• Definition:
  • A buffer overflow (or overrun) is a situation in which a program uses locations adjacent to a buffer (i.e., beyond one or both of the boundaries of a buffer).
• An Issue:
  • People frequently limit the definition of a buffer overflow to situations in which data is written to locations adjacent to the buffer
  • We will include both reading and writing since reading beyond the boundary can lead to harm (e.g., confidentiality and availability)
• Common (in Memory) Buffer Overflows in C:
  • When explicitly using an array
  • When using a string (i.e., implicitly using an array)

• A Common Idiom:
  • Pass an array and its length as different parameters
    (e.g., main(int argc, char* argv[]))
• The Problem:
  • A length that is too large might be passed in (or the length might be increased locally)

• Another Common Idiom:
  • void operation(int array[]) { int length = sizeof(array) / sizeof(array[0]); for (int i=0; i<length; i++) { // Operate on array[i] } }
• The Problem:
  • array is a parameter and, so, is a pointer type
  • Hence, sizeof(array) is the size of an int *, not the size of the array

• A Common Idiom:
  • Use a special value to indicate the last element of the array (e.g., -1 in an array of non-negative integers)
• The Problem:
  • The sentinel can be omitted or misspecified

• Recall:
  • C has character types char and wchar_t
  • A string in C is a contigous sequence of characters terminated by (and including) a sentinel character (the null character '\0')
• Data Structure:
  • Strings are stored in arrays
  • The length of the string is (i.e., should be) at least one less than the length of the array

• A Common Practice:
  • Use gets() to read characters (until it reaches a newline or end-of-stream character)
• An Example:
  • char line[81]; gets(line);
• The Problem:
  • An attacker can enter an indeterminate number of characters making it impossible to ensure that the buffer is large enough

• A Common Practice:
  • Copy a source string into a target/destination string using strcpy()
• An Example:
  • int main(int argc, char* argv[]) { char file_name[65]; char *temp; temp = argv[1] ? argv[1] : ""; strcpy(file_name, temp); }
• The Problem:
  • The attacker can supply a string that is too long (i.e., a file name that is longer than 64 characters in this example)

• A Common Practice:
  • Use strcat() to append a source string to a target string
• The Problem:
  • The target might not be long enough to hold the result

• A Common Practice:
  • Use sprintf() to create a (formatted) string
• An Example:
  • char line[81]; sprintf(line, "%2d: %s\n", i, user_input);
• The Problem:
  • sprintf() assumes that the buffer it is passed is large enough and the attacker might provide a string that is too long (i.e., longer than 80 - 2 - 2 - 1 = 75 characters in this example)

• Some Common Practices:
  • Not allocating memory for the null character (i.e., an off-by-one error when calculating the size)
  • Forgetting to include the null character (though there is space for it)
  • Using strncpy() to copy the first n characters from the source to the target when the source contains n or more characters (which results in a non-null terminated string)
• Some Problems That Arise:
  • strlen() uses the null character to determine the length of the string so any function that uses strlen() will have problems
  • Other functions (e.g., strcpy()) iterate until a null character is encountered so will have problems

• The Issue:
  • By overflowing a buffer the attacker can corrupt memory that is being used to store information of various kinds
• A Common Attack Vector:
  • User input

• The Issue:
  • By overflowing a buffer the attacker can corrupt the value of a pointer and, if the pointer is used in an assignment statement, corrupt the arbitrary address it now points to
• Sketch of an Example:
  • ... char buffer[BUFFER_SIZE]; long value = ...; long* p = ...; strncpy(buffer, argv[1], length); // Potential overflow into p *p = value; // Assign value to the address pointed to by p ...

• The Issue:
  • By overflowing a buffer the attacker can corrupt the value of function pointer and, if the function pointer is used subsequently, transfer control to arbitrary code
• Sketch of an Example:
  • ... static int value = ...; static char buffer[BUFFER_SIZE]; static void (*f)(int i); f = &some_function; strncpy(buffer, argv[1], length); // Potential overflow into f (void)(*f)(value); // Execute the code pointed to by f ...

