Data Execution Protection

esarjeant writes "In addition to a number of other security features, anti-virus vendors are starting to push buffer overflow detection. This will be part of Microsoft's future direction with Data Execution Prevention (DEP) and is already integrated with McAfee 8.0i. So it looks like everyone is going to upgrade all of their software again, will software vendors be able to keep up with the support calls?"

Comments

Virus vendors?

[King Of Chat]

Who buys viruses?

Re:Virus vendors?

[BorgCopyeditor]

I hope they don't have it on the same hard drive as their Norton Virus; if those two come into contact ... it would be bad.

support calls

[millahtime]

So it looks like everyone is going to upgrade all of their software again, will software vendors be able to keep up with the support calls?

Yes, with more automation, more people on the other end (most likely in India) and more cost passed onto the customer. When I used to work we used to have a saying. "If it weren't for Microsoft, we would all be out of jobs"

virus vendors?

[2020hindsight]

Virus vendors have been pushing buffer overflows for quite some time ...

Virus vendors eh?

[kevb]

Virus venders.. hmmm For just £39.95 a month, you too can recieve the latest virii, trojans and worms directly to your inbox.

Re:Virus vendors eh?

[RaguMS]

Virus venders.. hmmm For just £39.95 a month, you too can recieve the latest virii, trojans and worms directly to your inbox.

What a ripoff... I get all of mine for free.

Re:Virus vendors eh?

[penguinoid]

Please, do not pirate closed source viruses. Instead, use open source viruses, which you can get for free.

What is it with the buffer overflows?`

[Anonymous Coward]

I'm just a microcontroller guy, but can't the PC guys check their goddamn counters and pointers when using buffers? And why the hell do we still need to code buffers? Isn't there a library or a call to handle buffers in a safe way?

Re:What is it with the buffer overflows?`

[ThosLives]

I don't even think it's due to not checking pointers and NX bits or anything like that. The problem is the way in which our modern OSs map out the memory. Intel chips have the capability to map segments to be either code or data, and the chip will generate a fault if you try to execute anything in a data segment (inherent NX capability). This is part of the segment descriptors used in all programs. The problem is that, as far as I can tell, Windows maps both the code and data segments to the same logical addresses! This is kind of foolish; it should be possible to simply map these two segments to different areas and be completely transparent to the application. As long as applications are behaved and don't have segment overrides all over the place, this should be just fine. Then, when you try to jump to an address that's in the stack, the processor will trip a general protection fault (because the stack must be in a segment defined as data, well, stack to be precise).

Basically this is just laziness in the Windows architecture that overlaps the code and data segments. Separate these and the problem is solved with no new hardware, minimal application rework, and the like.

Incidentally, my perusal of the setup routines in Linux (well, it was version 1.0, so I don't know if this is still the case) show that it also maps code and data to the same actual addresses, which makes it vulnerable as well.

Sure, you can use "smart" languages and NX bits and stuff like that, but it's all assembly at some level, and the processor manufactures actually built in sufficient protection decades ago when they came up with segmented memory. (PowerPC architecture can also distinguish between code and non-code).

I am always amused at how the memory management community hasn't nipped this one in the bud ages ago when the tools to fix it already exist.

Re:What is it with the buffer overflows?`

[Swamii]

can't the PC guys check their goddamn counters and pointers when using buffers?

We try our best, but we're humans. We make mistakes.

And why the hell do we still need to code buffers? Isn't there a library or a call to handle buffers in a safe way?

Yes. In fact, most modern languages like Java and C# handle memory for us; no more deletes necessary, and buffer overflows, while not impossible, more much less likely to happen with higher level languages.

Re:What is it with the buffer overflows?`

[codegen]

Part of the problem is the reliance on langauges which are over permissive. There was a whole class of languages developed in the 80's and 90's such as Euclid, Turing (both from U of T), and Modula which were much more strongly checked. Indeed the semantics of the languages allowed for many of the runtime checks to be statically eliminated. See the papers "Proof Rules for the Programming Language Euclid", R.L. London et al., Acta Informatica, And "On Legality Assertions in Euclid", D.B. Wortman, IEEE Transactions on Software Engineering.

