Machine-Level Programming V Miscellaneous Topics n n class

Machine-Level Programming V: Miscellaneous Topics n n class 09. ppt Buffer Overflow Floating Point Code

Internet Worm and IM War November, 1988 n Internet Worm attacks thousands of Internet hosts. n How did it happen? July, 1999 n n Microsoft launches MSN Messenger (instant messaging system). Messenger clients can access popular AOL Instant Messaging Service (AIM) servers AIM client MSN server – 2– MSN client AIM server AIM client 15 -213, F’ 02

Internet Worm and IM War (cont. ) August 1999 n n Mysteriously, Messenger clients can no longer access AIM servers. Microsoft and AOL begin the IM war: l AOL changes server to disallow Messenger clients l Microsoft makes changes to clients to defeat AOL changes. l At least 13 such skirmishes. n How did it happen? The Internet Worm and AOL/Microsoft War were both based on stack buffer overflow exploits! l many Unix functions do not check argument sizes. l allows target buffers to overflow. – 3– 15 -213, F’ 02

String Library Code n Implementation of Unix function gets l No way to specify limit on number of characters to read /* Get string from stdin */ char *gets(char *dest) { int c = getc(); char *p = dest; while (c != EOF && c != 'n') { *p++ = c; c = getc(); } *p = ''; return dest; } n Similar problems with other Unix functions l strcpy: Copies string of arbitrary length l scanf, fscanf, sscanf, when given %s conversion specification – 4– 15 -213, F’ 02

Vulnerable Buffer Code /* Echo Line */ void echo() { char buf[4]; gets(buf); puts(buf); } /* Way too small! */ int main() { printf("Type a string: "); echo(); return 0; } – 5– 15 -213, F’ 02

Buffer Overflow Executions unix>. /bufdemo Type a string: 12345 Segmentation Fault unix>. /bufdemo Type a string: 12345678 Segmentation Fault – 6– 15 -213, F’ 02

Buffer Overflow Stack Frame for main Return Address Saved %ebp [3][2][1][0] buf Stack Frame for echo – 7– /* Echo Line */ void echo() { char buf[4]; gets(buf); puts(buf); } echo: pushl %ebp movl %esp, %ebp subl $20, %esp pushl %ebx addl $-12, %esp leal -4(%ebp), %ebx pushl %ebx call gets. . . /* Way too small! */ # Save %ebp on stack # # # Allocate space on stack Save %ebx Allocate space on stack Compute buf as %ebp-4 Push buf on stack Call gets 15 -213, F’ 02

Buffer Overflow Stack Example Stack Frame for main Return Address Saved %ebp [3][2][1][0] buf Stack Frame for echo unix> gdb bufdemo (gdb) break echo Breakpoint 1 at 0 x 8048583 (gdb) run Breakpoint 1, 0 x 8048583 in echo () (gdb) print /x *(unsigned *)$ebp $1 = 0 xbffff 8 f 8 (gdb) print /x *((unsigned *)$ebp + 1) $3 = 0 x 804864 d Stack Frame for main Before call to gets 08 04 86 4 d Return Address bf ff f 8 0 xbffff 8 d 8 Saved %ebp xx xx [3][2][1][0] buf Stack Frame for echo 8048648: call 804857 c 804864 d: mov 0 xffffffe 8(%ebp), %ebx # Return Point – 8– 15 -213, F’ 02

Buffer Overflow Example #1 Before Call to gets Stack Frame for main Return Address Saved %ebp [3][2][1][0] buf Stack Frame for echo Input = “ 123” Stack Frame for main 08 04 86 4 d Return Address bf ff f 8 0 xbffff 8 d 8 Saved %ebp 00 33 32 31 [3][2][1][0] buf Stack Frame for echo No Problem – 9– 15 -213, F’ 02

Buffer Overflow Stack Example #2 Stack Frame for main Return Address Saved %ebp [3][2][1][0] buf Stack Frame for echo 8048592: 8048593: 8048598: 804859 b: 804859 d: 804859 e: – 10 – 08 04 86 4 d Return Address bf ff 00 35 0 xbffff 8 d 8 Saved %ebp [3][2][1][0] buf 34 33 32 31 Stack Frame for echo code: push call mov pop ret Input = “ 12345” Saved value of %ebp set to 0 xbfff 0035 Bad news when later attempt to restore %ebp %ebx 80483 e 4 <_init+0 x 50> # gets 0 xffffffe 8(%ebp), %ebx %ebp, %esp %ebp # %ebp gets set to invalid value 15 -213, F’ 02

Buffer Overflow Stack Example #3 Stack Frame for main Return Address Saved %ebp [3][2][1][0] buf Stack Frame for echo Stack Frame for main Input = “ 12345678” 08 04 86 00 Return Address 38 37 36 35 0 xbffff 8 d 8 Saved %ebp 34 33 32 31 [3][2][1][0] buf Stack Frame for echo %ebp and return address corrupted Invalid address No longer pointing to desired return point 8048648: call 804857 c 804864 d: mov 0 xffffffe 8(%ebp), %ebx # Return Point – 11 – 15 -213, F’ 02

Malicious Use of Buffer Overflow Use of exploit code Stack after call to gets() return address A void foo(){ bar(); . . . } void bar() { char buf[64]; gets(buf); . . . } n n n – 12 – foo stack frame data written by gets() B 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 15 -213, F’ 02

Exploits Based on Buffer Overflows Buffer overflow bugs allow remote machines to execute arbitrary code on victim machines. Internet worm n Early versions of the finger server (fingerd) used gets() to read the argument sent by the client: l finger droh@cs. cmu. edu n Worm attacked fingerd server by sending phony argument: l finger “exploit-code padding new-return-address” l exploit code: executed a root shell on the victim machine with a direct TCP connection to the attacker. – 13 – 15 -213, F’ 02

