From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0A3EAC43331 for ; Mon, 11 Nov 2019 22:36:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id DA6EC20659 for ; Mon, 11 Nov 2019 22:36:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727484AbfKKWgE (ORCPT ); Mon, 11 Nov 2019 17:36:04 -0500 Received: from Galois.linutronix.de ([193.142.43.55]:60086 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727295AbfKKWfu (ORCPT ); Mon, 11 Nov 2019 17:35:50 -0500 Received: from localhost ([127.0.0.1] helo=nanos.tec.linutronix.de) by Galois.linutronix.de with esmtp (Exim 4.80) (envelope-from ) id 1iUIHk-0000vO-7v; Mon, 11 Nov 2019 23:35:48 +0100 Message-Id: <20191111223052.790754100@linutronix.de> User-Agent: quilt/0.65 Date: Mon, 11 Nov 2019 23:03:27 +0100 From: Thomas Gleixner To: LKML Cc: x86@kernel.org, Linus Torvalds , Andy Lutomirski , Stephen Hemminger , Willy Tarreau , Juergen Gross , Sean Christopherson , "H. Peter Anvin" Subject: [patch V2 13/16] x86/iopl: Fixup misleading comment References: <20191111220314.519933535@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Thomas Gleixner The comment for the sys_iopl() implementation is outdated and actively misleading in some parts. Fix it up. Signed-off-by: Thomas Gleixner --- arch/x86/kernel/ioport.c | 35 +++++++++++++++++++++++++++-------- 1 file changed, 27 insertions(+), 8 deletions(-) --- a/arch/x86/kernel/ioport.c +++ b/arch/x86/kernel/ioport.c @@ -41,7 +41,7 @@ void io_bitmap_exit(void) } /* - * this changes the io permissions bitmap in the current task. + * This changes the io permissions bitmap in the current task. */ long ksys_ioperm(unsigned long from, unsigned long num, int turn_on) { @@ -130,14 +130,24 @@ SYSCALL_DEFINE3(ioperm, unsigned long, f } /* - * sys_iopl has to be used when you want to access the IO ports - * beyond the 0x3ff range: to get the full 65536 ports bitmapped - * you'd need 8kB of bitmaps/process, which is a bit excessive. + * The sys_iopl functionality depends on the level argument, which if + * granted for the task is used by the CPU to check I/O instruction and + * CLI/STI against the current priviledge level (CPL). If CPL is less than + * or equal the tasks IOPL level the instructions take effect. If not a #GP + * is raised. The default IOPL is 0, i.e. no permissions. * - * Here we just change the flags value on the stack: we allow - * only the super-user to do it. This depends on the stack-layout - * on system-call entry - see also fork() and the signal handling - * code. + * Setting IOPL to level 0-2 is disabling the userspace access. Only level + * 3 enables it. If set it allows the user space thread: + * + * - Unrestricted access to all 65535 I/O ports + * - The usage of CLI/STI instructions + * + * The advantage over ioperm is that the context switch does not require to + * update the I/O bitmap which is especially true when a large number of + * ports is accessed. But the allowance of CLI/STI in userspace is + * considered a major problem. + * + * IOPL is strictly per thread and inherited on fork. */ SYSCALL_DEFINE1(iopl, unsigned int, level) { @@ -158,9 +168,18 @@ SYSCALL_DEFINE1(iopl, unsigned int, leve security_locked_down(LOCKDOWN_IOPORT)) return -EPERM; } + /* + * Change the flags value on the return stack, which has been set + * up on system-call entry. See also the fork and signal handling + * code how this is handled. + */ regs->flags = (regs->flags & ~X86_EFLAGS_IOPL) | (level << X86_EFLAGS_IOPL_BIT); + /* Store the new level in the thread struct */ t->iopl = level << X86_EFLAGS_IOPL_BIT; + /* + * X86_32 switches immediately and XEN handles it via emulation. + */ set_iopl_mask(t->iopl); return 0;