Machine-Level Programming III: Procedures Topics IA32 stack discipline Register saving conventions Creating pointers to local variables class07.ppt

Machine-Level Programming III:Procedures

Machine-Level Programming III:Procedures

TopicsTopics IA32 stack discipline Register saving

conventions Creating pointers to local

variables

class07.ppt

– 2 – 15-213, F’02

IA32/Linux Stack FrameIA32/Linux Stack FrameCurrent Stack Frame (“Top” Current Stack Frame (“Top”

to Bottom)to Bottom) Parameters for function

about to call“Argument build”

Local variables If can’t keep in registers

Saved register context Old frame pointer

Caller Stack FrameCaller Stack Frame Return address

Pushed by call instruction

Arguments for this call Stack Pointer(%esp)

Frame Pointer(%ebp)

Return Addr

SavedRegisters

+Local

Variables

ArgumentBuild

Old %ebp

Arguments

CallerFrame

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Pointer CodePointer Code

void s_helper (int x, int *accum){ if (x <= 1) return; else { int z = *accum * x; *accum = z; s_helper (x-1,accum); }}

int sfact(int x){ int val = 1; s_helper(x, &val); return val;}

Top-Level CallRecursive Procedure

Pass pointer to update location

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Temp.Space

%esp

Creating & Initializing PointerCreating & Initializing Pointer


_sfact:pushl %ebp # Save %ebpmovl %esp,%ebp # Set %ebpsubl $16,%esp # Add 16 bytes movl 8(%ebp),%edx # edx = xmovl $1,-4(%ebp) # val = 1

Using Stack for Local VariableUsing Stack for Local Variable Variable val must be stored

on stackNeed to create pointer to it

Compute pointer as -4(%ebp) Push on stack as second

argument

Initial part of sfact

x

Rtn adr

Old %ebp %ebp 0

4

8

-4 val = 1

Unused-12

-8

-16




– 5 – 15-213, F’02

Passing PointerPassing Pointer


leal -4(%ebp),%eax # Compute &valpushl %eax # Push on stackpushl %edx # Push xcall s_helper # callmovl -4(%ebp),%eax # Return val• • • # Finish

Calling s_helper from sfact

x

Rtn adr

Old %ebp %ebp 0

4

8

val = 1 -4

Unused-12

-8

-16

%espx

&val

Stack at time of call



val =x!

– 6 – 15-213, F’02

Using PointerUsing Pointer

• • •movl %ecx,%eax # z = ximull (%edx),%eax # z *= *accummovl %eax,(%edx) # *accum = z• • •

void s_helper (int x, int *accum){ • • • int z = *accum * x; *accum = z; • • •}

Register %ecx holds x Register %edx holds pointer to accum

Use access (%edx) to reference memory

%edxaccum

x

x%eax

%ecx

accum*x

accum*x

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SummarySummary

The Stack Makes Recursion WorkThe Stack Makes Recursion Work Private storage for each instance of procedure call

Instantiations don’t clobber each otherAddressing of locals + arguments can be relative to stack

positions

Can be managed by stack disciplineProcedures return in inverse order of calls

IA32 Procedures Combination of Instructions + IA32 Procedures Combination of Instructions + ConventionsConventions Call / Ret instructions Register usage conventions

Caller / Callee save %ebp and %esp

Stack frame organization conventions

– 8 – 15-213, F’02

Basic Data TypesBasic Data Types

IntegralIntegral Stored & operated on in general registers Signed vs. unsigned depends on instructions used

Intel GAS Bytes C

byte b 1 [unsigned] charword w 2 [unsigned] shortdouble word l 4 [unsigned] int

Floating PointFloating Point Stored & operated on in floating point registers

Intel GAS Bytes C

Single s 4 floatDouble l 8 doubleExtended t 10/12 long double

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Array AllocationArray AllocationBasic PrincipleBasic Principle

T A[L]; Array of data type T and length L Contiguously allocated region of L * sizeof(T) bytes

char string[12];

x x + 12int val[5];

x x + 4 x + 8 x + 12 x + 16 x + 20double a[4];

x + 32x + 24x x + 8 x + 16

char *p[3];

x x + 4 x + 8

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Array AccessArray AccessBasic PrincipleBasic Principle

T A[L]; Array of data type T and length L Identifier A can be used as a pointer to array element 0

ReferenceReference TypeType ValueValueval[4] int 3

val int * x

val+1 int * x + 4

&val[2] int * x + 8

val[5] int ??