• The Issue:
  • By overflowing a buffer the attacker can overwrite data in the memory allocated to the execution stack
• Ramifications:
  • The values of automatic variables can be modified
  • Program execution can be terminated
  • Arbitrary code can be executed

• A Memory Corruption Example:
  • cexamples/bufferoverflow/windows/string_overflow_data.c
     
• A Sample Windows/Intelx86 Execution:
  • Address of eid: 0x0804a030
  • Address of grade: 0x0804a024

• Notes:
  • Though eid only requires 9 bytes, space actually exists for 12 (for alignment reasons)
  • Intelx86 is little-endian
• Memory (Before and After):
  • 

If you compile using gcc -S smash.c a file containing the assembly language code (named smash.s) will be generated.

If you compile using gcc -o smash smash.c and then run gdb smash you can use the command disassemble main to see the assembly code that will be executed (i.e., with resolved addresses).

• The Result:
  • The program terminates abnormally
• Possible Reasons for an Abnormal Termination:
  • The new address is invalid
  • Memory at that address does not contain a valid CPU instruction
  • The CPU registers are not setup for the proper execution of the instruction
  • Memory at that address is not executable

• An Observation:
  • In the attack above, "random" characters were used to overflow the buffer
• A Question:
  • Can the buffer overflow be used to change the return to elsewhere in the program?
• The Answer:
  • Yes, if I can pass untypable characters (which I can using another program).

• Question:
  • Can control flow changes like those above be used to cause other kinds of harm?
• The Answer:
  • Yes, any code that exists in process memory can be executed (e.g., system() or exec()).

• A Question:
  • Is it possible to execute arbitrary code (i.e., code that isn't in process memory)?
• The Answer:
  • Yes, by using the buffer overflow to put instructions (often called shellcode for historical reasons) on the stack.

• A Question:
  • Isn't code injection difficult?
• An Answer:
  • It requires time, effort, and a knowledge of assembly language.
• Another Question:
  • Is there another way?
• The Answer:
  • Unfortunately, yes, using gadgets (or return-oriented programming).

• What Are Gadgets?
  • A gadget is a useful sequence of instructions.
• Are They Powerful?
  • A Turing-complete set of gadgets has been written for the IntelX86 architecture (i.e., any arbitrary program can be written using gadgets).
• How Are They Used?
  • A compiler can be used to create the gadgets from a higher-level language.

• Use a Single, Consistent Design:
  • Caller allocates, caller frees (C99, C11 Annex K)
  • Callee allocates, caller frees
  • Callee allocates, callee frees (C++ std::basic_string)
• Copying:
  • Avoid memcpy() and use memcpy_s() instead (C11) -- specifies the size and reports violations with the return value; reports NULL destination; reports the copying of overlapping segments
• Moving:
  • Avoid memmove() and use memmove_s() instead (C11) -- specifies the size and reports violations with the return value; reports NULL destinations

• Use a Single, Consistent Design:
  • Caller allocates, caller frees (C99, C11 Annex K)
  • Callee allocates, caller frees
  • Callee allocates, callee frees (C++ std::basic_string)
• Use the Right Functions (that Include the Maximum Size):
  • Never use gets(), use fgets() and/or getchar() (which are still vulnerable, but better) or gets_s() (C11)
  • Avoid strcpy() and strncpy() (which have different vulnerabilities) and use strcpy_s() instead (C11)
  • Avoid strcat() and strncat() (which have different vulnerabilities) and use strcat_s() instead (C11)

• An Observation:
  • It is awkward to validate data in every function (and, hence, is unlikely to be done in practice)
• A Reasonable Approach:
  • Any data that crosses a trust boundary should be validated
• Example Sources:
  • Command-line arguments
  • Environment variables
  • Sockets
  • Files
  • Pipes
  • Signals
  • Shared Memory
  • Devices

