Why would you want to zero a buffer ? Because it may contain sensitive information, I presume. If you don't have additional properties w.r.t allocated memory, what prevent a system with high load to temporarily put the given memory block on swap, leaking the information on disk ? Security is hard...
How to zero a buffer
111–120 of 216 posts
Re: How to zero a buffer
#112It is a little mind boggling that support for proper handling of this didn't arrive until c11. For a symmetric cipher without a demanding setup/init phase - would it make sense to just do a few rounds on a buffer using the zeroed key? Obviously quite a few more cycles, but should at least be a predictable (constant) overhead?
What do you mean? The solution that Percival presents compiles fine on my C89 compiler.
I didn't mean to imply that the solution as presented didn't work, I was just wondering if it would also work simply running the cipher with the zeroed key in order to avoid zeroing the key being optimized away. Obviously that'd be a lot more cycles; I'm just curious if it would be a viable solution ;-)
Re: How to zero a buffer
#113Interesting. This appears to solve a more general problem, which is: how to create a barrier against inter-procedural optimization and dead code elimination. I wonder if this trick could also be used to solve the double-checked locking problem. From the quintessential DCLP paper ( http://www.aristeia.com/Papers/DDJ_Jul_Aug_2004_revised.pdf ): Consider again the line that initializes pInstance: pInstance = new Singlet…
if the function must be called, then step 3 cannot possibly be performed before steps 1 and 2.
So just to clarify, there's no way the compiler could do "1, 3, 2" instead of "1, 2, 3"? It seems a naive implementation of a compiler could store the pointer to the allocated memory in the pInstance variable before calling the constructor, rather than using a temporary location for the pointer (e.g. a register). Does C++11 and later specify otherwise?
Re: How to zero a buffer
#114Why would you want to zero a buffer ? Because it may contain sensitive information, I presume. If you don't have additional properties w.r.t allocated memory, what prevent a system with high load to temporarily put the given memory block on swap, leaking the information on disk ? Security is hard...
what prevent a system with high load to temporarily put the given memory block on swap
mlock() or mlockall() is useful in this case, but those are POSIX functions, not C.
Re: How to zero a buffer
#115Why would you want to zero a buffer ? Because it may contain sensitive information, I presume. If you don't have additional properties w.r.t allocated memory, what prevent a system with high load to temporarily put the given memory block on swap, leaking the information on disk ? Security is hard...
There is mlock(2), which is supposed to prevent the memory from being swapped. The problem with that is that the call requires either root or CAP_IPC_LOCK.
If your user lacks the capability, you have to run the program setuid root. Allocate the sensitive buffers at the start, call mlock() on them and only then drop the privileges.
There's also the 10kg fine-tuning hammer, mlockall(2). That makes ALL the memory for the calling process to become unswappable. As it can lock either the "currently held" memory, or "all the memory to be allocated during process lifetime", it can provide for some additional amusement under memory pressure.
Re: How to zero a buffer
#116Why would you want to zero a buffer ? Because it may contain sensitive information, I presume. If you don't have additional properties w.r.t allocated memory, what prevent a system with high load to temporarily put the given memory block on swap, leaking the information on disk ? Security is hard...
There is mlock(2), which is supposed to prevent the memory from being swapped. The problem with that is that the call requires either root or CAP_IPC_LOCK. If your user lacks the capability, you have to run the program setuid root. Allocate the sensitive buffers at the start, call mlock() on them and only then drop the privileges. There's also the 10kg fine-tuning hammer, mlockall(2). That makes ALL the memory for th…
I assume this privilege problem can be solved now with UID namespaces in linux. However it's really ugly, depends on running multiple child processes and linux specific.
Re: How to zero a buffer
#117Earlier quoted context omitted.
Dead store elimination is a really important optimization, and it comes out of bog-standard compiler optimizations like SROA on SSA form IR. You really want your compiler to perform it for acceptable performance.
Yes. But dead store elimination combined with being allowed to dereference arbitrary pointers and leaving newly allocated blocks of memory uninitialized is problematic. Contrary to what the C standard would like us to believe, those other features do mean that dead store elimination alters the semantics of a program. It doesn't impact the semantics of the procedure whose dead stores are being eliminated, but it alter…
If your compiler couldn't perform dead store elimination on memory (remember, memory includes allocas), then you'd lose most of the benefits of performing standard SSA form optimizations after SROA has happened. Essentially you'd kill scalar replacement of aggregates, which is a critical optimization. It's very important for performance that it allowed to happen.
Re: How to zero a buffer
#118Why would you want to zero a buffer ? Because it may contain sensitive information, I presume. If you don't have additional properties w.r.t allocated memory, what prevent a system with high load to temporarily put the given memory block on swap, leaking the information on disk ? Security is hard...
>Why would you want to zero a buffer ? Because it may contain sensitive information, I presume.
You don't zero sensitive buffers. You randomize them, then free() them.
Re: How to zero a buffer
#119Earlier quoted context omitted.
I'm gonna go ahead and say it: Perhaps in retrospect C is an inappropriate choice of language for these kinds of applications. This "Performance at all costs, including safety and predictability" thing may be appropriate in video games, but for security-critical applications that philosophy is downright negligent.
Dead store elimination is a really important optimization, and it comes out of bog-standard compiler optimizations like SROA on SSA form IR. You really want your compiler to perform it for acceptable performance.
You want your compiler, and execution environment, to support it. In C's case, the compiler can do a lot of work in this department - but so can most modern OS's in the real-world, provide adequate protection:
sbuf = mmap(..,..,MAP_PRIVATE|MAP_ANON); // &etc.Re: How to zero a buffer
#120[deleted]