C and C++ put the reliance on the programmer to check the rules under the assumption that compiler provided checks are too expensive. They are only too expensive if you assume the everthing-is-a-pointer model that underlies these languages. Java and C# gain some safety since they do not allow arbitrary pointers, but, in my opinion, have still inherited too much from the parent laguages.

Part of the problem is the everything looks like a nail approach. There are some wonderful languages out there that are much more appropriate for many of the tasks, and have syntax and semantics that make many of the security problems much easier to solve. However, they are not the "mainstream" langauges and as such do not get the developer attention.

Re:What is it with the buffer overflows?`

[Nevo]

This description of the problem is flat-out wrong.

Nobody uses segments any more. Win32 programming uses a flat 32-bit address space.

The problem stems from the fact that, under the Intel architecture, procedure local variables are allocated on the stack right next to the return address pointer. If a lazy programmer allocates a 256 byte buffer and does a strcpy() that doesn't have a null within the first 256 bytes, strcpy() will keep copying data until it hits a null character, clobbering the return address (and other context information for the previous stack frame) as it does so. When the current function hits a RET instruction, the processor will jump to the overwritten return address.

DEP does nothing more than expose existing bugs. If your code triggers DEP, you already had a bug in your code... be thankful that DEP points this out to you!

Will software vendors be able to keep up?

[TheBrakShow]

"Will software vendors be able to keep up with the support calls?" No. Customers are going to have to wait on hold for... Oh... Nevermind.

I'm being optimistic

[hardcoredreamer]

"So it looks like everyone is going to upgrade all of their software again, will software vendors be able to keep up with the support calls" I will be optimistic that despite the development into a new direction, and the occasional headaches, things will be better in the future. That said, why are people so negative about change? So Microsoft's SP2 broke some programs, at least they finally released it. So we have more than 640K of memory and you had to use a memory manager, at least we got past conventional memory. So at least in theory, there will be less buffer under runs in patched/upgraded systems. Would you prefer they didn't try?

What is a Buffer Overflow?

[lecithin]

I hate to ask, but as a person that has never been into 'code', I have never understood what a buffer overflow was.

I am asking as a person that isn't a programmer but understands the concepts that go behind the smoke and mirrors.

Re:What is a Buffer Overflow?

[Anonymous Coward]

Wikipedia to the rescue

Re:What is a Buffer Overflow?

[RupW]

It's usually where you've assume that user input or decoded data won't exceed a certain length, and if the user deliberately enters too much data then they can scribble over the call stack and e.g. change the function return pointer and take control of the program. See Wikipedia.

Re:What is a Buffer Overflow?

[alc6379]

This is the way I understand it, and I'm not really a programmer. So, I know someone's going to clarify or refute:

You have some memory allocated for some type of variable, or something. That's called a buffer, and it's usually a certain number of bytes "big". There's a function in your program that puts a value into that variable. If you can feed more data into the buffer than it can handle, you can have a buffer overflow.

The reason why this is dangerous is because that data "spills" into another portion of the memory, which could already be occupied by anything from more data, to executable code. In the latter case, if you've overwritten executable code, you can replace that code with your own executable code, and do all kinds of nasty things that the original program wasn't intended to do.

...And again, this is from one layman to another-- that's how I understand it.

Re:What is a Buffer Overflow?

[TripMaster Monkey]

Great explanation of buffer overflows here

Re:What is a Buffer Overflow?

