Hacker News Reader

Comment by bmm6o | Hacker News Reader

"Random sleep" does not in general mean "sleep until the next timer tick". The best fix is making the function constant time, if you can achieve this with a sleep that makes the operation always take exactly one quantum then the sleep is really an implementation detail and quite far from "random sleep".

View on HN

I've realized this (and am certainly not alone in that, it's rather obvious, when one think about the nature of timing) -- but does anyone have some links on implementing this? Are there some (sequence of) x86_64 instructions that can be used to bound a procedure (in the pascal sense of the word) to a quantum, regardless of things like when the procedure is called (assuming the procedure is short enough, and depending on branching behaviour, I suppose instruction fetch/decode, data fetch/write and accompanying cache hit/miss can make it hard to a) select a worst-run time in terms of clocks cycles to target (and if so, for which concrete cpus) and b) be hard to make sure the cpu is actually busy for exactly that many cycles...)? Is this even possible to approach in this portable C99?

I suppose if one ignore the information leak due to possible change in cpu load, one might device a kind of evented "call back" model, where one wait to return the result of a procedure until an interrupt is triggered?

I don't expect a full answer, but if anyone has a link to some source code that isn't too complicated, I'd be very happy (either "real-world" or some good "example" code).

The most common way to avoid timing side channel attacks is to write the procedure in C or ASM in such a way that there are no data-dependent differences in execution path. You've probably seen e.g. the memcmp that doesn't exit early. This attack is a little different in that it's not that different instructions are executed, it's that different memory access patterns take different amounts of time. For that, you can maybe change the implementation to not have any data-dependent array accesses, or maybe you can do things with prefetching to make the memory accesses constant time.

An approach where you watch the clock will be inherently less portable and actually much harder. Not only will the timing calls be hardware or OS specific, but so will the worst-case time. Imagine having to deal with a chip going into low power mode during your computation. Also you probably don't want to count time that your thread wasn't scheduled to run, so now you're talking about integrating with the scheduler.

Ah, I missed that xiata said "random". My error.

Reply on news.ycombinator.com