Buffer overflow protection refers to various techniques used during software development to enhance the security of executable programs by detecting buffer overflow
Buffer overflow
In computer security and computer programming, a buffer overflow, or buffer overrun, is an Anomaly in software condition where a process attempts to store data beyond the boundaries of a fixed-length buffer .... s on stack
Call stack
In computer science, a call stack is a dynamic Stack data structure that stores information about the active subroutines of a computer program.... -allocated variables as they occur and preventing them from becoming serious security
Computer security
Computer security is a branch of technology known as information security as applied to computers. The objective of computer security can include protection of information from theft or corruption, or the preservation of availability, as defined in the security policy.... vulnerabilities. There have been several implementations of buffer overflow protection, two of the most common being StackGuard, and Stack-smashing Protection (SSP, also known as ProPolice).
cally, buffer overflow protection modifies the organization of data in the stack frame of a function call to include a "canary" value which, when destroyed, shows that a buffer preceding it in memory has been overflowed.
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Buffer overflow protection refers to various techniques used during software development to enhance the security of executable programs by detecting buffer overflow
Buffer overflow
In computer security and computer programming, a buffer overflow, or buffer overrun, is an Anomaly in software condition where a process attempts to store data beyond the boundaries of a fixed-length buffer .... s on stack
Call stack
In computer science, a call stack is a dynamic Stack data structure that stores information about the active subroutines of a computer program.... -allocated variables as they occur and preventing them from becoming serious security
Computer security
Computer security is a branch of technology known as information security as applied to computers. The objective of computer security can include protection of information from theft or corruption, or the preservation of availability, as defined in the security policy.... vulnerabilities. There have been several implementations of buffer overflow protection, two of the most common being StackGuard, and Stack-smashing Protection (SSP, also known as ProPolice).
How it works
Typically, buffer overflow protection modifies the organization of data in the stack frame of a function call to include a "canary" value which, when destroyed, shows that a buffer preceding it in memory has been overflowed. This gives the benefit of preventing an entire class of attacks. According to some software vendors, the performance impact of these techniques is negligible.
Canaries
Canaries or canary words are known values that are placed between a buffer and control data on the stack to monitor buffer overflows. When the buffer overflows, the first data to be corrupted will be the canary, and a failed verification of the canary data is therefore an alert of an overflow, which can then be handled, for example, by invalidating the corrupted data.
There is increasing recognition that wild, domestic, and companion animals may act as "sentinels" for environmental health hazards by providing early warning of human health hazards in the environment.... , since they would be affected by toxic gases earlier than the miners, thus providing a biological warning system.
There are three types of canaries in use: Terminator, Random, and Random XOR. Current versions of StackGuard support all three, while ProPolice supports Terminator and Random canaries.
Terminator canaries
Terminator Canaries use the observation that most buffer overflow attacks are based on certain string operations which end at terminators. The reaction to this observation is that the canaries are built of NULL
Null
Null is an English word meaning 'nothing' or without value or consequence. It is derived from the Latin word nullus meaning 'none'.Null may refer to:... terminators, CR, LF, and -1. The undesirable result is that the canary is known. Even with the protection, an attacker could potentially overwrite the canary and control information, then go back and write a shorter overflow to fix the canary. This is only effective in rare cases where double-overflows are possible.
Random canaries
Random canaries are randomly generated, usually from an entropy
Entropy (computing)
In computing, entropy is the randomness collected by an operating system or application for use in cryptography or other uses that require random data.... -gathering daemon
Daemon (computer software)
In Unix and other computer computer multitasking operating systems, a daemon is a computer program that runs in the background , rather than under the direct control of a user; they are usually initiated as background Computer processes.... , so an attacker doesn't know what it is. Usually, it is not logically possible or plausible to read the canary for exploiting; the canary is a secure value known only by those who need to know it—the buffer overflow protection code in this case.
Normally, a random canary is generated at program initialization, and stored in a global variable. This variable is usually padded by unmapped pages, so that attempting to read it using any kinds of tricks that exploit bugs to read off RAM cause a segmentation fault, terminating the program. It may still be possible to read the canary, if the attacker knows where it is, or can get the program to read from the stack.
Random XOR canaries
Random XOR Canaries are Random Canaries that are XOR scrambled using all or part of the control data. In this way, once the canary or the control data is clobbered, the canary value is wrong.