[goombah99]

The most common form is as follows. When a subroutine is called the return address is placed on the stack. Then all the local variables for the subroutine are placed on the stack. the subroutine runs and when it finishes it jumps to the return address on the stack. However if the subroutine were to write data into an array or string on the stack and tried to push more data into the string than space was allocated it would continue writing past the end of the array and eventually overwrite the return address. This allows a way to substitute a new return address for a virus maker. If this return address happened to jump right back onto the string itself then in principle the data string will now be exceuted as code.

partial remedial solutions include commands that prevent decleared data from being executed, having the return address stored on a different stack from the data stack, explicitly testing the stack integrity before executing a return from a subroutine, and putting up "electric fences" --basically buffer regions around every memory allocation that are not owned by the application requesting space.

Re:What is a Buffer Overflow?

[3.2.3]

See, I appreciate this explanation, and the one below which reframes the explanation as occuring on the stack. These are the explanations I've always understood. And which, frankly, didn't fully cut it with me.

*Many* moons ago, I took an OS writing course from Intel, on the 80286. The way I was taught, a buffer overflow is something that would not have been possible in the processor architecture. There were code segments, and data segments. If ever the twain should overlap, processor exceptions occur, whether on the heap on in the stack. Just intercepting these faults took a major act of god on the part of the processor's primary process.

The, a few years later, I got a reeducation in the same coure, only on the 80386. Heck, there was even more protection for processes on that processor. A lot of stuff that had to be implemented in software before, like determining which process caused the fault above would be done in hardware.

OS2 2.2 was so bullet proof mainly because it took such good advantage of these hardware protective mechanisms. When I read Gates's screeds against OS2, I find myself either laughing hysterically or yelling "liar" (because he tells some pretty huge whoppers about the development).

Add to all this, most OSes dynamically allocate memory to processes, so even if you could overlay code with data and manage to get it executed, getting it to overlay in the right place and on the right byte boundardy without causing a fault would seem pretty unlikely.

I guess as one who doesn't try to write malware, just the very idea of these overflow explanations seems so unlikely that even if I were wanting to write such programs, I wouldn't consider buffer or stack overflow as an idea.

I suppose I'm saying, the explanations I've heard about buffer overflow make sense to a point. And then they seem to run up against facts as I know them. I'd like to see explanations of buffer overflow that make complete sense.

i386 architecture as I understand it allows the OS programmer to place the processor in a mode which basically defeats all the code vs data and process control mechanisms in the hardware. In my understanding, this is something done only at boot time, and only for the period of time necessary to set up the code which kicks the processor into a hardware mode which supports memory and process partitioning for multitasking. Please don't tell me, not even as some karma raising "funny" joke, that Microsoft doesn't even use the hardware modes which I would presume would prevent buffer overflow from ever occuring?

Re:What is a Buffer Overflow?

[nudicle]

Quite a good writeup of stack buffer overflows can be found here.

Re:What is a Buffer Overflow?

[Just Some Guy]

You got it write, except that overwriting other data can be just as bad as overwriting executable code:

char buffer[100];
int dataHasBeenVirusChecked = 0;
gets(buffer);
if (dataHasBeenVirusChecked) { sendAsEmailAttachment(buffer); }

In this case, if "buffer" gets overfilled just so, then the program may incorrectly believe that the data it contains is safe to operate on even though it might not be. Remember, folks, there are other ways to exploit an overflowable buffer then the standard "write executable code to stack and jump to it" method.

Re:What is a Buffer Overflow?

[ayn0r]

I guess as one who doesn't try to write malware, just the very idea of these overflow explanations seems so unlikely that even if I were wanting to write such programs, I wouldn't consider buffer or stack overflow as an idea.

Dude, you're making it sound like it's a matter of faith whether stack/heap overflows can be done at all. :-)
Noone said it's easy and quickly done to write a working exploit. It takes time to find the vulnerabilities, and still much more time to write code exploiting them.

Add to all this, most OSes dynamically allocate memory to processes, so even if you could overlay code with data and manage to get it executed, getting it to overlay in the right place and on the right byte boundardy without causing a fault would seem pretty unlikely.

Not at all unlikely if you take advantage of offsets that already exist within the program. As soon as you've successfully determined where in memory the program data resides, you can use it as an offset simply put. You bring up a good point though, because this is one common misconception about exploits. A good portable exploit has to take use of memory offsets to work properly.

