diff options
| author | Stephan Müller <smueller@chronox.de> | 2021-11-21 15:14:20 +0100 |
|---|---|---|
| committer | Herbert Xu <herbert@gondor.apana.org.au> | 2021-11-26 16:25:18 +1100 |
| commit | 552d03a223eda3df84526ab2c1f4d82e15eaee7a (patch) | |
| tree | e680fe92fefee834aaadbd5b22ec0b0673ac145f /crypto | |
| parent | 13389403fe8a978bfc0a866b6d598451feaca212 (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.c | 3 |
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 */ |