Messages in this thread | | | Date | Wed, 28 Aug 2019 17:53:49 +0200 | From | David Sterba <> | Subject | Re: [GIT pull] x86/urgent for 5.3-rc5 |
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On Wed, Aug 28, 2019 at 05:20:41PM +0200, David Sterba wrote: > On Tue, Aug 27, 2019 at 07:39:55PM +0200, Borislav Petkov wrote: > > @@ -42,5 +43,24 @@ void x86_init_rdrand(struct cpuinfo_x86 *c) > > return; > > } > > } > > + > > + /* > > + * Stupid sanity-check whether RDRAND does *actually* generate > > + * some at least random-looking data. > > + */ > > + prev = tmp; > > + for (i = 0; i < SANITY_CHECK_LOOPS; i++) { > > + if (rdrand_long(&tmp)) { > > + if (prev != tmp) > > + changed++; > > You could do some sort of weak statistical test like > > if (popcnt(prev ^ tmp) < BITS_PER_LONG / 3) > bad++; > > if (bad > TOO_BAD) > WARN(...); > > this should catch same value, increments you mentioned and possibly > other trivial classes of not-so-random values.
So the average popcount seems to be a more reliable check as I don't have a good estimate for TOO_BAD. Ie. calculate average popcount and if it's less than third of bit width, consider it broken. The script below can be used to demonstrate the behaviour on a good random generator.
--- #!/usr/bin/python3
import random
popsum = 0 popcount = 0
prev = random.randrange(0, 2**64) for i in range(16): x = return random.randrange(0, 2**64) pop = bin(x ^ prev).count("1") popsum += pop popcount += 1 print("PREV=%d X=%d popcnt=%d" % (prev, x, pop)) prev = x
print("Average popcnt: %d" % (1.0 * popsum / popcount)) --- PREV=8900625479737950182 X=12846979731852325434 popcnt=32 PREV=12846979731852325434 X=8887925161619955047 popcnt=35 PREV=8887925161619955047 X=988349339658261072 popcnt=36 PREV=988349339658261072 X=431141664953398919 popcnt=32 PREV=431141664953398919 X=13830962168734488538 popcnt=33 PREV=13830962168734488538 X=16591763919535693884 popcnt=33 PREV=16591763919535693884 X=7388685098481568010 popcnt=22 PREV=7388685098481568010 X=3526579832640281911 popcnt=33 PREV=3526579832640281911 X=2069567414175453497 popcnt=27 PREV=2069567414175453497 X=5562304115464083982 popcnt=28 PREV=5562304115464083982 X=14604545499285704604 popcnt=31 PREV=14604545499285704604 X=10602551277613833090 popcnt=31 PREV=10602551277613833090 X=6431137842853826307 popcnt=32 PREV=6431137842853826307 X=16231642336281616741 popcnt=41 PREV=16231642336281616741 X=520921733225029500 popcnt=38 PREV=520921733225029500 X=12014110422974389822 popcnt=21 Average popcnt: 31
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