From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755855Ab1FCRGZ (ORCPT ); Fri, 3 Jun 2011 13:06:25 -0400 Received: from smtp-out.google.com ([74.125.121.67]:34089 "EHLO smtp-out.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754440Ab1FCRGY (ORCPT ); Fri, 3 Jun 2011 13:06:24 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=google.com; s=beta; h=date:from:x-x-sender:to:cc:subject:in-reply-to:message-id :references:user-agent:mime-version:content-type; b=M8rr9G/XcMikO358aiYcpIL/dHPZO0phH46sWNNXic7+gNgP1ZaMIZFi5mZxE2JEUK t6DEvtk0ch5p7NbfCfaA== Date: Fri, 3 Jun 2011 10:06:14 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@sister.anvils To: Andrea Arcangeli cc: Chris Wright , Andrea Righi , CAI Qian , Rik van Riel , Mel Gorman , KAMEZAWA Hiroyuki , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton Subject: Re: [BUG 3.0.0-rc1] ksm: NULL pointer dereference in ksm_do_scan() In-Reply-To: <20110602174305.GH19505@random.random> Message-ID: References: <20110601222032.GA2858@thinkpad> <2144269697.363041.1306998593180.JavaMail.root@zmail06.collab.prod.int.phx2.redhat.com> <20110602141927.GA2011@thinkpad> <20110602164841.GK23047@sequoia.sous-sol.org> <20110602174305.GH19505@random.random> User-Agent: Alpine 2.00 (LSU 1167 2008-08-23) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-System-Of-Record: true Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2 Jun 2011, Andrea Arcangeli wrote: > On Thu, Jun 02, 2011 at 10:29:39AM -0700, Hugh Dickins wrote: > > AndreaA, I didn't study the patch you posted half an hour ago, > > since by that time I'd worked it out and was preparing patch below. > > I think your patch would be for a different bug, hopefully one we > > don't have, it looks more complicated than we should need for this. > > I didn't expect two different bugs leading to double free. There wasn't a double free there, just failure to cope with race emptying the list, so accessing head when expecting a full entry. > > If you've time please review my other patch too because mmput runs > with no mmap_sem hold and I think the ksm scan code runs under the > assumption that __ksm_exit is waiting in down_write() when > ksm_mmlist_lock is released (before freeing the mm_slot), and that > assumption is wrong. ksm_test_exit may very well be true despite > __ksm_exit didn't run yet, and ksm scan will proceed freeing after > changing the mm_slot and ksm_exit will be free to run and free again > immediately after the ksm scan releases the ksm_mmlist_lock and before > it clears the MMF_VM_MERGEABLE (because the mm_slot has been changed > before releasing the ksm_mmlist_lock). > > The rmap_list being null will kind of hide it, the fact there's so > little time between the unlock of the ksm_mmlist_lock and the clearing > of MMF_VM_MERGEABLE (that will stop ksm_exit from calling __ksm_exit > at all) will also hide it. At least in > unmerge_and_remove_all_rmap_items remove_trailing_rmap_items will nuke > the rmap_list just before this race runs so making it more likely > possible. You'll see from the "beware" comment in scan_get_next_rmap_item() that this case is expected, that it sometimes reaches freeing the slots before the exiting task reaches __ksm_exit(). That race should already be handled. I believe your patch is unnecessary, because get_mm_slot() is a hashlist lookup, and will return NULL once either end has done the hlist_del(&mm_slot->link). Hugh