This is a note to let you know that I've just added the patch titled crypto: jitter - consider 32 LSB for APT to the 5.15-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: crypto-jitter-consider-32-lsb-for-apt.patch and it can be found in the queue-5.15 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. commit 67c3516c9f76709dc2f5a73c2c362276a0d7be9e Author: Stephan Müller <smueller@xxxxxxxxxx> Date: Sun Nov 21 15:14:20 2021 +0100 crypto: jitter - consider 32 LSB for APT [ 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@xxxxxxxxxx> Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> diff --git a/crypto/jitterentropy.c b/crypto/jitterentropy.c index a11b3208760f3..f6d3a84e3c214 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 */