X86 Assembly Buffer Overflow III:1

Transcription

1 X86 Assembly Buffer Overflow III:1

2 Admin Link to buffer overflow demo ASM quick-reference from Larry Zhang (thanks!) Need a operand argument summary too? Best tool (so far) for C and ASM exploration ddd (Linux) We will use this later in labs too III:2

3 Administrivia HW3 skip the farmer game portion, moved to HW5 Due today HW4 Endian swap in ASM and C Quiz2 please do your correction in RED Do not erase original answers Quiz3 due Wednesday How am I doing? Please know that there is no way we will cover everything you need to know in the book. So read. III:3

4 IA32 Linux Memory Layout Stack Runtime stack (8MB limit) Heap Dynamically allocated storage When call malloc(), calloc(), new() Data Statically allocated data E.g., arrays & strings declared in code Text Executable machine instructions Read-only FF not drawn to scale Stack 8MB Upper 2 hex digits = 8 bits of address Heap Data Text III:4

5 Memory Allocation Example FF not drawn to scale Stack char big_array[1<<24]; /* 16 MB */ char huge_array[1<<28]; /* 256 MB */ int beyond; char *p1, *p2, *p3, *p4; int useless() { return 0; } int main() { p1 = malloc(1 <<28); /* 256 MB */ p2 = malloc(1 << 8); /* 256 B */ p3 = malloc(1 <<28); /* 256 MB */ p4 = malloc(1 << 8); /* 256 B */ /* Some print statements... */ } Where does everything go? Heap Data Text III:5

6 IA32 Example Addresses address range ~2 32 FF not drawn to scale Stack $esp p3 p1 p4 p2 &p2 beyond big_array huge_array main() useless() final malloc() 0xffffbcd0 0x x x1904a110 0x1904a008 0x x x x x080483c6 0x x006be Heap malloc() is dynamically linked address determined at runtime Data Text III:6

7 Pointers How about some pointers to deal with the new boss? Sure. Try 0x0000A4F5, 0x00008EEF and 0x0000B32A. III:7

8 C operators Operators Associativity () [] ->. left to right! ~ * & (type) sizeof right to left * / % left to right + - left to right << >> left to right < <= > >= left to right ==!= left to right & left to right ^ left to right left to right && left to right left to right?: right to left = += -= *= /= %= &= ^=!= <<= >>= right to left, left to right -> has very high precedence ()has very high precedence monadic *just below III:8

9 C Pointer Declarations: Test Yourself! int *p int *p[13] int *(p[13]) int **p int (*p)[13] int *f() int (*f)() int (*(*f())[13])() int (*(*x[3])())[5] pis a pointer to int p is an array[13] of pointer to int p is an array[13] of pointer to int pis a pointer to a pointer to an int p is a pointer to an array[13] of int f is a function returning a pointer to int f is a pointer to a function returning int f is a function returning ptrto an array[13] of pointers to functions returning int x is an array[3] of pointers to functions returning pointers to array[5] of ints III:9

10 C Pointer Declarations: Test Yourself! int *p int *p[13] int *(p[13]) int **p int (*p)[13] int *f() int (*f)() int (*(*f())[13])() int (*(*x[3])())[5] pis a pointer to int p p is is an an array[13] of of pointer to to int int p is an array[13] of pointer to int pis a pointer to a pointer to an int p is a pointer to an array[13] of int f is a function returning a pointer to int f is a pointer to a function returning int f is a function returning ptrto an array[13] of pointers to functions returning int x is an array[3] of pointers to functions returning pointers to array[5] of ints III:10

11 C Pointer Declarations: Test Yourself! int *p int *p[13] int *(p[13]) int **p int (*p)[13] int *f() int (*f)() int (*(*f())[13])() int (*(*x[3])())[5] pis a pointer to int p p is is an an array[13] of of pointer to to int int p p is is an an array[13] of of pointer to to int int pis a pointer to a pointer to an int p is a pointer to an array[13] of int f is a function returning a pointer to int f is a pointer to a function returning int f is a function returning ptrto an array[13] of pointers to functions returning int x is an array[3] of pointers to functions returning pointers to array[5] of ints III:11

