ACK: [SRU][Focal][PATCH 1/1] crypto: jitter - consider 32 LSB for APT

[Marcelo Henrique Cerri]

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On Tue, Jun 18 2024, Gerald Yang wrote:
> From: Stephan Müller <smueller at chronox.de>
>
> BugLink: https://bugs.launchpad.net/bugs/2069715
>
> 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 at chronox.de>
> Signed-off-by: Herbert Xu <herbert at gondor.apana.org.au>
> (cherry picked from commit 552d03a223eda3df84526ab2c1f4d82e15eaee7a)
> Signed-off-by: Gerald Yang <gerald.yang at canonical.com>
> ---
>  crypto/jitterentropy.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/crypto/jitterentropy.c b/crypto/jitterentropy.c
> index 57f4a1ac738b..7d45a70111c8 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 */
> -- 
> 2.25.1

Looks good to me. The fix was introduced in 5.17, but the upstream 5.15
stable tree received the fix and it was applied to Jammy as well.

Target kernel is correct and we should apply it to the generic kernel.

Thank you!

Acked-by: Marcelo Henrique Cerri <marcelo.cerri at canonical.com>

- -- 
Regards,
Marcelo
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