From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1030466AbeCAMk2 (ORCPT ); Thu, 1 Mar 2018 07:40:28 -0500 Received: from mx2.suse.de ([195.135.220.15]:53589 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1030423AbeCAMk1 (ORCPT ); Thu, 1 Mar 2018 07:40:27 -0500 Date: Thu, 1 Mar 2018 13:40:24 +0100 From: Michal Hocko To: Mark Rutland Cc: linux-kernel@vger.kernel.org, Andrew Morton , Ingo Molnar , Mathieu Desnoyers , Peter Zijlstra , Rik van Riel , Will Deacon Subject: Re: [PATCH] Detect early free of a live mm Message-ID: <20180301124024.GE15057@dhcp22.suse.cz> References: <20180228121458.2230-1-mark.rutland@arm.com> <20180228121809.ztpcjb3256tj6tct@lakrids.cambridge.arm.com> <20180301093522.GC15057@dhcp22.suse.cz> <20180301112237.pjn5rmvkkpxcntl7@lakrids.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180301112237.pjn5rmvkkpxcntl7@lakrids.cambridge.arm.com> User-Agent: Mutt/1.9.3 (2018-01-21) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu 01-03-18 11:22:37, Mark Rutland wrote: > On Thu, Mar 01, 2018 at 10:35:22AM +0100, Michal Hocko wrote: > > On Wed 28-02-18 12:18:10, Mark Rutland wrote: > > > Ugh, I messed up Peter's email when sending this out. please > > > s/infraded/infradead/ if replying to the first mail. > > > > > > Sorry about that. > > > > > > Mark. > > > > > > On Wed, Feb 28, 2018 at 12:14:58PM +0000, Mark Rutland wrote: > > > > KASAN splats indicate that in some cases we free a live mm, then > > > > continue to access it, with potentially disastrous results. This is > > > > likely due to a mismatched mmdrop() somewhere in the kernel, but so far > > > > the culprit remains elusive. > > > > > > > > Let's have __mmdrop() verify that the mm isn't live for the current > > > > task, similar to the existing check for init_mm. This way, we can catch > > > > this class of issue earlier, and without requiring KASAN. > > > > Wouldn't it be better to check the mm_users count instead? > > {VM_}BUG_ON(atomic_read(&mm->mm_users)); > > Perhaps, but that won't catch a mismatched mmput(), which could > decrement mm_users (and mm_count) to zero early, before current->mm is > cleared in exit_mm(). true > Locally, I'm testing with: > > BUG_ON(mm == current->mm); > BUG_ON(mm == current->active_mm); > BUG_ON(refcount_read(&mm->mm_users) != 0); > BUG_ON(refcount_read(&mm->mm_count) != 0); > > ... so as to also catch an early free from another thread. I would be careful about active_mm. At least {un}use_mm does some tricks with it. > > Relying on current->mm resetting works currently but this is quite a > > subtle dependency. > > Do you mean because it only catches an early free in a thread using that > mm, or is there another subtlety you had in mind? No, I was more thinking about future changes when we stop clearing tsk->mm at exit. This is rather unlikely to be honest because that requires more changes. -- Michal Hocko SUSE Labs