From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753375AbbCXUIR (ORCPT ); Tue, 24 Mar 2015 16:08:17 -0400 Received: from mail-wi0-f181.google.com ([209.85.212.181]:38886 "EHLO mail-wi0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752293AbbCXUIP (ORCPT ); Tue, 24 Mar 2015 16:08:15 -0400 Date: Tue, 24 Mar 2015 21:08:11 +0100 From: Ingo Molnar To: Andy Lutomirski Cc: Takashi Iwai , Denys Vlasenko , Stefan Seyfried , X86 ML , Jiri Kosina , LKML , Tejun Heo Subject: Re: [PATCH] x86, entry: Check for syscall exit work with IRQs disabled Message-ID: <20150324200810.GA3753@gmail.com> References: <189320d42b4d671df78c10555976bb10af1ffc75.1427137498.git.luto@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <189320d42b4d671df78c10555976bb10af1ffc75.1427137498.git.luto@kernel.org> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Andy Lutomirski wrote: > We currently have a race: if we're preempted during syscall exit, we > can fail to process syscall return work that is queued up while > we're preempted in ret_from_sys_call after checking ti.flags. > > Fix it by disabling interrupts before checking ti.flags. > > Fixes: 96b6352c1271 x86_64, entry: Remove the syscall exit audit and schedule optimizations > Reported-by: Stefan Seyfried > Reported-by: Takashi Iwai > Signed-off-by: Andy Lutomirski > --- > > Ingo, I don't understand the LOCKDEP_SYS_EXIT stuff. Can you take a quick > look to confirm that it's okay to call it more than once? So the essence is that it wants to print this warning if we are holding a lock after a syscall: printk("[ BUG: lock held when returning to user space! ]\n"); it manipulates no state and is not sensitive to whether it's called before or after return-work processing. > arch/x86/kernel/entry_64.S | 18 ++++++++++++++---- > 1 file changed, 14 insertions(+), 4 deletions(-) > > diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S > index 1d74d161687c..2babb393915e 100644 > --- a/arch/x86/kernel/entry_64.S > +++ b/arch/x86/kernel/entry_64.S > @@ -364,12 +364,21 @@ system_call_fastpath: > * Has incomplete stack frame and undefined top of stack. > */ > ret_from_sys_call: > - testl $_TIF_ALLWORK_MASK,TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET) > - jnz int_ret_from_sys_call_fixup /* Go the the slow path */ > - > LOCKDEP_SYS_EXIT > DISABLE_INTERRUPTS(CLBR_NONE) > TRACE_IRQS_OFF > + > + /* > + * We must check ti flags with interrupts (or at least preemption) > + * off because we must *never* return to userspace without > + * processing exit work that is enqueued if we're preempted here. > + * In particular, returning to userspace with any of the one-shot > + * flags (TIF_NOTIFY_RESUME, TIF_USER_RETURN_NOTIFY, etc) set is > + * very bad. > + */ > + testl $_TIF_ALLWORK_MASK,TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET) > + jnz int_ret_from_sys_call_fixup /* Go the the slow path */ Should be safe to call it once again after user-work processing has been finished. I've picked up your fix for tip:x86/urgent. Thanks, Ingo