scp: a system call protector against buffer overflow attacks dove 邱秉誠

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

Buffer Overflow Attacks Buffer Overflow Attacks

dove邱秉誠

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OutlineOutline

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

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Introduction (1)Introduction (1)

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

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Introduction (2)Introduction (2)

Many countermeasures were publishedBut also were defeated

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

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Attacking Method (1)Attacking Method (1)

Stack Overflow Attack• Overflow the return address to launch injected

code (shell code)

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Return-into-libc Attack Overwrite the return address to execute system() functi

on 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

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Heap Overflow Attack• Similar to stack overflow, but on the heap area

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

• Usually used to bypass some protections

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Related Work (1)Related Work (1)

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

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Related Work (2)Related Work (2) StackGuard -- add canary word before return addre

ss 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

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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

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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

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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

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Binary Obfuscation• Change ELF layout (add new section)• Add new system call to notify the return ad

dress• Insert junk code in binary object

Attacker may use the new system callReturn into libc attack / scanning code attac

k can be launched

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PointGuard™• Encrypt pointer, decrypt when reference object

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Instruction Set Randomization• Hardware solution, encrypt / decrypt CPU

instructions

porting binaries

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

ue

Need recompile

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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

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

Assumptions• Malicious code have to use system call to damage s

ystem• vulnerable program is dynamic linked

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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

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The system call path

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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

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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

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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 Address

read();

jmp *GOT[__libc_read]

Address of __libc_read

__libc_read()

sys_read()

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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()


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Example with SCP system (lazy binding)

Loader (ld-linux.so.2) kernel

program

Glibc: printf() here

2. Loading program


Inject code

3. Load libc.so.6

6. Normal system call

4. Call printf()


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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

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Introduce fake pages

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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

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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

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Experimental Result (1)Experimental Result (1)

Efficiency test• Buffer overflow attack with injected code

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Efficiency test• Calling notify_kernel test

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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 %

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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

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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

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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

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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

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Macro test• 100,000 times per command

CommandOriginal 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 %

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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

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Future WorkFuture Work

More secure• Implement “executable but non-readable”

region in segment section on i386• The NX Bit chip• AMD 64 CPU

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scp: a system call protector against buffer overflow attacks dove 邱秉誠

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