Please don't tell me, not even as some karma raising "funny" joke, that Microsoft doesn't even use the hardware modes which I would presume would prevent buffer overflow from ever occuring?

This isn't limited to Windows. AFAIK all common OS:es share these problems. Now I haven't checked up on these CPU features you're talking about but it's nothing I've ever heard of...

For further reading I recommend "The Shellcoder's Handbook" by Jack Koziol and a bunch of others. It explains the basics on finding security holes and exploiting/securing them, and delves a bit deeper in a bunch of areas as well. Excellent read.

Re:What is a Buffer Overflow?

[TheLink]

It seems really silly and dangerous to mix code and data stacks. Why is it so common?

Maybe it will slow down CPUs, but I think that if a CPU knows that a stack will ONLY ever contain return addresses and another stack only contains data there can be a fair number of optimizations.

If you want to really be paranoid, have 3 stacks. One stack for code (return addresses), one stack for data (variables), and one stack for metadata - e.g. each entry could store the end location of the data (e.g. the data stack pointer before a subroutine call).

The popular method just seems really sloppy and error prone.

Re:great news

[JeanBaptiste]

question. this is _not_ a troll.

So MS is pushing for (what I'm guessing is) some sort of protection for application layer buffer overflows.

Does linux have any sort of thing like this? I know microsoft doesn't hold a monopoly on buffer overflows ;)

Seriously, I'm curious. Thanks.

Glad this is being addressed... :P

[TripMaster Monkey]

This will be part of Microsoft's future direction with Data Execution Prevention (DEP)

I feel safer already.

Exploits can be pure data

[redelm]

Malware doesn't need to bring in code, there's plenty of code in the target executable. All it needs is to be able to grab control via the return address on the stack. Then fill the stack with exploit data and set the return addr to something like an exec() syscall.

CSA already does this

[wschalle]

Cisco Systems CSA product does this and more.

Looks like...

[eno2001]

Microsoft and Intel are finally catching up to where DEC was back in 1992. DEC Alpha + OpenVMS = no such thing as a buffer overflow and 64 bit processing as well. Whatever happened to the future again? ;P

Re:Looks like...

[Florian Weimer]

DEC Alpha + OpenVMS = no such thing as a buffer overflow and 64 bit processing as well.

DEC ported the Microsoft DCOM implementation from Windows to OpenVMS, including its buffer overflow bugs.

Not a silver bullet

[TwistedSquare]

DEP will not prevent all buffer overflow attacks. It is intended to protect from the attack where the return address of the stack is overwritten to make the program jump into the stack. However, the program could still jump into a useful portion of existing code, or simply crash, or keep running but overflow a flag variable on the stack that will cause odd behaviour. It can also prevent things like JIT/HotSpot compilation. I'm not saying it's not useful at all, but it is one of many measures that all help a little.

A Flawed Architecture

[nurb432]

The basic architecture is fundamentally flawed in today's 'consumer grade' computers. Using a strict Harvard architecture, where data is *separate* from code, would eliminate a lot of today's troubles.

Is it too late to change? Well, we have had new chips arise ( like power , or CELL ) so, its not impossible.. just difficult.

Re:A Flawed Architecture

[iabervon]

Sure, you can have your code and data in separate spaces if you want. Atmel will sell you a very nice computer that works that way. Of course, you need to reprogram it externally if you want to run something different on it. The notion of running different programs without something external replacing the program memory doesn't work on a Harvard architecture.

And a Harvard architecture doesn't help, anyway, if your program contains routines that an attacker would like to run with chosen data, because the stack, which holds the return address of the current function, must be data memory. So the attacker can replace the return address with the address of the target function, arrange arguments appropriately, and return. The security advantages of a Harvard architecture does have are available already, with MMU flags which prevent instruction fetch from reading non-code pages; the issue is not treating code as data, but treating data as code (without some code telling the MMU that the pages should be code).

