SCP: SCP: A System Call Protector against A System Call Protector against

Buffer Overflow Attacks Buffer Overflow Attacks

dove邱秉誠

22

OutlineOutline Introduction Attacking Method Related Work SCP System Design Experimental Result Conclusion Future Work

33

Introduction (1)Introduction (1) Buffer Overflow Attack

• Easily launched• Huge amount of targets• Strongly damage• The most dangerous threat in the internet

44

Introduction (2)Introduction (2) Many countermeasures were published

But also were defeated

Developing an efficient and effective approach becomes a critical and emergent issue

55

Attacking Method (1)Attacking Method (1) Stack Overflow Attack

• Overflow the return address to launch injected code (shell code)

66

Attacking Method (2)Attacking Method (2) Return-into-libc Attack Overwrite the return address to execute system() function call

buffer

EBP

EIP (RA)

Fake RA

Pointer to system()’s arg

system()ESP

String format: AAA… address to system()

system()’s return address pointer to system()’s arg

“/bin/sh”High Address

Low Address

77

Attacking Method (3)Attacking Method (3) Heap Overflow Attack

• Similar to stack overflow, but on the heap area

88

Attacking Method (4)Attacking Method (4) Scanning Code Attack

• After overflow, scan the process image to find patterns and then jump into it

• Usually used to bypass some protections

99

Related Work (1)Related Work (1) Some Countermeasures

• StackGuard / StackShield• Address Obfuscation• Exec Shield• Binary Obfuscation• PointGuard™• Instruction Set Randomization• RAD

1010

Related Work (2)Related Work (2) StackGuard -- add canary word before return address StackShield --copy return address to confirm when return

Bypassing StackGuard and StackShield

return address

canary word

Buffer

High Address

Low Address

…

return address

Buffer

…

Saved return address

…

High Address

Low Address

1111

Related Work (3)Related Work (3) Address Obfuscation

• PaX ASLR project- Randomize the base address of memory regions

-- Randomize the base address of stack/heap-- Randomize the starting address of dynamic-linked libraries-- Randomize the locations of routines and static data

• ASLP• Internal fragmentation problem• Derandomization Attack

1212

Related Work (4)Related Work (4) Bound Checking

• Bound Checking for C

• Require source code / recompile• Runtime overhead are huge

- 4x / 5x when best case- 10x general case- 100x worst case

1313

Related Work (5)Related Work (5) Exec Shield

• Data/Stack section non-executable• Code section non-writable• Compatibility problem - ELF file format -- Add PT_GNU_STACK and PT_GNU_HEAP - Nested function - Recompile / porting - sigreturn() system call• Return into libc attack can be launched

1414

Related Work (6)Related Work (6) Binary Obfuscation

• Change ELF layout (add new section)• Add new system call to notify the return address• Insert junk code in binary objectAttacker may use the new system callReturn into libc attack / scanning code attack can be launched

1515

Related Work (7)Related Work (7) PointGuard™

• Encrypt pointer, decrypt when reference object

1616

Related Work (8)Related Work (8) Instruction Set Randomization

• Hardware solution, encrypt / decrypt CPU instructions

porting binaries

1717

Related Work (9)Related Work (9) RAD

• Return Address Defender• Compiler solution• Push return address to RAR when prologue• Pop return address from RAR when eprologue

Need recompile

1818

SCP System Design (1)SCP System Design (1) Principles

• Prevent attacker from executing int 80 offered by attacker• Prevent attacker from executing int 80 existed in system

Goal• Low overhead• Efficient to protect• Do not require source code• Compatibility• Use less system resource• Easy to maintain

1919

SCP System Design (2)SCP System Design (2) Assumptions

• Malicious code have to use system call to damage system• vulnerable program is dynamic linked

2020

SCP System Design (3)SCP System Design (3) System overview

• protect int 0x80’s return address

• Use secure enter kernel instead of all sysenter (int $0x80)

movl sys#, %eaxmovl arg1, %ebx…int $0x80

2121

SCP System Design (4)SCP System Design (4) The system call path

2222

SCP System Design (5)SCP System Design (5)

