Index: sys/arch/aarch64/aarch64/vm_machdep.c
===================================================================
RCS file: /home/netbsd/src/sys/arch/aarch64/aarch64/vm_machdep.c,v
retrieving revision 1.8
diff -p -u -r1.8 vm_machdep.c
--- sys/arch/aarch64/aarch64/vm_machdep.c	23 May 2020 18:08:59 -0000	1.8
+++ sys/arch/aarch64/aarch64/vm_machdep.c	14 Oct 2020 10:56:44 -0000
@@ -143,11 +143,17 @@ cpu_lwp_fork(struct lwp *l1, struct lwp 
 	*utf = *l1->l_md.md_utf;
 
 	/*
-	 * If specified, give the child a different stack
-	 * (make sure it's 16-byte aligned).
+	 * If specified, give the child a different stack (make sure it's
+	 * 16- or 8-byte aligned for 64- or 32-bit processes, respectively).
 	 */
-	if (stack != NULL)
-		utf->tf_sp = ((vaddr_t)(stack) + stacksize) & -16;
+	if (stack != NULL) {
+		utf->tf_sp = (vaddr_t)(stack) + stacksize;
+		if (__predict_false(l2->l_proc->p_flag & PK_32)) {
+			utf->tf_sp &= -8;
+			utf->tf_reg[13] = utf->tf_sp;
+		} else
+			utf->tf_sp &= -16;
+	}
 
 	/* build a new switchframe */
 	struct trapframe * const ktf = utf - 1;