A more significant security improvement is disallowing pointers into the stack, and modifying it only with special stack instructions with constant offsets from the top of the stack. This forces people to put their buffers in heap, where overflows are much harder to exploit. (See, for example, the JVM.)

Strengths and differences of this vs SELinux

[weave]

I just got done reading an interesting article about SELinux. I'm just curious as to the strengths and weaknesses of each approach.

The SELinux approach sounds to me like a far better way to approach this, actually controlling the permissions of a process with some high degree of precision, down to what files it can use and what other processes it can invoke.

Anyone learned in this stuff care to give a non-flamed opinion of the two approaches strengths and weaknesses? Also, do or will the newer Linux kernels do anything similar regarding stack protection?

Re:Strengths and differences of this vs SELinux

[kbielefe]

This is a good introduction to the main solutions to software exploits in Linux and the different kinds of protection they provide and why.

Most people recommend a combined approach including mandatory access control, chroot jails for services on the internet, stack smash protection, address space layout randomization, non-executable memory pages, firewalls, virus and spyware scanning, intrusion detection, regular vulnerability patching, and user education (did I leave anything out?). No one will tell you that you are safe after implementing just one of these solutions, but the more you do implement, the more secure your system will be.

All of the above have been available on Linux for some time, but are not implemented by default in any popular distribution that I am aware of, which is a shame because I believe it is only a matter of time before someone writes a really nasty worm for Linux. Most Linux users I know seem to believe they are safe with only regular patching and a firewall.

Gentoo is the best distro I have found for implementing these security measures and tries to build them in as an option wherever possible. Gentoo has great documentation on security and is all about custom configuration and compiling. Since some of the above solutions require special compiler technologies, Gentoo is a perfect fit.

Each of those solutions take a certain amount of effort to implement and will break certain existing applications in different ways. Basically, Microsoft is taking the next step and implementing the least disruptive and easiest solution that will provide some protection for all software running on the system. They should probably also compile their own software with stack smash protection and make address space layout randomization available as a next step.

I disagree.

[AtariAmarok]

Sometimes, serious crimes such as participating in the films "Star Trek 9" and "Star Trek 10" need serious punishment. Data is indeed guilty.

Re:great news

[King Of Chat]

Well, the 386 series processors already have to capability. Code and data segments are separate things - it's just never actually been set up in the operating system. Check out the type flag in the segment descriptor.

Time to buy a new computer again...

[the_skywise]

"Hey, my 3ghz computer is running as slow as a Pentium 1.5ghz... Why is that?"
"Oh that's all the new virus checking that runs the executables before they run to make sure they don't have any viruses in them."

So y'see... Viruses ARE good for the industry!

Re:Time to buy a new computer again...

[danheretic]

"Hey, my 3ghz computer is running as slow as a Pentium 1.5ghz... Why is that?"

It's because you bought a computer from Dell, Compaq, IBM or any number of vendors who bundle vast amounts of memory-resident crap on their already-crappy bargain-basement hardware with half the RAM it should have.

DEP is already in Windows

[hkb]

It was included with Windows XP SP2. It's also in the soon-to-be-released SP1 for Windows Server 2003.

It appears that if the hardware doesn't support DEP, it will enable some sort of software DEP, instead.

W2K3 SP! also includes a new, XPSP2-like firewall interface with some nice logging and an easy-to-use rules interface. There's also the new Security Configuration Wizard, which seems to do a pretty damned good job of really locking down 2003 for those that need it.

Software for software

[Doc Ruby]

Compilers should store data in separate protected memory segments, never embedded inside code, where overwrites can change adjacent instructions. JMPs to data segments should issue compiler warnings, and execution past original allocations should set a flag, at least in the VM. The compiler and existing VM can protect from most overflows, and is a centralized link in the chain to guard the software from every programmer, no matter how naive. If CPU vendors want to get on the bandwagon, they can offer an interrupt triggered by such boundary transgressions. Making buffer execution the exception, requiring handling, rather than the default, is a better model of coding suited best to the compilers that translate our directions to the computer into instructions to the CPU.