Random XOR Canaries have the same vulnerabilities as Random Canaries, except that the 'read from stack' method of getting the canary is a bit more complicated. The attacker must get the canary, the algorithm, and the control data to generate the original canary for re-encoding into the canary he needs to use to spoof the protection.
In addition, Random XOR Canaries can protect against a certain type of attack involving overflowing a buffer in a structure into a pointer to change the pointer to point at a piece of control data. Because of the XOR encoding, the canary will be wrong if the control data or return value is changed. Because of the pointer, the control data or return value can be changed without overflowing over the canary.
Although these canaries protect the control data from being altered by clobbered pointers, they do not protect any other data or the pointers themselves. Function pointers especially are a problem here, as they can be overflowed into and will execute shellcode when called.
Attacks that cannot be protected against
Stack-smashing protection is unable to protect against certain forms of attack. For example, it cannot protect against buffer overflows in heap.
StackGuard and ProPolice cannot protect against overflows in automatically allocated structures which overflow into function pointers. ProPolice at least will rearrange the allocation order to get such structures allocated before function pointers. A separate mechanism for pointer protection
Buffer overflow
In computer security and computer programming, a buffer overflow, or buffer overrun, is an Anomaly in software condition where a process attempts to store data beyond the boundaries of a fixed-length buffer .... was proposed in PointGuard and is available on Microsoft Windows.
There is no sane way to alter the structure of a structure; structures are expected to be the same between modules, especially with shared libraries. Any data in a structure after a buffer is impossible to protect with canaries; thus, programmers must be very careful about how they organize their variables and use their structures. Structures with buffers should be malloc
Malloc
In computing, malloc is a subroutine provided in the C and C++'s standard library for performing dynamic memory allocation.... ed or new[]ed pointers.
Implementations
StackGuard
StackGuard was the first canary stack protector, released for GCC in 1997, and published at . StackGuard is an extension to GCC
GNU Compiler Collection
The GNU Compiler Collection is a compiler system produced by the GNU Project supporting various programming languages. GCC is a key component of the GNU toolchain.... that provides buffer overflow protection. It was invented by Crispin Cowan, first implemented as a zero canary in the i386 backend for GCC 2.7.2.2 by Aaron Grier, and verified by Peat Bakke. Perry Wagle continued maintenance of StackGuard for the Immunix project, and implemented the Terminator, Random, and Random XOR canaries.
StackGuard was made available as a standard part of the Immunix
Immunix
Immunix was a commercial operating system that provided host-based application Computer security solutions. The last release of Immunix's GNU/Linux distribution was version 7.3 on November 27, 2003.... Linux distribution from 1998 to 2003, providing both Red Hat-compatible binary RPMs and patched GCC sources from GCC 2.7.2.3 through 2.96.
StackGuard was suggested for implementation in GCC according to the GCC 2003 Summit Proceedings and ; however, gcc
GNU Compiler Collection
The GNU Compiler Collection is a compiler system produced by the GNU Project supporting various programming languages. GCC is a key component of the GNU toolchain.... 3.x offers no official buffer overflow protection, and the SSP concept below has been adapted for GCC 4.1 instead.
GCC Stack-Smashing Protector (ProPolice)
The "Stack-Smashing Protector" or SSP, also known as ProPolice, is an enhancement of the StackGuard concept written and maintained by Hiroaki Etoh of IBM
IBM
International Business Machines Corporation, abbreviated IBM and nicknamed "Big Blue" , is a multinational corporation computer technology and consulting corporation headquartered in Armonk, New York, New York, United States.... . Its name derives from the word propolis
Propolis
Propolis is a resinous mixture that honey bees collect from tree buds, sap flows, or other botanical sources. It is used as a sealant for unwanted open spaces in the beehive .... . ProPolice differs from StackGuard in 3 ways:
ProPolice moves canary code generation from the back-end to the front-end of the compiler.
ProPolice also protects all registers saved in function's prologue (for example the frame pointer), and not only the Return Address
Return address
In postal mail, a return address is an explicit inclusion of the address of the person sending the message. It provides the recipient with a means to determine how to respond to the sender of the message if needed.... .
ProPolice, in addition to canary protection, also sorts array variables (where possible) to the highest part of the stack frame, to make it more difficult to overflow them and corrupt other variables. It also creates copies of arguments of the function, and relocates them together with local variables, effectively protecting the arguments.
