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Date:   Tue, 16 Apr 2019 18:07:17 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Theodore Ts'o <tytso@mit.edu>,
        David Laight <David.Laight@ACULAB.COM>,
        "Reshetova, Elena" <elena.reshetova@intel.com>,
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Subject: Re: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall
Message-ID: <20190416160717.GP4038@hirez.programming.kicks-ass.net>
References: <20190415060918.3766-1-elena.reshetova@intel.com>
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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@intel.com

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...