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authorStephan Müller <smueller@chronox.de>2021-11-21 15:14:20 +0100
committerHerbert Xu <herbert@gondor.apana.org.au>2021-11-26 16:25:18 +1100
commit552d03a223eda3df84526ab2c1f4d82e15eaee7a (patch)
treee680fe92fefee834aaadbd5b22ec0b0673ac145f /crypto
parent13389403fe8a978bfc0a866b6d598451feaca212 (diff)
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>
Diffstat (limited to 'crypto')
-rw-r--r--crypto/jitterentropy.c3
1 files changed, 1 insertions, 2 deletions
diff --git a/crypto/jitterentropy.c b/crypto/jitterentropy.c
index 4dc2261cdeef..788d90749715 100644
--- a/crypto/jitterentropy.c
+++ b/crypto/jitterentropy.c
@@ -265,7 +265,6 @@ static int jent_stuck(struct rand_data *ec, __u64 current_delta)
{
__u64 delta2 = jent_delta(ec->last_delta, current_delta);
__u64 delta3 = jent_delta(ec->last_delta2, delta2);
- unsigned int delta_masked = current_delta & JENT_APT_WORD_MASK;
ec->last_delta = current_delta;
ec->last_delta2 = delta2;
@@ -274,7 +273,7 @@ static int jent_stuck(struct rand_data *ec, __u64 current_delta)
* Insert the result of the comparison of two back-to-back time
* deltas.
*/
- jent_apt_insert(ec, delta_masked);
+ jent_apt_insert(ec, current_delta);
if (!current_delta || !delta2 || !delta3) {
/* RCT with a stuck bit */