12 C Pointer Declarations: Test Yourself! int *p int *p[13] int *(p[13]) int **p int (*p)[13] int *f() int (*f)() int (*(*f())[13])() int (*(*x[3])())[5] pis a pointer to int p p is is an an array[13] of of pointer to to int int p p is is an an array[13] of of pointer to to int int pis a pointer to a pointer to an int p p is is a a pointer to to an an array[13] of of int int f is a function returning a pointer to int f is a pointer to a function returning int f is a function returning ptrto an array[13] of pointers to functions returning int x is an array[3] of pointers to functions returning pointers to array[5] of ints III:12

13 C Pointer Declarations: Test Yourself! int *p int *p[13] int *(p[13]) int **p int (*p)[13] int *f() int (*f)() int (*(*f())[13])() int (*(*x[3])())[5] pis a pointer to int p p is is an an array[13] of of pointer to to int int p p is is an an array[13] of of pointer to to int int pis a pointer to a pointer to an int p p is is a a pointer to to an an array[13] of of int int f is a function returning a pointer to int f is a pointer to a function returning int f f is is a a function returning ptrto an an array[13] of of pointers to to functions returning int int x is an array[3] of pointers to functions returning pointers to array[5] of ints III:13

14 C Pointer Declarations int *p int *p[13] int *(p[13]) int **p int (*p)[13] int *f() int (*f)() int (*(*f())[13])() int (*(*x[3])())[5] pis a pointer to int p is an array[13] of pointer to int p is an array[13] of pointer to int pis a pointer to a pointer to an int p is a pointer to an array[13] of int f is a function returning a pointer to int f is a pointer to a function returning int f is a function returning ptrto an array[13] of pointers to functions returning int x is an array[3] of pointers to functions returning pointers to array[5] of ints III:14

15 Avoiding Complex Declarations Use typedef to build up the declaration Instead of int (*(*x[3])())[5]: typedef int fiveints[5]; typedef fiveints* p5i; typedef p5i (*f_of_p5is)(); f_of_p5is x[3]; xis an array of 3 elements, each of which is a pointer to a function returning an array of 5 ints III:15

16 Internet Worm and IM War November, 1988 Internet Worm attacks thousands of Internet hosts. How did it happen? July, 1999 Microsoft launches MSN Messenger (instant messaging system). Messenger clients can access popular AOL Instant Messaging Service (AIM) servers AIM client MSN server MSN client AIM server AIM client III:16

17 Internet Worm and IM War (cont.) August 1999 Mysteriously, Messenger clients can no longer access AIM servers. Microsoft and AOL begin the IM war: AOL changes server to disallow Messenger clients Microsoft makes changes to clients to defeat AOL changes. At least 13 such skirmishes. How did it happen? The Internet Worm and AOL/Microsoft War were both based on stack buffer overflow exploits! many Unix functions do not check argument sizes. allows target buffers to overflow. III:17

18 String Library Code Implementation of Unix function gets() /* Get string from stdin */ char *gets(char *dest) { int c = getchar(); char *p = dest; while (c!= EOF && c!= '\n') { *p++ = c; c = getchar(); } *p = '\0'; return dest; } No way to specify limit on number of characters to read Similar problems with other Unix functions strcpy: Copies string of arbitrary length scanf, fscanf, sscanf, when given %s conversion specification III:18

19 Vulnerable Buffer Code /* Echo Line */ void echo() { char buf[4]; /* Way too small! */ gets(buf); puts(buf); } int main() { printf("type a string:"); echo(); return 0; } unix>./bufdemo Type a string: unix>./bufdemo Type a string: Segmentation Fault unix>./bufdemo Type a string: abc Segmentation Fault III:19

