> It would be nice to get the exact version in which sha256 was fixed on ppc64, but it's not a requirements.

Ah, I think that I found the bugfix (8 Jan 2016):
https://github.com/torvalds/linux/commit/6de62f15b581

So it was fixed in the kernel 4.5.

I found also https://access.redhat.com/errata/RHSA-2017:2437 :

"The lrw_crypt() function in 'crypto/lrw.c' in the Linux kernel before 4.5 allows local users to cause a system crash and a denial of service by the NULL pointer dereference via accept(2) system call for AF_ALG socket without calling setkey() first to set a cipher key. (CVE-2015-8970, Moderate)"

So I will simply skip test_sha256() on all architectures for kernel older than 4.5. You can now ignore my questions in my previous comment ;-)