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=-10.3 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 DA204C10DCE for ; Fri, 6 Mar 2020 17:21:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A6B8D20709 for ; Fri, 6 Mar 2020 17:21:34 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=kernel-dk.20150623.gappssmtp.com header.i=@kernel-dk.20150623.gappssmtp.com header.b="O/6eGHTV" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726300AbgCFRVe (ORCPT ); Fri, 6 Mar 2020 12:21:34 -0500 Received: from mail-io1-f66.google.com ([209.85.166.66]:43912 "EHLO mail-io1-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726167AbgCFRVe (ORCPT ); Fri, 6 Mar 2020 12:21:34 -0500 Received: by mail-io1-f66.google.com with SMTP id n21so2803067ioo.10 for ; Fri, 06 Mar 2020 09:21:33 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel-dk.20150623.gappssmtp.com; s=20150623; h=subject:to:cc:references:from:message-id:date:user-agent :mime-version:in-reply-to:content-language:content-transfer-encoding; bh=UpJ/sy4Eqlqc62xJ9CGPbulBS93p2b28hl6znlt+s3U=; b=O/6eGHTVYcHkiG251FGLtEObmFqV0XFSf6lwS4Z+IVRMTSHQdIq547eK8C06y3PMoQ 4PN9yO+rlr/bvY0hcKXlQhEOltlEH985MxXSFTHKNrW8xwzFt4JEwmGpx6EzRJ8+8Sse G4MQsphafaUpkUhI15XT6cEyAzSF3ccgJRKXUw5ocgwNaBMTCmC7XOGdBesQRBm7dyC1 ieItttY+ZC3S/q6/uIEWofTLGnfszPsjfm4ay6VaFklIbiSQF4uHuDpcJtJqSa/D0ov1 McWnHnAYObmYXxAPocMHWUt4duL4Kb4rnTAsf3me+RM/q5K5MS/hgSE9Q+j8LnklJSrz 5Hug== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=UpJ/sy4Eqlqc62xJ9CGPbulBS93p2b28hl6znlt+s3U=; b=uMPA4iNueRUWCAeVflpVJ9+mG1QEqPoI6jaWbP2W2Gmbrkle0NYEWIVNg4HSG6dUjr SSZ9wmTi/LtPL4QdRUO9xFinop3lhn0dnGqLxwhcVeObs6+uy/uEsqtMD1qJspxpS64c NxomR7R31zmt3HEwGFm4puaxMR5V+qNUPVBdgmOJti16UGLDvmQeerd4/VF87Vtvu3rg ZyRN0Ly9AvFsZQeBKkTA1+b+E+BWutdHQXqlSjbloBw3vxdz5m/d8Gx3077XOFjtXyK6 Sd+HJgLMLVDTm7C031pfe4mTf230NMb6eZRkOX4256ganrIg0f4q+FF4y07gJNx0R3Gc 7iTQ== X-Gm-Message-State: ANhLgQ3uJDmnSduJHLPlBvhAq+dCzWePCcysZs+TaitCsv+byVx0gIPi TtZHC7z8nNWn1qBBt3tiFUcaAw== X-Google-Smtp-Source: ADFU+vubeWQ/bYcExUcJpxub9/GalDT9lSUcZrh8Uyp0P3zVAJZ0o+k+cKU0ijuUPObvYLcIczD2tg== X-Received: by 2002:a02:950d:: with SMTP id y13mr3949524jah.139.1583515293076; Fri, 06 Mar 2020 09:21:33 -0800 (PST) Received: from [192.168.1.159] ([65.144.74.34]) by smtp.gmail.com with ESMTPSA id w1sm4012794ilo.30.2020.03.06.09.21.31 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 06 Mar 2020 09:21:32 -0800 (PST) Subject: Re: KASAN: use-after-free Read in percpu_ref_switch_to_atomic_rcu To: paulmck@kernel.org Cc: Jann Horn , 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 References: <00000000000067c6df059df7f9f5@google.com> <3f805e51-1db7-3e57-c9a3-15a20699ea54@kernel.dk> <075e7fbe-aeec-cb7d-9338-8eb4e1576293@kernel.dk> <20200306164443.GU2935@paulmck-ThinkPad-P72> <11921f78-c6f2-660b-5e33-11599c2f9a4b@kernel.dk> <20200306171911.GA2935@paulmck-ThinkPad-P72> From: Jens Axboe Message-ID: Date: Fri, 6 Mar 2020 10:21:31 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 MIME-Version: 1.0 In-Reply-To: <20200306171911.GA2935@paulmck-ThinkPad-P72> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: io-uring-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 3/6/20 10:19 AM, Paul E. McKenney wrote: > On Fri, Mar 06, 2020 at 10:00:19AM -0700, Jens Axboe wrote: >> On 3/6/20 9:44 AM, Paul E. McKenney wrote: >>> 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. >> >> If the nested call_rcu() works, that seems greatly preferable to needing >> the rcu_barrier(), even if that would not be a showstopper for me. The >> nested call_rcu() is just a bit odd, but with a comment it should be OK. >> >> Incremental here I'm going to test, would just fold in of course. >> >> >> diff --git a/fs/io_uring.c b/fs/io_uring.c >> index f3218fc81943..95ba95b4d8ec 100644 >> --- a/fs/io_uring.c >> +++ b/fs/io_uring.c >> @@ -5330,7 +5330,7 @@ static void io_file_ref_kill(struct percpu_ref *ref) >> complete(&data->done); >> } >> >> -static void io_file_ref_exit_and_free(struct rcu_head *rcu) >> +static void __io_file_ref_exit_and_free(struct rcu_head *rcu) >> { >> struct fixed_file_data *data = container_of(rcu, struct fixed_file_data, >> rcu); >> @@ -5338,6 +5338,18 @@ static void io_file_ref_exit_and_free(struct rcu_head *rcu) >> kfree(data); >> } >> >> +static void io_file_ref_exit_and_free(struct rcu_head *rcu) >> +{ >> + /* >> + * We need to order our exit+free call again the potentially >> + * existing call_rcu() for switching to atomic. One way to do that >> + * is to have this rcu callback queue the final put and free, as we >> + * could otherwise a pre-existing atomic switch complete _after_ >> + * the free callback we queued. >> + */ >> + call_rcu(rcu, __io_file_ref_exit_and_free); >> +} >> + >> static int io_sqe_files_unregister(struct io_ring_ctx *ctx) >> { >> struct fixed_file_data *data = ctx->file_data; > > Looks good to me! Thanks Paul! I talked to Paul in private, but here's the final version with updated comment and attributions. https://git.kernel.dk/cgit/linux-block/commit/?h=io_uring-5.6&id=c1e2148f8ecb26863b899d402a823dab8e26efd1 -- Jens Axboe