Messages in this thread | | | From | Kees Cook <> | Date | Mon, 18 Mar 2019 14:07:57 -0700 | Subject | Re: [RFC PATCH] x86/entry/64: randomize kernel stack offset upon syscall |
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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
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