The system call path

6. return

5. return

User Space

Kernel Space

( user program )open();

( libc wrapper routine )__libc_open()

( kernel )sys_open()

High Address

Low Address1. go to PLT

4. trap into kernel

( PLT )jmp *GOT[__libc_open]

( GOT entry )Address of __libc_open

2. lookup GOT

3. call wrapper routine

2323

SCP System Design (6)SCP System Design (6)

Pseudo Code(b) Trap Code(a) Secure Enter Kernel

save_all_registers;page = 0; size = 0;if ( page == 0 ) { page = mmap2(); size = copy_trap_code(page); notify_kernel(size+6);}restore_all_registers;call page;

restore_sys#_in_eax;int $0x80; (sysenter)return_to_libc;

machine code:\x8B\x44\x24\x04\xCD\x80\x83\xC4\x08\xC3

2424

SCP System Design (7)SCP System Design (7) New system call path (with SCP system)

8. return

7. return

User Space

Kernel Space

( user program )open();

( libc wrapper routine )__libc_open()

( kernel )sys_open()

1. go to PLT

5. trap into kernel

( PLT )jmp *GOT[__libc_open]

( GOT entry )Address of __libc_open

2. lookup GOT

3. call wrapper routine

( trap page )int $0x80

4. call trap page

6. return

High Address

Low Addressread();

jmp *GOT[__libc_read]

Address of __libc_read

__libc_read()

sys_read()

2525

SCP System Design (8)SCP System Design (8)

Example with SCP system (lazy binding)

Loader (ld-linux.so.2) kernel

program

Glibc: printf() here

2. Loading program

1. Notify kernel the RA

Inject code

3. Load libc.so.6

6. Normal system call

4. Call printf()

5. Notify kernel the RA

2626

SCP System Design (9)SCP System Design (9)

Example with SCP system (lazy binding)

Loader (ld-linux.so.2) kernel

program

Glibc: printf() here

2. Loading program

1. Notify kernel the RA

Inject code

3. Load libc.so.6

6. Normal system call

4. Call printf()

5. Notify kernel the RA

2727

SCP System Design (10)SCP System Design (10)

Modify kernel• Add new system call notify_kernel() - It can only be called 2 times per process• Add new structures in task_struct to restore addresses - loader_return_address - syscall_return_address• do_fork() - Addresses copied from parent process• sys_execve() - loader_return_address = 0 - syscall_return_address = 0

2828

SCP System Design (11)SCP System Design (11) Introduce fake pages

2929

SCP System Design (12)SCP System Design (12) Attack analysis

• Attacker can launch scanning code attack to trace real int $0x80

• Possible solution: - Non-readable but executable code segment - … future work

(user program)system_call (PLT) (GOT) (libc)

wrapper routine(heap)

int $0x80

3030

SCP System Design (13)SCP System Design (13) SCP system analysis

• Allow executable stack - General debugger support

- Nested function- Do not require ELF modified- Do not require to recompile

• Allow execute programs without ASLR- No internal fragment problem- Process crashes are decided by process owners- Easy to maintain

3131

Experimental Result (1)Experimental Result (1) Efficiency test

• Buffer overflow attack with injected code

3232

Experimental Result (2)Experimental Result (2) Efficiency test

• Calling notify_kernel test

3333

Experimental Result (3)Experimental Result (3) Micro test

• 10,000,000 times per system call

System CallOriginal libc & Kerne

l(usec)

Secure libc & Secure Kernel

(usec)Increment

mmap 4.83598861 5.04285570 4.28 %

open 5.70100183 12.31045995 115.93 %

read 4.44757121 4.65731530 4.72 %

write 28.61905470 28.86815789 0.87 %

3434

Experimental Result (4)Experimental Result (4)

