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Date:   Thu, 6 Apr 2023 16:50:02 +0800
From:   Herbert Xu <herbert@gondor.apana.org.au>
To:     Stephan =?iso-8859-1?Q?M=FCller?= <smueller@chronox.de>
Cc:     linux-crypto@vger.kernel.org, Vladis Dronov <vdronov@redhat.com>
Subject: Re: [PATCH v4] crypto: jitter - permanent and intermittent health
 errors
Message-ID: <ZC6HuugQ9ddW0hed@gondor.apana.org.au>
References: <12194787.O9o76ZdvQC@positron.chronox.de>
 <4478169.LvFx2qVVIh@positron.chronox.de>
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On Mon, Mar 27, 2023 at 09:03:52AM +0200, Stephan M?ller wrote:
> According to SP800-90B, two health failures are allowed: the intermittend
> and the permanent failure. So far, only the intermittent failure was
> implemented. The permanent failure was achieved by resetting the entire
> entropy source including its health test state and waiting for two or
> more back-to-back health errors.
> 
> This approach is appropriate for RCT, but not for APT as APT has a
> non-linear cutoff value. Thus, this patch implements 2 cutoff values
> for both RCT/APT. This implies that the health state is left untouched
> when an intermittent failure occurs. The noise source is reset
> and a new APT powerup-self test is performed. Yet, whith the unchanged
> health test state, the counting of failures continues until a permanent
> failure is reached.
> 
> Any non-failing raw entropy value causes the health tests to reset.
> 
> The intermittent error has an unchanged significance level of 2^-30.
> The permanent error has a significance level of 2^-60. Considering that
> this level also indicates a false-positive rate (see SP800-90B section 4.2)
> a false-positive must only be incurred with a low probability when
> considering a fleet of Linux kernels as a whole. Hitting the permanent
> error may cause a panic(), the following calculation applies: Assuming
> that a fleet of 10^9 Linux kernels run concurrently with this patch in
> FIPS mode and on each kernel 2 health tests are performed every minute
> for one year, the chances of a false positive is about 1:1000
> based on the binomial distribution.
> 
> In addition, any power-up health test errors triggered with
> jent_entropy_init are treated as permanent errors.
> 
> A permanent failure causes the entire entropy source to permanently
> return an error. This implies that a caller can only remedy the situation
> by re-allocating a new instance of the Jitter RNG. In a subsequent
> patch, a transparent re-allocation will be provided which also changes
> the implied heuristic entropy assessment.
> 
> In addition, when the kernel is booted with fips=1, the Jitter RNG
> is defined to be part of a FIPS module. The permanent error of the
> Jitter RNG is translated as a FIPS module error. In this case, the entire
> FIPS module must cease operation. This is implemented in the kernel by
> invoking panic().
> 
> The patch also fixes an off-by-one in the RCT cutoff value which is now
> set to 30 instead of 31. This is because the counting of the values
> starts with 0.
> 
> Reviewed-by: Vladis Dronov <vdronov@redhat.com>
> Signed-off-by: Stephan Mueller <smueller@chronox.de>
> ---
> 
> v4:
>  - fix comment regarding fips=1
>  - update patch subject to match common naming schema
>  - remove now unused jent_panic function
>  - added Reviewed-by line
> 
> v3:
>  - remove an unused goto target
> 
> v2:
>  - Drop the enforcement of permanent disabling the entropy source
> 
>  crypto/jitterentropy-kcapi.c |  51 ++++++-------
>  crypto/jitterentropy.c       | 144 +++++++++++++----------------------
>  crypto/jitterentropy.h       |   1 -
>  3 files changed, 76 insertions(+), 120 deletions(-)

Patch applied.  Thanks.
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt