From: Stephan Müller smueller@chronox.de
[ Upstream commit 552d03a223eda3df84526ab2c1f4d82e15eaee7a ]
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 Signed-off-by: Sasha Levin sashal@kernel.org --- crypto/jitterentropy.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-)
diff --git a/crypto/jitterentropy.c b/crypto/jitterentropy.c index 6e147c43fc186..37c4c308339e4 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 */