Original libc & kernelexecve("./micro-test.open", ["./micro-test.open"], [/* 29 vars */]) = 0% time seconds usecs/call calls errors syscall------ ----------- ----------- --------- --------- ---------------- 49.40 828.885786 41 20000000 gettimeofday 25.81 432.1044308 43 10000003 1 open 24.59 411.1630811 41 10000002 close 0.04 0.700248 700248 1 brk 0.04 0.700211 700211 1 mprotect 0.04 0.700202 700202 1 read 0.02 0.402909 201455 2 write 0.02 0.400418 200209 2 munmap 0.01 0.201221 33537 6 old_mmap 0.01 0.100622 33541 3 fstat64------ ----------- ----------- --------- --------- ----------------100.00 1677.766736 40000021 1 total

3535

avg timediff = 0.0000073216265842318534575253186069687672 sec = 7.32162658 usecCommand exited with non-zero status 81 Command being timed: "./micro-test.open" User time (seconds): 0.69 System time (seconds): 122.31 Percent of CPU this job got: 100% Elapsed (wall clock) time (h:mm:ss or m:ss): 2:03.00 Average shared text size (kbytes): 0 Average unshared data size (kbytes): 0 Average stack size (kbytes): 0 Average total size (kbytes): 0 Maximum resident set size (kbytes): 0 Average resident set size (kbytes): 0 Major (requiring I/O) page faults: 82 Minor (reclaiming a frame) page faults: 7 Voluntary context switches: 0 Involuntary context switches: 0 Swaps: 0 File system inputs: 0 File system outputs: 0 Socket messages sent: 0 Socket messages received: 0 Signals delivered: 0 Page size (bytes): 4096 Exit status: 81

3636

Experimental Result (5)Experimental Result (5)

Modified libc & kernelexecve("./micro-test.open", ["./micro-test.open"], [/* 29 vars */]) = 0% time seconds usecs/call calls errors syscall------ ----------- ----------- --------- --------- ---------------- 47.44 821.1515639 41 20000000 gettimeofday 28.54 494.834607 49 10000003 1 open 23.82 412.936479 41 10000002 close 0.04 0.700382 700382 1 read 0.04 0.700380 700380 1 brk 0.02 0.403299 201650 2 write 0.02 0.400813 200407 2 mmap2 0.02 0.400782 200391 2 munmap 0.02 0.400761 200381 2 utimes 0.01 0.202308 33718 6 old_mmap 0.01 0.101257 33752 3 mprotect 0.01 0.101186 33729 3 fstat64------ ----------- ----------- --------- --------- ----------------100.00 1733.697893 40000027 1 total

3737

avg timediff = 0.0000123534624044317750067014868853298992 sec = 12.35346240 usecCommand exited with non-zero status 82 Command being timed: "./micro-test.open" User time (seconds): 0.68 System time (seconds): 173.26 Percent of CPU this job got: 99% Elapsed (wall clock) time (h:mm:ss or m:ss): 2:53.94 Average shared text size (kbytes): 0 Average unshared data size (kbytes): 0 Average stack size (kbytes): 0 Average total size (kbytes): 0 Maximum resident set size (kbytes): 0 Average resident set size (kbytes): 0 Major (requiring I/O) page faults: 104 Minor (reclaiming a frame) page faults: 13 Voluntary context switches: 0 Involuntary context switches: 0 Swaps: 0 File system inputs: 0 File system outputs: 0 Socket messages sent: 0 Socket messages received: 0 Signals delivered: 0 Page size (bytes): 4096 Exit status: 82

3838

Experimental Result (5)Experimental Result (5) Macro test

• 100,000 times per command

Command Original libc & Kernel(sec)

Secure libc & Secure Kernel(sec) Increment

ls 0.00781023 0.00922095 18.06 %

make 0.01481522 0.01697969 14.61 %

sysctl 0.02905236 0.03447007 18.65 %

tar 0.00804451 0.00940219 16.88 %

gcc 0.98855523 1.00293709 01.45 %

3939

ConclusionConclusion We propose a new method to protect system calls by registering valid int 80 on premise

• Without recompile• Allow executable stack• Use ASLR optionally• Compatible with other protections

4040

Future WorkFuture Work More secure

• Implement “executable but non-readable” region in segment section on i386

• The NX Bit chip• AMD 64 CPU

4141

Thanks Q & A