crypto: jitter - consider 32 LSB for APT
The APT compares the current time stamp with a pre-set value. The current code only considered the 4 LSB only. Yet, after reviews by mathematicians of the user space Jitter RNG version >= 3.1.0, it was concluded that the APT can be calculated on the 32 LSB of the time delta. Thi change is applied to the kernel. This fixes a bug where an AMD EPYC fails this test as its RDTSC value contains zeros in the LSB. The most appropriate fix would have been to apply a GCD calculation and divide the time stamp by the GCD. Yet, this is a significant code change that will be considered for a future update. Note, tests showed that constantly the GCD always was 32 on these systems, i.e. the 5 LSB were always zero (thus failing the APT since it only considered the 4 LSB for its calculation). Signed-off-by: Stephan Mueller <smueller@chronox.de> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
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@ -265,7 +265,6 @@ static int jent_stuck(struct rand_data *ec, __u64 current_delta)
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{
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{
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__u64 delta2 = jent_delta(ec->last_delta, current_delta);
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__u64 delta2 = jent_delta(ec->last_delta, current_delta);
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__u64 delta3 = jent_delta(ec->last_delta2, delta2);
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__u64 delta3 = jent_delta(ec->last_delta2, delta2);
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unsigned int delta_masked = current_delta & JENT_APT_WORD_MASK;
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ec->last_delta = current_delta;
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ec->last_delta = current_delta;
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ec->last_delta2 = delta2;
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ec->last_delta2 = delta2;
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@ -274,7 +273,7 @@ static int jent_stuck(struct rand_data *ec, __u64 current_delta)
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* Insert the result of the comparison of two back-to-back time
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* Insert the result of the comparison of two back-to-back time
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* deltas.
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* deltas.
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*/
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*/
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jent_apt_insert(ec, delta_masked);
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jent_apt_insert(ec, current_delta);
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if (!current_delta || !delta2 || !delta3) {
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if (!current_delta || !delta2 || !delta3) {
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/* RCT with a stuck bit */
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/* RCT with a stuck bit */
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