How does gcc stack protector work

This does not mean that the buffer cannot be exploited. For example, if 16 bytes are written to the buffer above and it is not null-terminated, unintended behaviour can still take place later on during program execution. Note how there is only a single protective value, not every variable is protected in this manner.

One heuristic ordering often used, with the stack growing downwards, is first storing the canary, then buffers that might overflow into each other and finally all the small variables unaffected by overruns.

This is based on the idea that it is generally less dangerous if arrays are modified, compared to variables that hold flags, pointers and function pointers, which may more seriously alter execution. Some compilers randomize the order of stack variables and randomize the stack frame layout, which further complicates determining the right input with the intended malicious effect.

Compilers such as GCC enable this feature if requested through compiler options, or if the compiler supplier enabled it by default. It is worth considering enabling it by default if your operating system is security conscious and you provide support. It is possible to use it in your entire operating system even kernel and standard library, perhaps excluding ports with really poor code quality. A feature enabled with a -ffoo option can be disabled with the -fno-foo counterpart.

Several options exist that provide different variants of SSP:. This includes functions with buffers larger than 8 bytes or calls to alloca. This helps find bugs. This is disabled if you pass -nostdlib as you do when linking a kernel and you'll need to supply your own implementation. For user-space, you have two options:. If a function has been inlined, then stack smash protection will not work for that function. To prevent this, one must use the noinline attribute like so:.

Disabling inlining in GCC can be done with the -fno-inline compile flag, however, that will not inline functions with the inline attribute. If any tests do not work when trying to trip the protective mechanism, this may be the reason why it does not work! Run-time support needs only two components: A global variable and a check failure handler. For instance, a minimal implementation could be:. Note how the secret guard value is hard-coded rather than being decided during program load. You should have the program loader the bootloader in the case of the kernel randomize the values.

You can do this by putting the guard value in a special segment that the loader knows to randomize. The numbers shown here are not special, they are just examples of randomly generated numbers. You can still take advantage of the bug-discovering properties of SSP even if the guard value is not cryptographically secure unless you anticipate sufficiently obscure bugs that intelligently circumvent SSP.

If the compiler detects custom overflow detection or whatever, just change the flavor of 'add' to use. There's nothing against such optimizations and other targets could implement it too, but if you now have to stall the pipeline to fetch a flag, it might not really be worth it overall.

The problem is in the C implementations. I'd be curious to hear about any that do. When C runtimes generally don't provide a way to trap overflows, then it becomes very difficult for any other languages or runtimes to do so, unless they bypass C.

This is probably a lamentable state of affairs. Down to decisions made in the days when performance was king and other factors like correctness and security weren't really a consideration relative to today. Decisions which, in my view, certainly don't serve us well anymore. There's a longer argument about whether traps would have been more efficient than flags that have to be checked, and whether traps might have been more likely to be implemented.

Still, there is surely lots of code where the security benefits of runtime overflow-checking would outweigh any performance costs?

I'd be curious to read more about the relative costs of the overheads, and the effectiveness of overflow flag checking on current ISAs, if anyone knows. GCC has an "-ftrapv" argument which I was hoping might do this, and use hardware flags when possible.

With -O it seems to be using leaq to generate the addition. To be more precise, signed overflow is undefined behaviour in C. Unsigned integers are defined to wrap around. It needs to be a bit signed integer. An int is likely to be only 32 bits. On second thoughts, that may be a compiler bug. At any rate, I could only get it to work on GCC 4. Hmm, that's pretty limited in usefulness so, and buggy with respect to what the documentation suggests it does the docs don't qualify when overflow checks will actually be done.

I am using Yocto project for our BSP. Please let me know if am missing anything. User: Password:. And if the answer is no.. There is no reason to assume stack-smashing is only on char buffers using bytewise traversal. A good example is CVE, where you could overwrite the variable which indicated that the user has authenticated successfully. I haven't found specifics of CVE, but presumably it would solve the problem. If your corruption target is referenced before function return then you can write over the canary without fear of triggering a stack check.

In this case the target can be anywhere higher in memory than the overflow even in another stack frame completely. Using this, it has 4 variants of, e.

I'd like to see the day where the CPU assists with over and underflow detection. The primary responsibility for arithmetic overflow detection not being ubiquitous in real-world software lies not with CPU designers and implementers, but with the designers and implementers of languages that don't take advantage of that assistance and the programmers who chose not to use, or not to press for the creation of, languages that do.

If you overflow an integer but never use it…who cares? The nice thing about it is that it also doesn't increase code size or extra time, so there's no overhead for the check and you don't need an instrumented build to find the problem s. Not even as GCC builtins. There are tricks with pushf but they're workarounds. That said, I think it'd still be very worthwhile on certain lumps of code. Sometimes correctness and helping programmers guard against their own omniscience matters a lot more than performance!

These could also provide protection. If so, are there any downsides to putting the size to 1? It appears that -Wstack-protector is not the kind of flag you want enabled at all times if you want a warning-free build. Is this right? Jonathan Leffler k gold badges silver badges bronze badges.

Guillaume Guillaume 4, 4 4 gold badges 22 22 silver badges 18 18 bronze badges. These articles should help: - What's the stack smashing protector - SSP — dirkgently. Do you need to use the variable-length arrays VLA feature of C99? If you didn't use it, you wouldn't get the warning - and you would get the protection.

Are you really sure you gain enough from using VLA that the loss of protection is acceptable? VLAs have been removed, the warning was useful in this case. Thanks for the links. I still don't see the big picture though. Is stack protection really useful for a commercial game? Will the game experience any performance hit if -fstack-protector is enabled? And the miminum buffer size set to 1? Or if -fstack-protector-all is enabled?

Do you expect people to try and crack your code? If so, it's probably a good idea. As for performance, you'll have to calculate. Here's a starting point: trl. Show 3 more comments. Active Oldest Votes. For debugging-oriented solutions, look at things like mudflap.

As to your specific questions: Use stack protector if you get data from uncontrolled sources. The answer to this is probably yes.

So use it. Even if you don't have data from uncontrolled sources, you probably will eventually or already do and don't realize it. The warnings tell you what buffers the stack protection can't protect. It is only pointing it out to you so that you can, if you decide redesign the code so that buffer is protected. No, those warnings don't represent issues, they just point out information to you.

Don't use them regularly.