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

[Kees Cook <keescook@chromium.org>]

On Mon, Mar 18, 2019 at 1:16 PM Andy Lutomirski <luto@kernel.org> wrote:
> On Mon, Mar 18, 2019 at 2:41 AM Elena Reshetova
> <elena.reshetova@intel.com> wrote:
> > Performance:
> >
> > 1) lmbench: ./lat_syscall -N 1000000 null
> >     base:                     Simple syscall: 0.1774 microseconds
> >     random_offset (rdtsc):     Simple syscall: 0.1803 microseconds
> >     random_offset (rdrand): Simple syscall: 0.3702 microseconds
> >
> > 2)  Andy's tests, misc-tests: ./timing_test_64 10M sys_enosys
> >     base:                     10000000 loops in 1.62224s = 162.22 nsec / loop
> >     random_offset (rdtsc):     10000000 loops in 1.64660s = 164.66 nsec / loop
> >     random_offset (rdrand): 10000000 loops in 3.51315s = 351.32 nsec / loop
> >
>
> Egads!  RDTSC is nice and fast but probably fairly easy to defeat.
> RDRAND is awful.  I had hoped for better.

RDRAND can also fail.

> So perhaps we need a little percpu buffer that collects 64 bits of
> randomness at a time, shifts out the needed bits, and refills the
> buffer when we run out.

I'd like to avoid saving the _exact_ details of where the next offset
will be, but if nothing else works, this should be okay. We can use 8
bits at a time and call prandom_u32() every 4th call. Something like
prandom_bytes(), but where it doesn't throw away the unused bytes.

-- 
Kees Cook