
eax, #0
mov cr0, eax
; Warning: you need to make sure that all of
; the segment-registers have proper access; rights and segment-limits in their caches to
; work correctly when back in real-mode!
Unmodified segment-registers
• If you can arrange for your program not to
change any segment-registers while PE=1
then your code can safely enter and leave
protected-mode without creating GDT/IDT
• This means you will have to have to make
sure no interrupts or exceptions can occur
while the PE-bit is set for protected-mode
• Can use cli and sti to control interrupts
• Avoid exceptions by doing nothing illegal
In-class Exercise
• The ‘sysregs.s’ bootsector demo-program
displays the current value found in the two
48-bit system registers: GDTR and IDTR
• Your job is to add modifications that demo
• Modify the ‘sysregs.s’ bootsector program
so it will display registers LDTR and TR in
addition to displaying GDTR and IDTR
• Can you enter and leave protected-mode
without causing a system “crash”?