20 Buffer Overflow Disassembly f0 <echo>: 80484f0: 55 push %ebp 80484f1: 89 e5 mov %esp,%ebp 80484f3: 53 push %ebx 80484f4: 8d 5d f8 lea 0xfffffff8(%ebp),%ebx 80484f7: 83 ec 14 sub $0x14,%esp 80484fa: 89 1c 24 mov %ebx,(%esp) 80484fd: e8 ae ff ff ff call 80484b0 <gets> : 89 1c 24 mov %ebx,(%esp) : e8 8a fe ff ff call a: 83 c4 14 add $0x14,%esp d: 5b pop %ebx e: c9 leave f: c3 ret 80485f2: e8 f9 fe ff ff call 80484f0 <echo> 80485f7: 8b 5d fc mov 0xfffffffc(%ebp),%ebx 80485fa: c9 leave 80485fb: 31 c0 xor %eax,%eax 80485fd: c3 ret III:20

21 Buffer Overflow Stack Before call to gets Stack Frame for main Return Address Saved %ebp [3][2][1][0] buf Stack Frame for echo %ebp /* Echo Line */ void echo() { char buf[4]; /* Way too small! */ gets(buf); puts(buf); } echo: pushl %ebp movl %esp, %ebp pushl %ebx leal -8(%ebp),%ebx subl $20, %esp movl %ebx, (%esp) call gets... # Save %ebp on stack # Save %ebx # Compute buf as %ebp-8 # Allocate stack space # Push buf on stack # Call gets III:21

22 Buffer Overflow Stack Example unix> gdb bufdemo (gdb) break echo Breakpoint 1 at 0x (gdb) run Breakpoint 1, 0x in echo () (gdb) print /x $ebp $1 = 0xffffc638 (gdb) print /x *(unsigned *)$ebp $2 = 0xffffc658 (gdb) print /x *((unsigned *)$ebp + 1) $3 = 0x80485f7 Before call to gets Stack Frame for main Before call to gets Stack Frame for main 0xffffc658 Return Address Saved %ebp f7 ff ff c6 58 0xffffc638 [3][2][1][0] Stack Frame for echo buf xx xx xx xx buf Stack Frame for echo 80485f2:call 80484f0 <echo> 80485f7:mov 0xfffffffc(%ebp),%ebx # Return Point III:22

23 Buffer Overflow Example #1 Before call to gets Input Stack Frame for main 0xffffc658 Stack Frame for main 0xffffc f7 ff ff c6 58 xx xx xx xx buf Stack Frame for echo 0xffffc f7 ff ff c6 58 0xffffc buf Stack Frame for echo Overflow bufs, but no problem III:23

24 Buffer Overflow Example #2 Before call to gets Input Stack Frame for main 0xffffc658 Stack Frame for main 0xffffc f7 ff ff c6 58 xx xx xx xx buf Stack Frame for echo 0xffffc f7 ff ff c6 00 0xffffc buf Stack Frame for echo Base pointer corrupted a: 83 c4 14 add $0x14,%esp # deallocate space d: 5b pop %ebx # restore %ebx e: c9 leave # movl %ebp, %esp; popl %ebp f: c3 ret # Return III:24

25 Buffer Overflow Example #3 Before call to gets Input Stack Frame for main 0xffffc658 Stack Frame for main 0xffffc f7 ff ff c6 58 xx xx xx xx buf Stack Frame for echo 0xffffc xffffc buf Stack Frame for echo Return address corrupted 80485f2: call 80484f0 <echo> 80485f7: mov 0xfffffffc(%ebp),%ebx # Return Point III:25

26 Malicious Use of Buffer Overflow Stack after call to gets() void foo(){ bar();... } return address A B foo stack frame int bar() { char buf[64]; gets(buf);... return...; } data written by gets() B pad exploit code bar stack frame Input string contains byte representation of executable code Overwrite return address with address of buffer When bar() executes ret, will jump to exploit code III:26