The GNU Compiler Collection is a compiler system produced by the GNU Project supporting various programming languages. GCC is a key component of the GNU toolchain.... 3.x; a less intrusive reimplementation is included in the GCC 4.1 release. Currently, SSP is standard in OpenBSD
OpenBSD
OpenBSD is a Unix-like computer operating system descended from Berkeley Software Distribution , a Unix derivative developed at the University of California, Berkeley.... , FreeBSD
FreeBSD
FreeBSD is a Unix-like free software operating system descended from AT&T Unix via the Berkeley Software Distribution branch through the 386BSD and Berkeley Software Distribution#4.4BSD and descendants operating systems.... (since 8.0), Ubuntu (since 6.10), DragonFly BSD
DragonFly BSD
DragonFly BSD is a Free software Unix-like operating system created as a fork of FreeBSD 4.8. Matthew Dillon , a FreeBSD and Amiga developer since 1994, began work on DragonFly BSD in June 2003 and announced it on the FreeBSD mailing lists on July 16, 2003.... and the IPCop
IPCop
IPCop is a Linux distribution which aims to provide a simple-to-manage firewall appliance based on IBM PC compatible hardware. IPCop is a Stateful firewall built on the Linux netfilter framework.... Linux
Linux
Linux is a generic term referring to Unix-like computer operating systems based on the Linux kernel. Their development is one of the most prominent examples of free and open source software collaboration; typically all the underlying source code can be used, freely modified, and redistributed by anyone under the terms of the GNU GPL license... distribution. It is also available in NetBSD
NetBSD
NetBSD is a freely redistributable, open source version of the Unix-derivative Berkeley Software Distribution computer operating system. It was the second open source BSD descendant to be formally released, after 386BSD, and continues to be actively developed.... , Debian
Debian
DebianGNU/Linux is one of the most popular and influential computer operating systems composed of free software and open source software.... and Gentoo
Gentoo Linux
Gentoo is a computer operating system built on top of the Linux Kernel and based on the Portage package management system. It is distributed as Free software.... , disabled by default.
By default, stack-smashing protection can be attained by adding the -fstack-protector flag for string protection, or -fstack-protector-all for protection of all types. On some systems, as under OpenBSD, ProPolice is enabled by default, and the -fno-stack-protector flag disables it. With GCC 4.1 and above the array size threshold for stack-smashing protection to be enabled can be tuned with --param ssp-buffer-size=....
Microsoft Visual Studio is an integrated development environment from Microsoft. It can be used to develop Console application and graphical user interface applications along with Windows Forms applications, web sites, web applications, and web services in both native code together with managed code for all platforms supported by Microsoft W... /GS
The compiler suite from Microsoft also implements buffer overflow protection since version 2003. It is enabled by using the "/GS" flag in compiler options.
StackGhost (hardware-based)
Invented by Mike Frantzen, StackGhost is a simple tweak to the register window spill/fill routines which makes buffer overflows much more difficult to exploit. It uses a unique hardware feature of the Sun Microsystems
Sun Microsystems
Sun Microsystems, Inc. is a multinational corporation vendor of computers, computer components, computer software, and information technology services, founded on February 24, 1982.... SPARC
SPARC
SPARC is a Reduced Instruction Set Computer microprocessor instruction set Computer architecture originally designed in 1985 by Sun Microsystems.... and SPARC64 architecture (that being: deferred on-stack in-frame register window spill/fill) to detect modifications of return pointers (a common way for an exploit
Exploit
Exploit can mean:*Exploit *Exploit *Exploit *Exploitation*An achievement. The first summit of mount Everest was a stunning exploit.*The longest river on the island of Newfoundland is called the Exploits River.... to hijack execution paths) transparently, automatically protecting all applications without requiring binary or source modifications. The performance impact is negligible, less than one percent. The resulting gdb issues were resolved by Mark Kettenis two years later, allowing enabling of the feature. Following this event, the StackGhost code was integrated (and optimized) into OpenBSD
OpenBSD
OpenBSD is a Unix-like computer operating system descended from Berkeley Software Distribution , a Unix derivative developed at the University of California, Berkeley.... /SPARC.
An example of canaries
Normal buffer allocation for x86 architectures and other similar architectures is shown in the buffer overflow
Buffer overflow
In computer security and computer programming, a buffer overflow, or buffer overrun, is an Anomaly in software condition where a process attempts to store data beyond the boundaries of a fixed-length buffer .... entry. Here, we will show the modified process as it pertains to StackGuard.
