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

[Peter Zijlstra]

On Tue, Apr 16, 2019 at 11:43:49AM -0400, Theodore Ts'o wrote:
> If it's x86 specific, maybe the simplest thing to do is to use RDRAND
> if it exists, and fall back to something involving a TSC and maybe
> prandom_u32 (assuming on how bad you think the stack leak is going to
> be) if RDRAND isn't available?

>From https://lkml.kernel.org/r/20190320072715.3857-1-elena.reshetova@xxxxxxxxx

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


Basically, RDRAND is frigging slow...