27 Exploits Based on Buffer Overflows Buffer overflow bugs allow remote machines to execute arbitrary code on victim machines Internet worm Early versions of the finger server (fingerd) used gets()to read the argument sent by the client: finger Worm attacked fingerd server by sending phony argument: finger exploit-code padding new-returnaddress Exploit code: executed a root shell on the victim machine with a direct TCP connection to the attacker. III:27

28 Exploits Based on Buffer Overflows Buffer overflow bugs allow remote machines to execute arbitrary code on victim machines IM War AOL exploited existing buffer overflow bug in AIM clients Exploit code: returned 4-byte signature (the bytes at some location in the AIM client) to server. When Microsoft changed code to match signature, AOL changed signature location. III:28

29 Example Virus: Code Red Worm History June 18, Microsoft announces buffer overflow vulnerability in IIS Internet server July 19, over 250,000 machines infected by new virus in 9 hours White house must change its IP address. Pentagon shut down public WWW servers for day WhenCS:APP Web Site was set up Received strings of form GET /default.ida?nnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn N...NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNN%u909 0%u6858%ucbd3%u7801%u9090%u6858%ucbd3%u7801%u9090%u 6858%ucbd3%u7801%u9090%u9090%u8190%u00c3%u0003%u8b0 0%u531b%u53ff%u0078%u0000%u00=a HTTP/1.0" "-" "-" III:29

30 Code Red Exploit Code Starts 100 threads running Spread self Generate random IP addresses & send attack string Between 1st & 19th of month Attack Send 98,304 packets; sleep for 4-1/2 hours; repeat Denial of service attack Between 21st & 27th of month Deface server s home page After waiting 2 hours III:30

31 Avoiding Overflow Vulnerability /* Echo Line */ void echo() { char buf[4]; /* Way too small! */ fgets(buf, 4, stdin); puts(buf); } Use library routines that limit string lengths fgetsinstead of gets strncpy instead of strcpy Don t use scanf with %s conversion specification Use fgetsto read the string Or use %ns where nis a suitable integer III:31

32 System-Level Protections Randomized stack offsets At start of program, allocate random amount of space on stack Makes it difficult for hacker to predict beginning of inserted code Non-executable code segments In traditional x86, segments are either readonly or writeable Can execute anything readable Better: add explicit execute permission Stack corruption detection Compiler inserts canary just beyond array and code to check it before function return Code aborts with error if canary changed unix> gdb bufdemo (gdb) break echo (gdb) run (gdb) print /x $ebp $1 = 0xffffc638 (gdb) run (gdb) print /x $ebp $2 = 0xffffbb08 (gdb) run (gdb) print /x $ebp $3 = 0xffffc6a8 III:32

33 Web-aside: Assembly & C Requirements to have asm + C integrated together? In assembly, follow register usage, parameter passing, and return value conventions Problems with inline assembly Not portable Knowing what registers are safe to use Advantages of GCC s extended asm You inform compiler of source values required, results your code generates, and registers that will be overwritten GCC generates code to correctly set up source values, execute desired instructions, and correctly use computed results III:33

34 BONUS SLIDES III:34

35 IA32 Floating Point (x87) History 8086: first computer to implement IEEE FP separate 8087 FPU (floating point unit) 486: merged FPU and Integer Unit onto one chip Becoming obsolete with x86-64 Summary Hardware to add, multiply, and divide Floating point data registers Various control & status registers Floating Point Formats single precision (C float): 32 bits double precision (C double): 64 bits extended precision (C long double): 80 bits Integer Unit Instruction decoder and sequencer Memory FPU III:35

36 FPU Data Register Stack (x87) FPU register format (80 bit extended precision) s exp frac 0 FPU registers 8 registers %st(0) -%st(7) Logically form stack Top: %st(0) Bottom disappears (drops out) after too many pushs Top %st(3) %st(2) %st(1) %st(0) III:36

