arm64: set POR_EL0 for kernel threads

Restrict kernel threads to only have RWX overlays for pkey 0.  This matches
what arch/x86 does, by defaulting to a restrictive PKRU.
Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Kevin Brodsky <Kevin.Brodsky@arm.com>
Link: https://lore.kernel.org/r/20241001133618.1547996-2-joey.gouly@arm.comSigned-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/process.c

@@ -412,6 +412,9 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 
 		p->thread.cpu_context.x19 = (unsigned long)args->fn;
 		p->thread.cpu_context.x20 = (unsigned long)args->fn_arg;
+
+		if (system_supports_poe())
+			p->thread.por_el0 = POR_EL0_INIT;
 	}
 	p->thread.cpu_context.pc = (unsigned long)ret_from_fork;
 	p->thread.cpu_context.sp = (unsigned long)childregs;