Re:Software for software

[bani]

compilers already do this.

the problem is not jmp to data segments. the problem largely is executable stacks. which is exactly what stack smashing is about.

the other problem is that executable stacks are required for some legitimate compiler functions such as trampolines.

the real solutions are:
_complete_ separation of code and data segments.
code is _never_ writable, under any circumstances.
data is _never_ executable, under any circumstances.
no executable stack.
no more mprotect().

this will solve the arbitrary executable code issue, but won't solve other issues such as logic/sql/scripting exploits. it should solve most of the rootshell exploits though.

Keeping your developers happy

[Aslan72]

The huge problem with McAfee 8.0i has been figuring out a policy that protects from buffer overruns and keeps your developers happy; I've had to loosen the restrictions for those folks because as you put together stuff in vstudio and attempt to debug it, McAfee's Buffer Overrun flags it and doesn't allow it to run :(.

--pete

Re:great news

[mchawi]

Check Google with a string like Linux NX AMD. There have also been several slashdot stories about it. The short answer is yes it is available, but I don't know how widely used it is.

Re:great news

[TripMaster Monkey]

The main protection Linux has is the developers looking at Microsoft and saying, "See those guys? Let's not be like those guys."

Re:great news

[TheRaven64]

Not sure about Linux, but OpenBSD has a number of features which protect from this kind of vulnerability. This is why a lot of arbitrary code execution vulnerabilities become DoS vulnerabilities on OpenBSD.

People Still Writing Code in C

[billstewart]

C is one of the best languages out there for many things, but nobody should still be using it, because there are too many people who are careless about subtle things and shoot themselves in the foot with it. Yes, if you're writing device drivers, C is probably still the language of choice, but the number of people who do that is pretty limited, and they can run lint and doublecheck their code to make sure they don't get overrun errors. C++ isn't much better - you _can_ write code using constructs that don't get buffer overflows, but you don't have to (if anything, the nicest thing about C++ is being able to fall back to C when you need it), so a random C++ program is no more trustable than a random C program. It's not the 20th century any more - stop doing dangerous things!

(And yes, I still write C/C++ when I need it, but that's laziness after 25 years of habitual use, and usually I use shel when I need to program :-)

Re:People Still Writing Code in C

[Anonymous Coward]

With respect, this is complete rubbish.

You can't blame a programming language for sloppy coding. Sloppy code is sloppy code and it makes no difference what language you use, if you're a crap coder then you are going to have problems.

And stating that "C is probably still the language of choice, but the number of people who do that is pretty limited" is just plain wrong. Back in the REAL world, C is used more now than it has ever been.

And besides, what do you suggest you write an OS is ? Perl ? TCL ? Vusual Basic ?

Oh hang on though, surely the Perl interpreter is written in C isn;t it ? Oh, and TCL's ? And Vidual Basic's ? Or is that C++ ?

As I said, back in the real world....

Re:Umm...

[ctr2sprt]

This may sound really dumb, but isn't it up to the guy who wrote the vulnerability in the first place to fix it?

There is a time gap between when a bug is first discovered and when it is fixed. There is an even bigger gap between when a bug is fixed and when users actually bother to install the patch. Helping to prevent buffer overflows and the like will limit the problems caused by those gaps.

how the hell do you detect an overflow?

Memory is allocated using a library call like malloc(). Debugging tools will trap malloc() and actually allocate slightly more memory than is asked for, then write a signature before and after the buffer. It will then periodically check those signatures to see if they are still there. If they aren't - like because a program overwrote them with its own data - it means there's a buffer overflow. You can also use the CPU's virtualization hardware to spot some kinds of buffer overflows or other errors (like trying to read from a page that was allocated but never written to). There are other methods, but that's the most common and probably the easiest to understand.

Re:great news

[ColdGrits]

It's built-in to OpenBSD and has been since V3.3 (currently shipping 3.6, 3.7 due in 2 months).

