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=-11.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS 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 21ED8C433E6 for ; Fri, 28 Aug 2020 17:54:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0D43520DD4 for ; Fri, 28 Aug 2020 17:54:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727888AbgH1RyU (ORCPT ); Fri, 28 Aug 2020 13:54:20 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59548 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726928AbgH1RyT (ORCPT ); Fri, 28 Aug 2020 13:54:19 -0400 Received: from ZenIV.linux.org.uk (zeniv.linux.org.uk [IPv6:2002:c35c:fd02::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5201FC061264; Fri, 28 Aug 2020 10:54:19 -0700 (PDT) Received: from viro by ZenIV.linux.org.uk with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1kBiZp-006Goc-QX; Fri, 28 Aug 2020 17:54:13 +0000 Date: Fri, 28 Aug 2020 18:54:13 +0100 From: Al Viro To: syzbot Cc: axboe@kernel.dk, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, maz@kernel.org, oleg@redhat.com, peterz@infradead.org, syzkaller-bugs@googlegroups.com Subject: Re: kernel BUG at fs/inode.c:LINE! (2) Message-ID: <20200828175413.GL1236603@ZenIV.linux.org.uk> References: <000000000000c8fcd905adefe24b@google.com> <20200828153825.GI1236603@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200828153825.GI1236603@ZenIV.linux.org.uk> Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Fri, Aug 28, 2020 at 04:38:25PM +0100, Al Viro wrote: > On Fri, Aug 28, 2020 at 06:18:17AM -0700, syzbot wrote: > > Hello, > > > > syzbot found the following issue on: > > > > HEAD commit: d012a719 Linux 5.9-rc2 > > git tree: upstream > > console output: https://syzkaller.appspot.com/x/log.txt?x=15aa650e900000 > > kernel config: https://syzkaller.appspot.com/x/.config?x=891ca5711a9f1650 > > dashboard link: https://syzkaller.appspot.com/bug?extid=c92c93d1f1aaaacdb9db > > compiler: clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81) > > syz repro: https://syzkaller.appspot.com/x/repro.syz?x=12ecb939900000 > > C reproducer: https://syzkaller.appspot.com/x/repro.c?x=140a19a9900000 > > Trying to reproduce... OK, I think I see what's going on. ep_loop_check_proc() runs into an already doomed file that has already committed to getting killed (->f_count is already at 0), but still hadn't gotten through its epitems removal (e.g. has its eventpoll_release_file() sitting there trying to get epmutex). Blindly bumping refcount here is worse than useless. Try this, to verify that this is what's going on; it's _not_ a proper fix, but it should at least tell if we have something else going on. diff --git a/fs/eventpoll.c b/fs/eventpoll.c index e0decff22ae2..4590223b2de9 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -1995,7 +1995,7 @@ static int ep_loop_check_proc(void *priv, void *cookie, int call_nests) * during ep_insert(). */ if (list_empty(&epi->ffd.file->f_tfile_llink)) { - get_file(epi->ffd.file); + if (get_file_rcu(epi->ffd.file)) list_add(&epi->ffd.file->f_tfile_llink, &tfile_check_list); }