*(val+1) int 5

val + i int * x + 4 i

1 5 2 1 3int val[5];

x x + 4 x + 8 x + 12 x + 16 x + 20

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Array ExampleArray Example

NotesNotes Declaration “zip_dig cmu” equivalent to “int cmu[5]” Example arrays were allocated in successive 20 byte blocks

Not guaranteed to happen in general

typedef int zip_dig[5];

zip_dig cmu = { 1, 5, 2, 1, 3 };zip_dig mit = { 0, 2, 1, 3, 9 };zip_dig ucb = { 9, 4, 7, 2, 0 };

zip_dig cmu; 1 5 2 1 3

16 20 24 28 32 36zip_dig mit; 0 2 1 3 9

36 40 44 48 52 56zip_dig ucb; 9 4 7 2 0

56 60 64 68 72 76

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Array Accessing ExampleArray Accessing Example

Memory Reference CodeMemory Reference Code

int get_digit (zip_dig z, int dig){ return z[dig];}

# %edx = z # %eax = dig

movl (%edx,%eax,4),%eax # z[dig]

ComputationComputation Register %edx contains starting

address of array Register %eax contains array

index Desired digit at 4*%eax + %edx Use memory reference (%edx,%eax,4)

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Referencing ExamplesReferencing Examples

Code Does Not Do Any Bounds Checking!Code Does Not Do Any Bounds Checking!

ReferenceReference AddressAddress ValueValue Guaranteed?Guaranteed?mit[3] 36 + 4* 3 = 48 3

mit[5] 36 + 4* 5 = 56 9

mit[-1] 36 + 4*-1 = 32 3

cmu[15] 16 + 4*15 = 76 ?? Out of range behavior implementation-dependent

No guaranteed relative allocation of different arrays

zip_dig cmu; 1 5 2 1 3

16 20 24 28 32 36zip_dig mit; 0 2 1 3 9

36 40 44 48 52 56zip_dig ucb; 9 4 7 2 0

56 60 64 68 72 76

YesYes

NoNo

NoNo

NoNo

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int zd2int(zip_dig z){ int i; int zi = 0; for (i = 0; i < 5; i++) { zi = 10 * zi + z[i]; } return zi;}

Array Loop ExampleArray Loop Example

Original SourceOriginal Source

int zd2int(zip_dig z){ int zi = 0; int *zend = z + 4; do { zi = 10 * zi + *z; z++; } while(z <= zend); return zi;}

Transformed VersionTransformed Version As generated by GCC Eliminate loop variable i Convert array code to

pointer code Express in do-while form

No need to test at entrance

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# %ecx = zxorl %eax,%eax # zi = 0leal 16(%ecx),%ebx # zend = z+4

.L59:leal (%eax,%eax,4),%edx # 5*zimovl (%ecx),%eax # *zaddl $4,%ecx # z++leal (%eax,%edx,2),%eax # zi = *z + 2*(5*zi)cmpl %ebx,%ecx # z : zendjle .L59 # if <= goto loop

Array Loop ImplementationArray Loop ImplementationRegistersRegisters

%ecx z%eax zi%ebx zend

ComputationsComputations 10*zi + *z implemented as *z + 2*(zi+4*zi)

z++ increments by 4














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Nested Array ExampleNested Array Example

Declaration “zip_dig pgh[4]” equivalent to “int pgh[4][5]” Variable pgh denotes array of 4 elements

» Allocated contiguously Each element is an array of 5 int’s

» Allocated contiguously

“Row-Major” ordering of all elements guaranteed

#define PCOUNT 4zip_dig pgh[PCOUNT] = {{1, 5, 2, 0, 6}, {1, 5, 2, 1, 3 }, {1, 5, 2, 1, 7 }, {1, 5, 2, 2, 1 }};

zip_digpgh[4];

76 96 116 136 156

1 5 2 0 6 1 5 2 1 3 1 5 2 1 7 1 5 2 2 1

– 17 – 15-213, F’02

Nested Array AllocationNested Array AllocationDeclarationDeclaration

T A[R][C]; Array of data type T R rows, C columns Type T element requires K bytes

Array SizeArray Size R * C * K bytes

ArrangementArrangement Row-Major Ordering

A[0][0] A[0][C-1]

A[R-1][0]

• • •

• • •A[R-1][C-1]

•••

•••

int A[R][C];

A[0][0]

A[0]

[C-1]• • •

A[1][0]

A[1][C-1]

• • •A

[R-1][0]

A[R-1][C-1]

• • •• • •

4*R*C Bytes

– 18 – 15-213, F’02

• • •

Nested Array Row AccessNested Array Row Access

Row VectorsRow Vectors A[i] is array of C elements Each element of type T Starting address A + i * C * K

A[i][0]

A[i]

[C-1]• • •

A[i]

A[R-1][0]

A[R-1][C-1]

• • •

A[R-1]

• • •

A

A[0][0]

A[0]

[C-1]• • •

A[0]

int A[R][C];

A+i*C*4 A+(R-1)*C*4

– 19 – 15-213, F’02

struct rec { int i; int a[3]; int *p;};

Assembly

# %eax = val# %edx = rmovl %eax,(%edx) # Mem[r] = val

void set_i(struct rec *r, int val){ r->i = val;}

StructuresStructuresConceptConcept

Contiguously-allocated region of memory Refer to members within structure by names Members may be of different types

Accessing Structure MemberAccessing Structure Member

Memory Layout

i a p

0 4 16 20

– 20 – 15-213, F’02


# %ecx = idx# %edx = rleal 0(,%ecx,4),%eax # 4*idxleal 4(%eax,%edx),%eax # r+4*idx+4

int *find_a (struct rec *r, int idx){ return &r->a[idx];}

Generating Pointer to Struct. MemberGenerating Pointer to Struct. Member

Generating Pointer to Generating Pointer to Array ElementArray Element Offset of each structure

member determined at compile time

i a p

0 4 16

r + 4 + 4*idx

r

– 21 – 15-213, F’02


# %edx = rmovl (%edx),%ecx # r->ileal 0(,%ecx,4),%eax # 4*(r->i)leal 4(%edx,%eax),%eax # r+4+4*(r->i)movl %eax,16(%edx) # Update r->p

void set_p(struct rec *r){ r->p = &r->a[r->i];}

Structure Referencing (Cont.)Structure Referencing (Cont.)C CodeC Code

i a

0 4 16

Element i

i a p

0 4 16

– 22 – 15-213, F’02

AlignmentAlignment

Aligned DataAligned Data Primitive data type requires K bytes Address must be multiple of K Required on some machines; advised on IA32

treated differently by Linux and Windows!

Motivation for Aligning DataMotivation for Aligning Data Memory accessed by (aligned) double or quad-words

Inefficient to load or store datum that spans quad word boundaries

Virtual memory very tricky when datum spans 2 pages

CompilerCompiler Inserts gaps in structure to ensure correct alignment of

fields

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Specific Cases of AlignmentSpecific Cases of AlignmentSize of Primitive Data Type:Size of Primitive Data Type:

1 byte (e.g., char) no restrictions on address

2 bytes (e.g., short) lowest 1 bit of address must be 02

4 bytes (e.g., int, float, char *, etc.) lowest 2 bits of address must be 002

8 bytes (e.g., double) Windows (and most other OS’s & instruction sets):

» lowest 3 bits of address must be 0002

Linux:» lowest 2 bits of address must be 002

» i.e., treated the same as a 4-byte primitive data type 12 bytes (long double)

Linux:» lowest 2 bits of address must be 002

» i.e., treated the same as a 4-byte primitive data type

– 24 – 15-213, F’02

struct S1 { char c; int i[2]; double v;} *p;

Satisfying Alignment with StructuresSatisfying Alignment with StructuresOffsets Within StructureOffsets Within Structure

Must satisfy element’s alignment requirement

Overall Structure PlacementOverall Structure Placement Each structure has alignment requirement K

Largest alignment of any element

Initial address & structure length must be multiples of K

Example (under Windows):Example (under Windows): K = 8, due to double elementc i[0] i[1] v

p+0 p+4 p+8 p+16 p+24

Multiple of 4 Multiple of 8

Multiple of 8 Multiple of 8

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Linux vs. WindowsLinux vs. Windows

Windows (including Cygwin):Windows (including Cygwin): K = 8, due to double element