37 FPU instructions (x87) Large number of floating point instructions and formats ~50 basic instruction types load, store, add, multiply sin, cos, tan, arctan, and log Instruction Effect Description fldz push 0.0 Load zero flds Addr push Mem[Addr] fmuls Addr %st(0) %st(0)*m[addr]multiply faddp Often slower than math lib (!) Sample instructions: Load single precision real %st(1) %st(0)+%st(1);pop Add and pop III:37

38 FP Code Example (x87) Compute inner product of two vectors Single precision arithmetic Common computation float ipf (float x[], float y[], int n) { int i; float result = 0.0; for (i = 0; i < n; i++) result += x[i]*y[i]; return result; } pushl %ebp movl %esp,%ebp pushl %ebx # setup movl 8(%ebp),%ebx # %ebx=&x movl 12(%ebp),%ecx # %ecx=&y movl 16(%ebp),%edx # %edx=n fldz # push +0.0 xorl %eax,%eax # i=0 cmpl %edx,%eax # if i>=n done jge.l3.l5: flds (%ebx,%eax,4) # push x[i] fmuls (%ecx,%eax,4) # st(0)*=y[i] faddp # st(1)+=st(0); pop incl %eax # i++ cmpl %edx,%eax # if i<n repeat jl.l5.l3: movl -4(%ebp),%ebx # finish movl %ebp, %esp popl %ebp ret # st(0) = result III:38

39 Inner Product Stack Trace Initialization 1. fldz eax = i ebx = *x ecx = *y 0.0 %st(0) Iteration 0 Iteration 1 2. flds (%ebx,%eax,4) 0.0 %st(1) x[0] %st(0) 3. fmuls (%ecx,%eax,4) 0.0 %st(1) x[0]*y[0] %st(0) 5. flds (%ebx,%eax,4) x[0]*y[0] x[1] 6. fmuls (%ecx,%eax,4) x[0]*y[0] x[1]*y[1] %st(1) %st(0) %st(1) %st(0) 4. faddp 0.0+x[0]*y[0] %st(0) 7. faddp x[0]*y[0]+x[1]*y[1] %st(0) III:39

40 x87 Floating Point Problems One of least elegant features of x86 Remnants of original co-processor design Compiler technology is much better today Stack registers are problematic All must be treated as caller-saved, too much memory traffic Can t treat as true stack over multiple function calls Separate FP status word is problematic Set by FP comparison instructions, outcome used by branches Must transfer to int word, test bits explicitly Even experienced programmers find this code arcane and difficult to read. III:40

41 SSE Floating Point Architecture Better compiler target than x87 Includes new registers and instructions In Intel CPUs since Pentium III Default for x86-64, but not IA32 III:41

42 Vector Instructions: SSE Family SIMD (single-instruction, multiple data) vector instructions New data types, registers, operations Parallel operation on small (length 2-8) vectors of integers or floats Example: + x 4-way Floating point vector instructions Available with Intel s SSE (streaming SIMD extensions) family SSE starting with Pentium III: 4-way single precision SSE2 starting with Pentium 4: 2-way double precision All x86-64 have SSE3 (superset of SSE2, SSE) III:42

43 Intel Architectures (Focus Floating Point) Processors Architectures Features 8086 x Pentium Pentium MMX x86-32 MMX Pentium III SSE 4-way single precision FP Pentium 4 SSE2 2-way double precision FP time Pentium 4E Pentium 4F Core 2 Duo SSE3 x86-64 / em64t SSE4 Our focus: SSE3 used for scalar (non-vector) floating point III:43

44 SSE3 Registers All caller saved %xmm0 for floating point return value 128 bit = 2 doubles = 4 singles %xmm0 %xmm1 %xmm2 %xmm3 %xmm4 %xmm5 %xmm6 %xmm7 Argument #1 Argument #2 Argument #3 Argument #4 Argument #5 Argument #6 Argument #7 Argument #8 %xmm8 %xmm9 %xmm10 %xmm11 %xmm12 %xmm13 %xmm14 %xmm15 III:44

