Re: [PATCH v2 5/5] arm64: entry: Enable random_kstack_offset support

[Mark Rutland]

On Wed, Mar 25, 2020 at 01:22:07PM -0700, Kees Cook wrote:
> On Wed, Mar 25, 2020 at 01:21:27PM +0000, Mark Rutland wrote:
> > On Tue, Mar 24, 2020 at 01:32:31PM -0700, Kees Cook wrote:
> > > Allow for a randomized stack offset on a per-syscall basis, with roughly
> > > 5 bits of entropy.
> > > 
> > > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > 
> > Just to check, do you have an idea of the impact on arm64? Patch 3 had
> > figures for x86 where it reads the TSC, and it's unclear to me how
> > get_random_int() compares to that.
> 
> I didn't do a measurement on arm64 since I don't have a good bare-metal
> test environment. I know Andy Lutomirki has plans for making
> get_random_get() as fast as possible, so that's why I used it here.

Ok. I suspect I also won't get the chance to test that in the next few
days, but if I do I'll try to share the results.

My concern here was that, get_random_int() has to grab a spinlock and
mess with IRQ masking, so has the potential to block for much longer,
but that might not be an issue in practice, and I don't think that
should block these patches.

> I couldn't figure out if there was a comparable instruction like rdtsc
> in aarch64 (it seems there's a cycle counter, but I found nothing in
> the kernel that seemed to actually use it)?

AArch64 doesn't have a direct equivalent. The generic counter
(CNTxCT_EL0) is the closest thing, but its nominal frequency is
typically much lower than the nominal CPU clock frequency (unlike TSC
where they're the same). The cycle counter (PMCCNTR_EL0) is part of the
PMU, and can't be relied on in the same way (e.g. as perf reprograms it
to generate overflow events, and it can stop for things like WFI/WFE).

Thanks,
Mark.