When a function is called, a stack frame is created. A stack frame is built from the end of memory to the beginning; and each stack frame is placed on the top of the stack, closest to the beginning of memory. Thus, running off the end of a piece of data in a stack frame alters data previously entered into the stack frame; and running off the end of a stack frame places data into the previous stack frame. A typical stack frame may look as below, having a Return Address
Return address
In postal mail, a return address is an explicit inclusion of the address of the person sending the message. It provides the recipient with a means to determine how to respond to the sender of the message if needed.... (RETA) placed first, followed by other control information (CTLI).
C is a general-purpose computer programming language originally developed in 1972 by Dennis Ritchie at the Bell Telephone Laboratories to implement the Unix operating system.... , a function may contain many different per-call data structures. Each piece of data created on call is placed in the stack frame in order, and is thus ordered from the end to the beginning of memory. Below is a hypothetical function and its stack frame.
int foo
(d..)(c.........)(b...)(a...)(CTLI)(RETA)
In this hypothetical situation, if more than ten bytes are written to the array c, or more than 13 to the character array d, the excess will overflow into integer pointer b, then into integer a, then into the control information, and finally the return address. By overwriting b, the pointer is made to reference any position in memory, causing a read from an arbitrary address. By overwriting RETA, the function can be made to execute other code (when it attempts to return), either existing functions (ret2libc
Return-to-libc attack
A return-to-libc attack is a computer security attack usually starting with a buffer overflow in which the return statement on the stack is replaced by the address of another function and an additional portion of the stack is overwritten to provide arguments to this function.... ) or code written into the stack during the overflow.
In a nutshell, poor handling of c and d, such as the unbounded strcpy
Strcpy
The C offers a C standard library called strcpy, defined in the string.h header file, that allows null-terminated memory blocks to be copied from one location to another.... calls above, may allow an attacker to control a program by influencing the values assigned to c and d directly. The goal of buffer overflow protection is to detect this issue in the least intrusive way possible. This is done by removing what can be out of harms way and placing a sort of tripwire, or canary, after the buffer.
Buffer overflow protection is implemented as a change to the compiler. As such, it is possible for the protection to alter the structure of the data on the stack frame. This is exactly the case in systems such as ProPolice. The above function's automatic variables are rearranged more safely: arrays c and d are allocated first in the stack frame, which places integer a and integer pointer b before them in memory. So the stack frame becomes
(b...)(a...)(d..)(c.........)(CTLI)(RETA)
As it is impossible to move CTLI or RETA without breaking the produced code, another tactic is employed. An extra piece of information, called a "canary" (CNRY), is placed after the buffers in the stack frame. When the buffers overflow, the canary value is changed. Thus, to effectively attack the program, an attacker must leave definite indication of his attack. The stack frame is
(b...)(a...)(d..)(c.........)(CNRY)(CTLI)(RETA)
At the end of every function there is an instruction which continues execution from the memory address indicated by RETA. Before this instruction is executed, a check of CNRY ensures it has not been altered. If the value of CNRY fails the test, program execution is ended immediately. In essence, both serious attacks and harmless programming bugs result in a program abort.
The canary technique adds a few instructions of overhead for every function call with an automatic array, immediately before all dynamic buffer allocation and after dynamic buffer deallocation. The overhead generated in this technique is not significant. It does work, though, unless the canary remains unchanged. If the attacker knows that it's there, he may simply copy over it with itself. This is usually difficult to arrange intentionally, and highly improbable in unintentional situations.
The position of the canary is implementation specific, but it is always between the buffers and the protected data. Varied positions and lengths have varied benefits.
Address space layout randomization is a Computer insecurity technique which involves randomly arranging the positions of key data areas, usually including the base of the executable and position of Library , Dynamic memory allocation, and Stack-based memory allocation, in a Process 's address space....
Pax may refer to:* the Latin language word for peace, used in phrases such as Pax Romana ; also, its personification, Pax , goddess of peace in Roman mythology...
Static code analysis is the Program analysis that is performed without actually executing programs built from that software . In most cases the analysis is performed on some version of the source code and in the other cases some form of the object code....
A memory debugger is a programming tool for finding memory leaks and buffer overflows. These are due to Software bug related to the allocation and deallocation of dynamic memory....
Elias Levy , was the moderator of the full disclosure vulnerability mailing list Bugtraq from May 14 1996, until he stepped down on October 15 2001....
