When and how to use GCC's stack protection feature?

Question

I have enabled the -Wstack-protector warning when compiling the project I'm working on (a commercial multi-platform C++ game engine, compiling on Mac OS X 10.6 with GCC 4.2). This flag warns about functions that will not be protected against stack smashing even though -fstack-protector is enabled. GCC emits some warnings when building the project:

not protecting function: no buffer at least 8 bytes long
not protecting local variables: variable length buffer

For the first warning, I found that it is possible to adjust the minimum size a buffer must have when used in a function, for this function to be protected against stack smashing: --param ssp-buffer-size=X can be used, where X is 8 by default and can be as low as 1.

For the second warning, I can't suppress its occurrences unless I stop using -Wstack-protector.

1. When should -fstack-protector be used? (as in, for instance, all the time during dev, or just when tracking bugs down?)
2. When should -fstack-protector-all be used?
3. What is -Wstack-protector telling me? Is it suggesting that I decrease the buffer minimum size?
4. If so, are there any downsides to putting the size to 1?
5. 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?

Answer 1

Stack-protection is a hardening strategy, not a debugging strategy. If your game is network-aware or otherwise has data coming from an uncontrolled source, turn it on. If it doesn't have data coming from somewhere uncontrolled, don't turn it on.

Here's how it plays out: If you have a bug and make a buffer change based on something an attacker can control, that attacker can overwrite the return address or similar portions of the stack to cause it to execute their code instead of your code. Stack protection will abort your program if it detects this happening. Your users won't be happy, but they won't be hacked either. This isn't the sort of hacking that is about cheating in the game, it's the sort of hacking that is about someone using a vulnerability in your code to create an exploit that potentially infects your user.

For debugging-oriented solutions, look at things like mudflap.

As to your specific questions:

1. 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.

2. Stack protections for all buffers can be used if you want extra protection in exchange for some performance hit. From gcc4.4.2 manual:

   -fstack-protector

   Emit extra code to check for buffer overflows, such as stack smashing attacks. This is done by adding a guard variable to functions with vulnerable objects. This includes functions that call alloca, and functions with buffers larger than 8 bytes. The guards are initialized when a function is entered and then checked when the function exits. If a guard check fails, an error message is printed and the program exits.
   

   -fstack-protector-all

   Like -fstack-protector except that all functions are protected.

3. The warnings tell you what buffers the stack protection can't protect.

4. It is not necessarily suggesting you decrease your minimum buffer size, and at a size of 0/1, it is the same as stack-protector-all. It is only pointing it out to you so that you can, if you decide redesign the code so that buffer is protected.
5. No, those warnings don't represent issues, they just point out information to you. Don't use them regularly.

Answer 2

You indeed should not care about the warning for normal builds. It's really more of an informational message. I hope it's obvious that you do have an inherent security concern with variable-sized buffers on the stack; get the size calculation wrong and you're opening a big hole.