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=-7.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 7F872C10F25 for ; Fri, 6 Mar 2020 16:44:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 58E2A2084E for ; Fri, 6 Mar 2020 16:44:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1583513088; bh=uM6BBzCXa6HAUuOEGiA8P2jf81J77c6GVb5q3IeM3R4=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=rETapDG0T0R79SoW22FhOfmVofiQt1/dG4jVZdVkhBRIZVCnFS82lgEQEel5LhzxV h22OI7GFFGY0Wwu1vZWIQAUR4Sn7+7FGRtDsBHDGEAlwJu+b2lPXojbxXti9Wz/2ur 5GQRzwK3+BKRqxScqMqM4sEpCsCWxDDSTamQ4DGs= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726702AbgCFQoo (ORCPT ); Fri, 6 Mar 2020 11:44:44 -0500 Received: from mail.kernel.org ([198.145.29.99]:43274 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726167AbgCFQoo (ORCPT ); Fri, 6 Mar 2020 11:44:44 -0500 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 39A3A2072A; Fri, 6 Mar 2020 16:44:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1583513083; bh=uM6BBzCXa6HAUuOEGiA8P2jf81J77c6GVb5q3IeM3R4=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=suKLdZ7u99Tu+TZXXLBW/DTPMhhpoblbweU/aMPVupcPWtgSo/+q6IAK70+gMPfRL ah5o8KFpGEyp2Ul18AA3nzRoMfgJYb3P5A7YF5u3e5JqpvT3J9JTC8iW7bZG8EUK/I jZckRz4TQB2vJtmbOVkawf2lRhrHScoF9i7hpsxg= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 0E85935226BF; Fri, 6 Mar 2020 08:44:43 -0800 (PST) Date: Fri, 6 Mar 2020 08:44:43 -0800 From: "Paul E. McKenney" To: Jann Horn Cc: Jens Axboe , Dmitry Vyukov , syzbot , Al Viro , io-uring , linux-fsdevel , Borislav Petkov , "H. Peter Anvin" , LKML , Ingo Molnar , Peter Zijlstra , syzkaller-bugs , Thomas Gleixner , tony.luck@intel.com, the arch/x86 maintainers , Dan Carpenter Subject: Re: KASAN: use-after-free Read in percpu_ref_switch_to_atomic_rcu Message-ID: <20200306164443.GU2935@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <00000000000067c6df059df7f9f5@google.com> <3f805e51-1db7-3e57-c9a3-15a20699ea54@kernel.dk> <075e7fbe-aeec-cb7d-9338-8eb4e1576293@kernel.dk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.4 (2018-02-28) Sender: io-uring-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On Fri, Mar 06, 2020 at 04:36:20PM +0100, Jann Horn wrote: > On Fri, Mar 6, 2020 at 4:34 PM Jens Axboe wrote: > > On 3/6/20 7:57 AM, Jann Horn wrote: > > > +paulmck > > > > > > On Wed, Mar 4, 2020 at 3:40 PM Jens Axboe wrote: > > >> On 3/4/20 12:59 AM, Dmitry Vyukov wrote: > > >>> On Fri, Feb 7, 2020 at 9:14 AM syzbot > > >>> wrote: > > >>>> > > >>>> Hello, > > >>>> > > >>>> syzbot found the following crash on: > > >>>> > > >>>> HEAD commit: 4c7d00cc Merge tag 'pwm/for-5.6-rc1' of git://git.kernel.o.. > > >>>> git tree: upstream > > >>>> console output: https://syzkaller.appspot.com/x/log.txt?x=12fec785e00000 > > >>>> kernel config: https://syzkaller.appspot.com/x/.config?x=e162021ddededa72 > > >>>> dashboard link: https://syzkaller.appspot.com/bug?extid=e017e49c39ab484ac87a > > >>>> compiler: clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81) > > >>>> > > >>>> Unfortunately, I don't have any reproducer for this crash yet. > > >>>> > > >>>> IMPORTANT: if you fix the bug, please add the following tag to the commit: > > >>>> Reported-by: syzbot+e017e49c39ab484ac87a@syzkaller.appspotmail.com > > >>> > > >>> +io_uring maintainers > > >>> > > >>> Here is a repro: > > >>> https://gist.githubusercontent.com/dvyukov/6b340beab6483a036f4186e7378882ce/raw/cd1922185516453c201df8eded1d4b006a6d6a3a/gistfile1.txt > > >> > > >> I've queued up a fix for this: > > >> > > >> https://git.kernel.dk/cgit/linux-block/commit/?h=io_uring-5.6&id=9875fe3dc4b8cff1f1b440fb925054a5124403c3 > > > > > > I believe that this fix relies on call_rcu() having FIFO ordering; but > > > > > > says: > > > > > > | call_rcu() normally acts only on CPU-local state[...] It simply > > > enqueues the rcu_head structure on a per-CPU list, Indeed. For but one example, if there was a CPU-to-CPU migration between the two call_rcu() invocations, it would not be at all surprising for the two callbacks to execute out of order. > > > Is this fix really correct? > > > > That's a good point, there's a potentially stronger guarantee we need > > here that isn't "nobody is inside an RCU critical section", but rather > > that we're depending on a previous call_rcu() to have happened. Hence I > > think you are right - it'll shrink the window drastically, since the > > previous callback is already queued up, but it's not a full close. > > > > Hmm... > > You could potentially hack up the semantics you want by doing a > call_rcu() whose callback does another call_rcu(), or something like > that - but I'd like to hear paulmck's opinion on this first. That would work! Or, alternatively, do an rcu_barrier() between the two calls to call_rcu(), assuming that the use case can tolerate rcu_barrier() overhead and latency. Thanx, Paul