A Tough Transition

[cyngus]

Having recently gotten my hands on a Windows XP box with a P4 that supported the NX bit I thought I'd turn it on, good idea right? yeah, great idea if you don't want to use half your applications. The NX bit stayed used for about five minutes. I wonder how many of Microsoft's apps will actually work with this protection turned on.

Boldly going where Linux went back in 2000

[mccrew]

Sounds like folks are reinventing Avaya Labs' incredibly useful Libsafe. This is a library that you can set up to preload before libc, either on a process-by-process or system wide basis, and it defines its own set of functions (strcpy, et al) to override those in the standard C library. It is able to detect many stack smashing attacks. When a stack smashing attack is detected, the offending process is terminated, and the administrator is sent an e-mail with copious technical detail.

I am surprised that major distributions have not picked up and run with this great tool. One of the first things I do on any new machine is to ensure that all internet-facing services are being run with libsafe preloaded.

No Execute = snake oil

[ajs318]

Sorry, but the whole "No Execute" thang is aceite de serpiente, as they say in Madrid. Even the much-vaunted {by people who don't understand it, anyway} Harvard Architecture {i.e. using separate buses for data and instructions, thereby breaking the Neumann principle totally} doesn't work. If the computer can make some kind of decision based on the content of memory location x, then this is tantamount to x being an executable location.

Now, if you had a "Take no action whatsoever based on the content of this location, in fact, whenever you are asked even to read it, always return the same value" flag -- that might prevent the execution of unwanted code. Chances are your system would also be computationally incomplete.

As it stands, NX is trivially defeated by persuading the user to install a simple piece of code -- effectively an emulator.

Basically, NX is answering the wrong question. The question that needs to be asked is "How can we best persuade users not to run arbitrary code when they don't know what the hell it does?" My own answer would be for every processor to have its own, unique instruction set; so only code compiled for that one particular individual processor would ever run on it. {Obviously you'd have to have a compatibility mode for bootstrapping, so you could compile the compiler to compile the unique-ified software; but this would have to be accessed by some deliberate hardware action that no software could get around.} I'm sure that is not impossible; but I'm not sure that it's feasible as long as the likes of Microsoft want to do things their way.

DEP has nothing

[bluefoxlucid]

DEP actually can be evaded because it supplies no ASLR. If the attacker can reasonably know where some data exists in memory--particularly, his exploit and msvcrt.dll for memcpy() and VirtualAlloc()--he can basically switch DEP off during an attack. Believe it or not, this is pretty easy if everything is in the same place every program run.

Fortunately in Linux we have PaX, which supplies much better protection than W^X, Exec Shield, or DEP with "competetive" (i.e. comparable, potentially lower; it can actually viably compete) compatibility. Red Hat of course has convinced the GCC devs to make GCC mark everything to have an executable stack if the compiler is at all not sure that it can operate without one; but PaX ignores that and still only "breaks" a few packages (and nVidia's glx).

PaX, GrSecurity, IBM's SSP (ProPolice), and PIE executable binaries should pave the future on Linux; but people are trying so hard to avoid them. It's not even much work to maintain a distro using those.

DEP is basically like vanilla Linux on AMD64.

Better idea?

[CODiNE]

I'm kind of a jr programmer and here's the idea I had. Could be done by the compiler and is probably already out there in some form.

Character arrays have an extra byte stuck on the end of them. When the compiler sees that it's being called by an unsafe method or some sort of strcpy it puts a random value into that byte, and rechecks it after the call. There is no way for the buffer overflow code to know what the value was and when it is changed the program is immediately killed. Then again your overflows still have a 1 in 256 chance of working. ;-)

So is this already being done somewhere or is there any reason why this just wouldn't work?

Seems to me OSS along with GCC has the potential to fix overflow problems a LOT easier than a commerical OS vender could.

-Don.