Linux:Linux: K = 4; double treated like a 4-byte data type

struct S1 { char c; int i[2]; double v;} *p;

c i[0] i[1] v

p+0 p+4 p+8 p+16 p+24

Multiple of 4 Multiple of 8Multiple of 8 Multiple of 8

c i[0] i[1]

p+0 p+4 p+8

Multiple of 4 Multiple of 4Multiple of 4

v

p+12 p+20

Multiple of 4

– 26 – 15-213, F’02

Overall Alignment RequirementOverall Alignment Requirementstruct S2 { double x; int i[2]; char c;} *p;

struct S3 { float x[2]; int i[2]; char c;} *p;

p+0 p+12p+8 p+16 Windows: p+24Linux: p+20

ci[0] i[1]x

ci[0] i[1]

p+0 p+12p+8 p+16 p+20

x[0] x[1]

p+4

p must be multiple of: 8 for Windows4 for Linux

p must be multiple of 4 (in either OS)

– 27 – 15-213, F’02

Ordering Elements Within StructureOrdering Elements Within Structurestruct S4 { char c1; double v; char c2; int i;} *p;

struct S5 { double v; char c1; char c2; int i;} *p;

c1 iv

p+0 p+20p+8 p+16 p+24

c2

c1 iv

p+0 p+12p+8 p+16

c2

10 bytes wasted space in Windows

2 bytes wasted space

– 28 – 15-213, F’02

Arrays of StructuresArrays of Structures

PrinciplePrinciple Allocated by repeating allocation

for array type In general, may nest arrays &

structures to arbitrary depth

a[0]

a+0

a[1] a[2]

a+12 a+24 a+36

• • •

a+12 a+20a+16 a+24

struct S6 { short i; float v; short j;} a[10];

a[1].i a[1].ja[1].v

– 29 – 15-213, F’02

Accessing Element within ArrayAccessing Element within Array Compute offset to start of structure

Compute 12*i as 4*(i+2i) Access element according to its offset

within structureOffset by 8Assembler gives displacement as a + 8

» Linker must set actual value

a[0]

a+0

a[i]

a+12i

• • • • • •

short get_j(int idx){ return a[idx].j;}

# %eax = idxleal (%eax,%eax,2),%eax # 3*idxmovswl a+8(,%eax,4),%eax

a+12i a+12i+8


a[i].i a[i].ja[i].v

– 30 – 15-213, F’02

Satisfying Alignment within StructureSatisfying Alignment within StructureAchieving AlignmentAchieving Alignment

Starting address of structure array must be multiple of worst-case alignment for any element

a must be multiple of 4

Offset of element within structure must be multiple of element’s alignment requirement

v’s offset of 4 is a multiple of 4

Overall size of structure must be multiple of worst-case alignment for any element

Structure padded with unused space to be 12 bytes


a[0]

a+0

a[i]

a+12i

• • • • • •

a+12i a+12i+4

a[1].i a[1].ja[1].v

Multiple of 4

Multiple of 4

– 31 – 15-213, F’02

SummarySummaryArrays in CArrays in C

Contiguous allocation of memory Pointer to first element No bounds checking

Compiler OptimizationsCompiler Optimizations Compiler often turns array code into pointer code (zd2int) Uses addressing modes to scale array indices Lots of tricks to improve array indexing in loops

StructuresStructures Allocate bytes in order declared Pad in middle and at end to satisfy alignment

UnionsUnions Overlay declarations Way to circumvent type system

– 32 – 15-213, F’02

Internet Worm and IM WarInternet Worm and IM WarNovember, 1988November, 1988

Internet Worm attacks thousands of Internet hosts. How did it happen?

July, 1999July, 1999 Microsoft launches MSN Messenger (instant messaging system). Messenger clients can access popular AOL Instant Messaging Service (AIM)

servers

AIMserver

AIMclient

AIMclient

MSNclient

MSNserver

– 33 – 15-213, F’02

Internet Worm and IM War (cont.)Internet Worm and IM War (cont.)August 1999August 1999

Mysteriously, Messenger clients can no longer access AIM servers.

Microsoft and AOL begin the IM war:AOL changes server to disallow Messenger clientsMicrosoft 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 The Internet Worm and AOL/Microsoft War were both based on based on stack buffer overflowstack buffer overflow exploits! exploits!

many Unix functions do not check argument sizes.allows target buffers to overflow.

