3 ms·

Special primitives in programming languages should be provided to solve this problem, just like how Linux kernel uses a "barrier()" macro to mark non-reorderabl

Special primitives in programming languages should be provided to solve this problem, just like how Linux kernel uses a "barrier()" macro to mark non-reorderable code, and "wmb()" and "rmb()" to mark non-reorderable I/O. OpenBSD provides void explicit_bzero(void *s, size_t n); In libc, which is used for this purpose. Recently, glibc added function, presumably because it's a good idea. But it still has limitations due to lack of compiler support. I think the problem can only be solved at the compiler/OS level. > The explicit_bzero() function addresses a problem that security-conscious applications may run into when using bzero(): if the compiler can deduce that the location to zeroed will never again be touched by a correct program, then it may remove the bzero() call altogether. This is a problem if the intent of the bzero() call was to erase sensitive data (e.g., passwords) to prevent the possibility that the data was leaked by an incorrect or compromised program. Calls to explicit_bzero() are never optimized away by the compiler. > The explicit_bzero() function does not solve all problems associated with erasing sensitive data: > 1. The explicit_bzero() function does not guarantee that sensitive data is completely erased from memory. (The same is true of bzero().) For example, there may be copies of the sensitive data in a register and in "scratch" stack areas. The explicit_bzero() function is not aware of these copies, and can't erase them. > 2. In some circumstances, explicit_bzero() can decrease security. If the compiler determined that the variable containing the sensitive data could be optimized to be stored in a register (because it is small enough to fit in a register, and no operation other than the explicit_bzero() call would need to take the address of the variable), then the explicit_bzero() call will force the data to be copied from the register to a location in RAM that is then immediately erased (while the copy in the register remains unaffected). The problem here is that data in RAM is more likely to be exposed by a bug than data in a register, and thus the explicit_bzero() call creates a brief time window where the sensitive data is more vulnerable than it would otherwise have been if no attempt had been made to erase the data.

ncmncm 8y ago
> I think the problem can only be solved at the compiler/OS level. There's the problem. That is a lot of levels. There is support at certain levels; nowadays you can force writebacks from certain cache lines, and OSes let you keep pages from being swapped. The problem is that it needs coordinated support at all levels, but there is no money behind it, unlike features that may improve performance, and hardly anybody knows there is even a problem. Spectre and Meltdown have raised awareness of security threats in infrastructure, but are so big they also absorb all the budget for it, and will continue indefinitely.
- segfaultbuserr 8y ago
  Exactly. It's definitely possible, but something like this needs great amount of coordination and money. I've once read something about Remote Direct Memory Access, it allows one to completely bypass the kernel, CPU, cache, and stream data directly to the network adapter of another computer - you break a lot of essential abstractions of modern computer system, meanwhile still be able to provide a consistent and usable infrastructure. Its security equivalent (something allows low-level control of the protected data) is unlikely to happen, except for DRM, I guess.

ncmncm 8y ago

> I think the problem can only be solved at the compiler/OS level. There's the problem. That is a lot of levels. There is support at certain levels; nowadays you can force writebacks from certain cache lines, and OSes let you keep pages from being swapped. The problem is that it needs coordinated support at all levels, but there is no money behind it, unlike features that may improve performance, and hardly anybody knows there is even a problem. Spectre and Meltdown have raised awareness of security threats in infrastructure, but are so big they also absorb all the budget for it, and will continue indefinitely.

segfaultbuserr 8y ago
Exactly. It's definitely possible, but something like this needs great amount of coordination and money. I've once read something about Remote Direct Memory Access, it allows one to completely bypass the kernel, CPU, cache, and stream data directly to the network adapter of another computer - you break a lot of essential abstractions of modern computer system, meanwhile still be able to provide a consistent and usable infrastructure. Its security equivalent (something allows low-level control of the protected data) is unlikely to happen, except for DRM, I guess.