• An Observation:
  • The size of some entites can be determined reliably
• Obtaining this Information:
  • size_t __builtin_object_size(void* ptr, int type)
• Error Codes:
  • Returns 0 (if type is in {0,1}) or -1 or (if type is in {2,3}) if the size can't be determined

• Some Details:
  • The pointer needn't point to the start of the entity, it can point to a member
  • The value of type determines what is considered the "end of" the entity
• An Example:
  • struct Account {char name[10]; int id; char branch[20];} a; void* p = &a.id;
• Behavior:
  • If type is 0 or 2 then size_t __builtin_object_size(p, type) returns sizeof(int) plus 20 (i.e., the size to the end of a; the maximum remaining size)
  • If type is 1 or 3 then size_t __builtin_object_size(p, type) returns sizeof(int) (i.e., the size to the end of a.id; the minimum remaining size)

• Using Size Checking "By Hand":
  • Many functions that operate on arrays can make use of size checking to provide some protection
• Using Size Checking More Widely:
  • In GCC, many string functions will use size checking if the symbol _FORTIFY_SOURCE is defined

• The Goal:
  • Protect the return address on the stack from being written to
• The Approach:
  • Write a value that is difficult to insert/spoof to an address "before" that of the memory being protected
• The Analogy:
  • Like a canary in a coal mine, the special value would be "killed" by any attempt to write to the memory being protected

• Values that are Difficult to Insert:
  • CR, LF, NULL
• Values that are Difficult to Spoof:
  • 32-bit random number
  • 32-bit random number XORed with the return address

• Stack Canaries in GCC (SSP/ProPolice):
  • Use the -fstack-protector-all flag
  • Note: This tool also changes the organization of the stack (i.e., places the canary "after" arrays)
• Stack Canaries in Visual C:
  • Use the /GS flag

• Purpose:
  • Prevent the execution of arbitrary code
• How It Works:
  • Randomizing the addresses of stack pages (e.g., using complete randomization or a randomly-sized gap) makes it more difficult for attackers to predict the addresses (e.g., of shellcode, system functions, and/or gadgets) that they want to return to
• Examples:
  • Linux (Debian since 2007, Ubuntu since 2008): See sysctl -w kernel.randomize_va_space
  • MS Windows (Since Vista): See the /DYNAMICBASE linker option

• Purpose:
  • Prevent the execution of malicious code from the stack
• Limitations:
  • Doesn't prevent the execution of malicious code from the heap
  • Doesn't prevent the execution of malicious code from data segments

• Purpose:
  • Prevent the execution of malicious code
• How it Works:
  • Parts of the process memory space are marked as writable (W) or executable (X), but not both (e.g., W xor X)
• Hardware Support:
  • Include the ability to mark memory pages as data only, disabling the execution of code in those pages

• Hardware Support (cont.):
  • AMD - NoeXecute (NX) bit
  • Intel - eXecute Disable (XD) bit
  • ARM - eXecute Never (XN) bit
• Operating System Support:
  • MS Windows - Data Execution Prevention (DEP) using /NXCOMPAT
  • Linux - PaX

• Oracle - Silicon Secured Memory (SSM):
  • 64-bit pointers use some of the bits for a "color" and the chip checks to ensure that it points to memory of the appropriate "color"
  • Sometimes called Application Data Integrity (ADI)

• An Observation:
  • There are now many compile-time and run-time mitigation strategies in place to help protect against buffer overflow vulnerabilities
• One Implication:
  • You may need to temporarily disable some of these mitigation strategies while experimenting
• Examples:
  • gcc: -fno-stack-protector
  • gcc: -fno-defer-pop
  • shell: su echo 0 > /proc/sys/kernel/randomize_va_space (it normally contains the value 2)
  • shell: execstack -s
  • Visual C: Don't use /GS