45 SSE3 Registers Different data types and associated instructions Integer vectors: 16-way byte 8-way 2 bytes 4-way 4 bytes Floating point vectors: 4-way single 2-way double Floating point scalars: single double 128 bit LSB III:45

46 SSE3 Instructions: Examples Single precision 4-way vector add: addps %xmm0 %xmm1 %xmm0 + %xmm1 Single precision scalar add: addss %xmm0 %xmm1 %xmm0 + %xmm1 III:46

47 SSE3 Instruction Names packed (vector) single slot (scalar) addps addss single precision addpd addsd double precision our focus III:47

48 SSE3 Basic Instructions Moves Single Double Effect movss movsd D S Usual operand form: reg reg, reg mem, mem reg Arithmetic Single Double Effect addss addsd D D + S subss subsd D D S mulss mulsd D D x S divss divsd D D / S maxss maxsd D max(d,s) minss minsd D min(d,s) sqrtss sqrtsd D sqrt(s) III:48

49 x86-64 FP Code Example Compute inner product of two vectors Single precision arithmetic Uses SSE3 instructions ipf: xorps %xmm1, %xmm1 # result = 0.0 xorl %ecx, %ecx # i = 0 jmp.l8 # goto middle.l10: # loop: movslq %ecx,%rax # icpy = i incl %ecx # i++ movss (%rsi,%rax,4), %xmm0# t = y[icpy] mulss (%rdi,%rax,4), %xmm0# t *= x[icpy] addss %xmm0, %xmm1 # result += t.l8: # middle: cmpl %edx, %ecx # i:n jl.l10 # if < goto loop movaps %xmm1, %xmm0 # return result ret float ipf (float x[], float y[], int n) { int i; float result = 0.0; for (i = 0; i < n; i++) result += x[i]*y[i]; return result; } III:49

50 SSE3 Conversion Instructions Conversions Same operand forms as moves Instruction cvtss2sd cvtsd2ss cvtsi2ss cvtsi2sd cvtsi2ssq cvtsi2sdq cvttss2si cvttsd2si cvttss2siq cvttss2siq Description single double double single int single int double quad int single quad int double single int(truncation) double int(truncation) single quad int(truncation) double quad int(truncation) III:50

51 x86-64 FP Code Example double funct(double a, float x, double b, int i) { return a*x - b/i; } a %xmm0 double x %xmm1 float b %xmm2 double i %edi int funct: cvtss2sd %xmm1, %xmm1 # %xmm1 = (double) x mulsd %xmm0, %xmm1 # %xmm1 = a*x cvtsi2sd %edi, %xmm0 # %xmm0 = (double) i divsd %xmm0, %xmm2 # %xmm2 = b/i movsd %xmm1, %xmm0 # %xmm0 = a*x subsd %xmm2, %xmm0 # return a*x - b/i ret III:51

52 Constants double cel2fahr(double temp) { return 1.8 * temp ; } Here: Constants in decimal format compiler decision hex more readable # Constant declarations.lc2:.long # Low order four bytes of 1.8.long # High order four bytes of 1.8.LC4:.long 0 # Low order four bytes of 32.0.long # High order four bytes of 32.0 # Code cel2fahr: mulsd.lc2(%rip), %xmm0 # Multiply by 1.8 addsd.lc4(%rip), %xmm0 # Add 32.0 ret III:52

53 Comments SSE3 floating point Uses lower ½ (double) or ¼ (single) of vector Finally departure from awkward x87 Assembly very similar to integer code x87 still supported Even mixing with SSE3 possible Not recommended For highest floating point performance Vectorization a must (but not in this course ) See next slide III:53

54 Vector Instructions Starting with version 4.1.1, GCC can auto-vectorizeto some extent -O3 or ftree-vectorize No speed-up guaranteed limited capability Intel s C++ compiler (icc) currently much better Spice machines have installed (May 2010) For highest performance vectorize yourself using intrinsics Intrinsics = C interface to vector instructions Future Intel AVX announced: 4-way double, 8-way single III:54