From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756117AbZBTQC0 (ORCPT ); Fri, 20 Feb 2009 11:02:26 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752379AbZBTQCR (ORCPT ); Fri, 20 Feb 2009 11:02:17 -0500 Received: from mx2.mail.elte.hu ([157.181.151.9]:34412 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752128AbZBTQCQ (ORCPT ); Fri, 20 Feb 2009 11:02:16 -0500 Date: Fri, 20 Feb 2009 17:01:57 +0100 From: Ingo Molnar To: Vegard Nossum Cc: stable@kernel.org, Andrew Morton , Nick Piggin , Pekka Enberg , linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm: fix lazy vmap purging (use-after-free error) Message-ID: <20090220160157.GA11294@elte.hu> References: <20090220134121.GA19575@damson.getinternet.no> <20090220135000.GA9616@elte.hu> <20090220140157.GA12799@elte.hu> <19f34abd0902200651k7e86aebay5398ef5ac0578561@mail.gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <19f34abd0902200651k7e86aebay5398ef5ac0578561@mail.gmail.com> User-Agent: Mutt/1.5.18 (2008-05-17) X-ELTE-VirusStatus: clean X-ELTE-SpamScore: -1.5 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.3 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Vegard Nossum wrote: > 2009/2/20 Ingo Molnar : > > > > * Ingo Molnar wrote: > > > >> ah, indeed: > >> > >> list_del_rcu(&va->list); > >> > >> i suspect it could be hit big time in a workload that opens > >> more than 512 files, as expand_files() uses a > >> vmalloc()+vfree() pair in that case. > > > > hm, perhaps it's not a problem after all. The freeing is done > > via rcu, and list_del_rcu() leaves the forward pointer intact. > > Well, it's not the particular line that you posted, in any case. > That's &va->list, but the traversed list is &va->purge_list. > > I thought it would be the line: > > call_rcu(&va->rcu_head, rcu_free_va); > > (which does kfree() in the callback) that was the problem. > > > > > So how did it happen that the entry got kfree()d before the loop > > was done? We are in a spinlocked section so the CPU should not > > have entered rcu processing. > > I added some printks to __free_vmap_area() and rcu_free_va(), and it > shows that the kfree() is being called immediately (inside the list > traversal). So the call_rcu() is happening immediately (or almost > immediately). > > If I've understood correctly, the RCU processing can happen inside a > spinlock, as long as interrupts are enabled. (Won't the timer IRQ > trigger softirq processing, which triggers RCU callback processing, > for example?) > > And interrupts are enabled when this happens: EFLAGS: 00000292 > > Please correct me if I am wrong! The timer irq will do RCU garbage-collection - but only of entries where a grace period has passed. Otherwise there would be no point in using RCU at all, if the kfree() can happen immediately. RCU is about delaying action, and doing it at a point in time when we sure are in a quiescent state. (we have done a context-switch or scheduled to idle) The question is, is this piece of loop traversal code preemptible? I dont think it is since it's embedded in a spinlock: spin_lock(&vmap_area_lock); - list_for_each_entry(va, &valist, purge_list) + list_for_each_entry_safe(va, n_va, &valist, purge_list) __free_vmap_area(va); spin_unlock(&vmap_area_lock); [ on -rt this could be preemptible and this would be a real fix there. I just dont see how the kfree() can execute on mainline. Obviously it did, since you got the kmemcheck assert. ] Ingo