Re:great news

[x0n]

Yes, but nothing stops user apps from ignoring segment descriptors -- and the operating system cannot easily check the type flag before executing the code. On the other hand, the NX (no execute) flag causes a _hardware_ interrupt which cannot be ignored by the user app if the O/S decides to act on it.

- Oisin

Re:great news

[mscnln]

Yep, it does. With the grsecurity patchset, you can disable memory from being executed.

Re:Time to change.... OS

[kyojin the clown]

you need to run huge globs of anti-virus software just to stop it being compromised

everyone loves to bash MS here, fair enough. i dont really give two monkeys.

however, i just dont think this is true. i run NAV2004 on my XP box, and it never ever flags anything. now, either this means its just cack (corporate edition is certainly pretty cack), or i just dont get all these viruses. i spend plenty of time surfing around the shady underbelly of the web, with firefox admittedly, and i use my common sense with the email i get, however i havent had a virus in 7 years of varying MS systems.

are you sure this isnt just anti-MS hysteria?

Better Late Than Never

[SunFan]

I've had stack protection for quite some time with Solaris and OpenBSD. The Windows platform is a few years late to the party; doesn't Microsoft realize how much easier their life would be if they acted earlier?

Companies with Windows are like a person persisting to wear worn-out shoes. They're uncomfortable, they cause blisters, they don't keep water out, yet they keep them, because going barefoot is worse, I guess. The software industry still has a lot of growing-up to do.

Now, a translation into non-technical terms

[dsplat]

You were at your buddy's place to watch the Super Bowl. The chips ran out in the second quarter. Since you knew there was no way the half time show was going to be as interesting this year, you volunteered to go get more.

On your way out you made a mental note to come back to your buddy's place, rather than your own. This is the return address. You also made a mental note that you needed potato chips and another case of beer. That list is in your buffer.

Your other "friend", a known sponge who still owes several people for beer at various events dating back 3 years now, comes along. While you are at the store, he throws an extra case into the cart. He'll pay you back later.

That annoyed you enough that you forgot whose house you were going back to. You ended up at your place before you realized it. As a result, you missed the first few minutes of the third quarter.

It's an intel flaw

[spitzak]

The problem you are describing is not with Windows or Linux. What you are describing is in fact exactly the lack of a "NX" bit. The Intel processors could not make memory readable and not be executable. Thus if you want to read the data on your stack then it was also possible to jump to it and execute it. The fact that Windows or Linux were unable to fix this problem is not their fault.

Possibly you are confused by 80286 segments, which could make memory readable without being executable (because you could only execute by loading the PC segment register, and the OS could apply totally different rules depending on which segment register was being loaded). However apparently the 80286 scheme has a lot of problems which is why neither Windows or Linux use it for virtual memory (I am not sure what the problems are but it is obvious nobody wanted to work with it).

Re:It's an intel flaw

[RaVPup]

Intel and AMD chips have only just added a seperate read and execute bit. Solaris 2.6 first introduced the non_exec_stack parameter and that was defeated using return to libc. The SPARC V9 architecture has had seperate XR bits for a while and return to libc works on that architecture. I think a lot of people are falling for marketing hype.

Segmentation is better! (not)

[Anonymous Coward]

You obviously don't remember how painful it was to program for 16-bit Windows. Segments and thunks and far pointers and all that other bullshit.

Windows uses a 4GB flat address space. The same memory model used by Linux and all other modern OSes. Segmentation, though supported by the hardware, is (1) inefficient and (2) more difficult to program for. Even CPU vendors realize it was bad technology and are moving away from it. Example: The new AMD64 chips support all the segmentation crap in compatibility mode, but if you switch them to 64-bit mode they ONLY SUPPORT A FLAT ADDRESS SPACE. There is a word for this: "Progress".

The real answer, as another poster pointed out, is to add NX bits to the page protection hardware. One of the odd quirks of the x86 page protection hardware was that if you can read it, you can execute it. That turned out to be a bad design. :)

Re:great news

[FooBarWidget]

Does Linux have something like this? Fedora has had Exec-Shield long before Windows had DEP.