– 34 – 15-213, F’02

String Library CodeString Library Code Implementation of Unix function gets

No way to specify limit on number of characters to read

Similar problems with other Unix functionsstrcpy: Copies string of arbitrary lengthscanf, fscanf, sscanf, when given %s conversion specification

/* Get string from stdin */char *gets(char *dest){ int c = getc(); char *p = dest; while (c != EOF && c != '\n') { *p++ = c; c = getc(); } *p = '\0'; return dest;}

– 35 – 15-213, F’02

Vulnerable Buffer CodeVulnerable Buffer Code

int main(){ printf("Type a string:"); echo(); return 0;}

/* Echo Line */void echo(){ char buf[4]; /* Way too small! */ gets(buf); puts(buf);}

– 36 – 15-213, F’02

Buffer Overflow ExecutionsBuffer Overflow Executions

unix>./bufdemoType a string:123123

unix>./bufdemoType a string:12345Segmentation Fault

unix>./bufdemoType a string:12345678Segmentation Fault

– 37 – 15-213, F’02

Buffer Overflow StackBuffer Overflow Stack

echo:pushl %ebp # Save %ebp on stackmovl %esp,%ebpsubl $20,%esp # Allocate space on stackpushl %ebx # Save %ebxaddl $-12,%esp # Allocate space on stackleal -4(%ebp),%ebx # Compute buf as %ebp-4pushl %ebx # Push buf on stackcall gets # Call gets. . .

/* Echo Line */void echo(){ char buf[4]; /* Way too small! */ gets(buf); puts(buf);}

Return Address

Saved %ebp

[3][2][1][0] buf

%ebp

StackFrame

for main

StackFrame

for echo

– 38 – 15-213, F’02

Buffer Overflow Stack Example

Buffer Overflow Stack Example

Before call to gets

unix> gdb bufdemo(gdb) break echoBreakpoint 1 at 0x8048583(gdb) runBreakpoint 1, 0x8048583 in echo ()(gdb) print /x *(unsigned *)$ebp$1 = 0xbffff8f8(gdb) print /x *((unsigned *)$ebp + 1)$3 = 0x804864d

8048648: call 804857c <echo> 804864d: mov 0xffffffe8(%ebp),%ebx # Return Point

Return Address

Saved %ebp

[3][2][1][0] buf

%ebp

StackFrame

for main

StackFrame

for echo

0xbffff8d8

Return Address

Saved %ebp

[3][2][1][0] buf

StackFrame

for main

StackFrame

for echo

bf ff f8 f8

08 04 86 4d

xx xx xx xx

– 39 – 15-213, F’02

Buffer Overflow Example #1Buffer Overflow Example #1

Before Call to gets Input = “123”

No Problem

0xbffff8d8

Return Address

Saved %ebp

[3][2][1][0] buf

StackFrame

for main

StackFrame

for echo

bf ff f8 f8

08 04 86 4d

00 33 32 31

Return Address

Saved %ebp

[3][2][1][0] buf

%ebp

StackFrame

for main

StackFrame

for echo

– 40 – 15-213, F’02

Buffer Overflow Stack Example #2Buffer Overflow Stack Example #2

Input = “12345”

8048592: push %ebx 8048593: call 80483e4 <_init+0x50> # gets 8048598: mov 0xffffffe8(%ebp),%ebx 804859b: mov %ebp,%esp 804859d: pop %ebp # %ebp gets set to invalid value 804859e: ret

echo code:

0xbffff8d8

Return Address

Saved %ebp

[3][2][1][0] buf

StackFrame

for main

StackFrame

for echo

bf ff 00 35

08 04 86 4d

34 33 32 31

Return Address

Saved %ebp

[3][2][1][0] buf

%ebp

StackFrame

for main

StackFrame

for echo

Saved value of %ebp set to 0xbfff0035

Bad news when later attempt to restore %ebp

– 41 – 15-213, F’02

Buffer Overflow Stack Example #3Buffer Overflow Stack Example #3

Input = “12345678”

Return Address

Saved %ebp

[3][2][1][0] buf

%ebp

StackFrame

for main

StackFrame

for echo

8048648: call 804857c <echo> 804864d: mov 0xffffffe8(%ebp),%ebx # Return Point

0xbffff8d8

Return Address

Saved %ebp

[3][2][1][0] buf

StackFrame

for main

StackFrame

for echo

38 37 36 35

08 04 86 00

34 33 32 31

Invalid address

No longer pointing to desired return point

%ebp and return address corrupted

– 42 – 15-213, F’02

Malicious Use of Buffer OverflowMalicious Use of Buffer Overflow

Input string contains byte representation of executable code Overwrite return address with address of buffer When bar() executes ret, will jump to exploit code

void bar() { char buf[64]; gets(buf); ... }

void foo(){ bar(); ...}

Stack after call to gets()

B

returnaddress

A

foo stack frame

bar stack frame

B

exploitcode

pad

data written

bygets()

– 43 – 15-213, F’02

Exploits Based on Buffer OverflowsExploits Based on Buffer OverflowsBuffer overflow bugs allow remote machines to execute Buffer overflow bugs allow remote machines to execute

arbitrary code on victim machines.arbitrary code on victim machines.

