Re: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall

[Ingo Molnar <mingo@kernel.org>]

* Andy Lutomirski <luto@kernel.org> wrote:

> > 8 gigabits/sec sounds good throughput in principle, if there's no
> > scalability pathologies with that.
> 
> The latency is horrible.

Latency would be amortized via batching anyway, so 8 gigabits/sec 
suggests something on the order of magnitude of 4 bits per cycle, right? 
With 64 bits extraction at a time that would be 16 cycles per 64-bit 
word, which isn't too bad, is it?

But you are right that get_random_bytes() is probably faster, and also 
more generic.

> > It would also be nice to know whether RDRAND does buffering 
> > *internally*,
> 
> Not in a useful way :(

Too bad ...

> > Any non-CPU source of randomness for system calls and plans to add 
> > several extra function calls to every x86 system call is crazy talk I 
> > believe...
> 
> I think that, in practice, the only real downside to enabling this 
> thing will be the jitter in syscall times. Although we could decide 
> that the benefit is a bit dubious and the whole thing isn't worth it. 
> But it will definitely be optional.

Making it "optional" is not really a technical argument in any way 
though, either distros enable it in which case it's a de-facto default 
setting, or they don't, in which case it de-facto almost doesn't exist.

Thanks,

	Ingo