Hi Kees,

Kees Cook [off-list ref] writes:

On Thu, Apr 29, 2021 at 11:14:51AM +0200, Sven Schnelle wrote:

quoted

enough and has much less performance penalty compared to using
get_random_int(). The patch also adds randomization in pgm_check_handler()
as the sigreturn/rt_sigreturn system calls might be called from there.

Ah, interesting. Is this path to syscalls unique to s390? (As in, should
x86 and arm64 gain coverage over a path that got missed?)

Yes, it's unique to s390. So there should be no need to do anything
similar on other architectures.

quoted

+static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs,
+						  unsigned long ti_work)
+{
+	choose_random_kstack_offset(get_tod_clock_fast() & 0xff);

What's the stack alignment on s390? Or, better question, what's the
expected number of entropy bits?

The stack alignement on s390 is 8 bytes, so this should give us 5 bits
of entropy.