Internet wormInternet worm Early versions of the finger server (fingerd) used gets() to

read the argument sent by the client:finger [email protected]

Worm attacked fingerd server by sending phony argument:finger “exploit-code padding new-return-address”exploit code: executed a root shell on the victim machine with a

direct TCP connection to the attacker.

– 44 – 15-213, F’02

Exploits Based on Buffer OverflowsExploits Based on Buffer OverflowsBuffer overflow bugs allow remote machines to execute Buffer overflow bugs allow remote machines to execute

arbitrary code on victim machines.arbitrary code on victim machines.

IM WarIM 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.

– 45 – 15-213, F’02

Date: Wed, 11 Aug 1999 11:30:57 -0700 (PDT) Date: Wed, 11 Aug 1999 11:30:57 -0700 (PDT) From: Phil Bucking <[email protected]> From: Phil Bucking <[email protected]> Subject: AOL exploiting buffer overrun bug in their own software! Subject: AOL exploiting buffer overrun bug in their own software! To: [email protected] To: [email protected]

Mr. Smith,Mr. Smith,

I am writing you because I have discovered something that I think you I am writing you because I have discovered something that I think you might find interesting because you are an Internet security expert with might find interesting because you are an Internet security expert with experience in this area. I have also tried to contact AOL but received experience in this area. I have also tried to contact AOL but received no response.no response.

I am a developer who has been working on a revolutionary new instant I am a developer who has been working on a revolutionary new instant messaging client that should be released later this year.messaging client that should be released later this year.......It appears that the AIM client has a buffer overrun bug. By itself 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. 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 But AOL is now *exploiting their own buffer overrun bug* to help in its efforts to block MS Instant Messenger.its efforts to block MS Instant Messenger.........Since you have significant credibility with the press I hope that youSince you have significant credibility with the press I hope that youcan use this information to help inform people that behind AOL'scan use this information to help inform people that behind AOL'sfriendly exterior they are nefariously compromising peoples' security.friendly exterior they are nefariously compromising peoples' security.

Sincerely,Sincerely,Phil Bucking Phil Bucking Founder, Bucking Consulting Founder, Bucking Consulting [email protected]@yahoo.com It was later determined that this email

originated from within Microsoft!

– 46 – 15-213, F’02

Code Red WormCode Red Worm

HistoryHistory 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 SiteWhen We Set Up CS:APP Web Site Received strings of formGET /default.ida?

NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNN....NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNN%u9090%u6858%ucbd3%u7801%u9090%u6858%ucbd3%u7801%u9090%u6858%ucbd3%u7801%u9090%u9090%u8190%u00c3%u0003%u8b00%u531b%u53ff%u0078%u0000%u00=a

HTTP/1.0" 400 325 "-" "-"

– 47 – 15-213, F’02

Code Red Exploit CodeCode Red Exploit Code

Starts 100 threads running Spread self

Generate random IP addresses & send attack stringBetween 1st & 19th of month

Attack www.whitehouse.govSend 98,304 packets; sleep for 4-1/2 hours; repeat

» Denial of service attackBetween 21st & 27th of month

Deface server’s home pageAfter waiting 2 hours

– 48 – 15-213, F’02

Code Red EffectsCode Red Effects

Later Version Even More MaliciousLater Version Even More Malicious Code Red II As of April, 2002, over 18,000 machines infected Still spreading

Paved Way for NIMDAPaved Way for NIMDA Variety of propagation methods One was to exploit vulnerabilities left behind by Code Red II

– 49 – 15-213, F’02

Avoiding Overflow VulnerabilityAvoiding Overflow Vulnerability

Use Library Routines that Limit String LengthsUse Library Routines that Limit String Lengths fgets instead of gets strncpy instead of strcpy Don’t use scanf with %s conversion specification

Use fgets to read the string

/* Echo Line */void echo(){ char buf[4]; /* Way too small! */ fgets(buf, 4, stdin); puts(buf);}

Documents

Machine-Level Programming III: Procedures Topics IA32 stack discipline Register saving conventions Creating pointers to local variables class07.ppt