kern/58149: aarch64: Cannot return from a signal handler if SP was misaligned when the signal arrived

[pho%cielonegro.org@localhost]

>Number:         58149
>Category:       kern
>Synopsis:       aarch64: Cannot return from a signal handler if SP was misaligned when the signal arrived
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Apr 14 03:40:00 +0000 2024
>Originator:     PHO
>Release:        10.0
>Organization:
>Environment:
NetBSD yukari.cielonegro.org 10.0 NetBSD 10.0 (GENERIC64) #0: Thu Mar 28 08:33:33 UTC 2024  mkrepro%mkrepro.NetBSD.org@localhost:/usr/src/sys/arch/evbarm/compile/GENERIC64 evbarm
>Description:
Copied from the commit message of my patch to fix it:

aarch64: Don't trip up on misaligned SP when a signal is sent to a process

1. sendsig_siginfo() previously assumed that user SP was always aligned to
   16 bytes and could call signal handlers with SP misaligned. This is a
   wrong assumption because aarch64 demands that SP is aligned *only while*
   it's being used to access memory. Now it properly aligns it before
   pusing anything on the stack.

2. cpu_mcontext_validate() used to check if _REG_SP was aligned and
   considered the ucontext invalid otherwise. This meant if a signal was
   sent to a process whose SP was misaligned, the signal handler would fail
   to return because the ucontext passed from the kernel was an invalid
   one. Now setcontext(2) doesn't complain about misaligned SP.

>How-To-Repeat:

>Fix:
Apply this patch:
https://github.com/NetBSD/src/commit/f672806f26384c093c73b4ee0fb94f1fa309ca4a