Exploits Based on Buffer Overflows Buffer overflow bugs allow remote machines to execute arbitrary code on victim machines. IM War n n n – 14 – 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. 15 -213, F’ 02

Date: Wed, 11 Aug 1999 11: 30: 57 -0700 (PDT) From: Phil Bucking Subject: AOL exploiting buffer overrun bug in their own software! To: rms@pharlap. com Mr. Smith, I am writing you because I have discovered something that I think you might find interesting because you are an Internet security expert with experience in this area. I have also tried to contact AOL but received no response. I am a developer who has been working on a revolutionary new instant messaging client that should be released later this year. . It appears that the AIM client has a buffer overrun bug. By itself this might not be the end of the world, as MS surely has had its share. But AOL is now *exploiting their own buffer overrun bug* to help in its efforts to block MS Instant Messenger. . . Since you have significant credibility with the press I hope that you can use this information to help inform people that behind AOL's friendly exterior they are nefariously compromising peoples' security. Sincerely, Phil Bucking Founder, Bucking Consulting philbucking@yahoo. com – 15 – It was later determined that this email originated from within Microsoft! 15 -213, F’ 02

Code Red Worm History n n n June 18, 2001. Microsoft announces buffer overflow vulnerability in IIS Internet server July 19, 2001. 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 When We Set Up CS: APP Web Site n Received strings of form GET /default. ida? NNNNNNNNNNNNNNNNNNNN. . NNNNNNNNNNNNNNNNNNN%u 9090%u 6858%ucbd 3%u 780 1%u 9090%u 6858%ucbd 3%u 7801%u 9090%u 909 0%u 8190%u 00 c 3%u 0003%u 8 b 00%u 531 b%u 53 ff%u 0078%u 0000%u 00=a HTTP/1. 0" 400 325 "-" – 16 – 15 -213, F’ 02

Code Red Exploit Code n n Starts 100 threads running Spread self l Generate random IP addresses & send attack string l Between 1 st & 19 th of month n Attack www. whitehouse. gov l Send 98, 304 packets; sleep for 4 -1/2 hours; repeat » Denial of service attack l Between 21 st & 27 th of month n Deface server’s home page l After waiting 2 hours – 17 – 15 -213, F’ 02

Code Red Effects Later Version Even More Malicious n Code Red II n As of April, 2002, over 18, 000 machines infected Still spreading n Paved Way for NIMDA n n – 18 – Variety of propagation methods One was to exploit vulnerabilities left behind by Code Red II 15 -213, F’ 02

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 n n n – 19 – fgets instead of gets strncpy instead of strcpy Don’t use scanf with %s conversion specification l Use fgets to read the string 15 -213, F’ 02

IA 32 Floating Point History n Instruction decoder and sequencer 8086: first computer to implement IEEE FP l separate 8087 FPU (floating point unit) n 486: merged FPU and Integer Unit onto one chip Summary n n n Hardware to add, multiply, and divide Floating point data registers Various control & status registers Integer Unit FPU Floating Point Formats n n n – 20 – single precision (C float): 32 bits double precision (C double): 64 bits extended precision (C long double): 80 bits Memory 15 -213, F’ 02

FPU Data Register Stack FPU register format (extended precision) 79 78 s 0 64 63 exp frac FPU registers n 8 registers n Logically forms shallow stack Top called %st(0) n n When push too many, bottom values disappear %st(3) %st(2) %st(1) %st(0) “Top” stack grows down – 21 – 15 -213, F’ 02

FPU instructions Large number of floating point instructions and formats n ~50 basic instruction types n load, store, add, multiply sin, cos, tan, arctan, and log! n Sample instructions: Instruction Effect fldz flds Addr fmuls Addr faddp push 0. 0 Load zero push M[Addr] Load single precision real %st(0) <- %st(0)*M[Addr] Multiply %st(1) <- %st(0)+%st(1); pop Add and pop – 22 – Description 15 -213, F’ 02

Floating Point Code Example Compute Inner Product of Two Vectors n n 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; } – 23 – pushl %ebp movl %esp, %ebp pushl %ebx movl 8(%ebp), %ebx movl 12(%ebp), %ecx movl 16(%ebp), %edx fldz xorl %eax, %eax cmpl %edx, %eax jge. L 3. L 5: flds (%ebx, %eax, 4) fmuls (%ecx, %eax, 4) faddp incl %eax cmpl %edx, %eax jl. L 5. L 3: movl -4(%ebp), %ebx movl %ebp, %esp popl %ebp ret # setup # # # %ebx=&x %ecx=&y %edx=n push +0. 0 i=0 if i>=n done # # # push x[i] st(0)*=y[i] st(1)+=st(0); pop i++ if i

Inner Product Stack Trace Initialization 1. fldz 0. 0 %st(0) Iteration 0 Iteration 1 2. flds (%ebx, %eax, 4) 0. 0 x[0] 5. flds (%ebx, %eax, 4) %st(1) %st(0) 3. fmuls (%ecx, %eax, 4) 0. 0 x[0]*y[0] %st(1) %st(0) 4. faddp 0. 0+x[0]*y[0] x[1] %st(1) %st(0) 6. fmuls (%ecx, %eax, 4) x[0]*y[0] x[1]*y[1] %st(1) %st(0) 7. faddp %st(0) x[0]*y[0]+x[1]*y[1] – 24 – 15 -213, F’ 02

Final Observations Working with Strange Code n Important to analyze nonstandard cases l E. g. , what happens when stack corrupted due to buffer overflow n Helps to step through with GDB IA 32 Floating Point n – 25 – Strange “shallow stack” architecture 15 -213, F’ 02