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=-3.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no 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 65C85C3565B for ; Fri, 21 Feb 2020 18:54:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3245B2071E for ; Fri, 21 Feb 2020 18:54:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729477AbgBUSyE (ORCPT ); Fri, 21 Feb 2020 13:54:04 -0500 Received: from outgoing-auth-1.mit.edu ([18.9.28.11]:53829 "EHLO outgoing.mit.edu" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726150AbgBUSyD (ORCPT ); Fri, 21 Feb 2020 13:54:03 -0500 Received: from callcc.thunk.org (guestnat-104-133-8-109.corp.google.com [104.133.8.109] (may be forged)) (authenticated bits=0) (User authenticated as tytso@ATHENA.MIT.EDU) by outgoing.mit.edu (8.14.7/8.12.4) with ESMTP id 01LIrvZF014282 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Fri, 21 Feb 2020 13:53:58 -0500 Received: by callcc.thunk.org (Postfix, from userid 15806) id B88524211EF; Fri, 21 Feb 2020 13:53:56 -0500 (EST) Date: Fri, 21 Feb 2020 13:53:56 -0500 From: "Theodore Y. Ts'o" To: Jan Kara Cc: Eric Biggers , linux-ext4@vger.kernel.org Subject: Re: [PATCH v3 2/2] ext4: fix race between writepages and enabling EXT4_EXTENTS_FL Message-ID: <20200221185356.GC741939@mit.edu> References: <20200219183047.47417-1-ebiggers@kernel.org> <20200219183047.47417-3-ebiggers@kernel.org> <20200220091548.GB13232@quack2.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200220091548.GB13232@quack2.suse.cz> Sender: linux-ext4-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org On Thu, Feb 20, 2020 at 10:15:48AM +0100, Jan Kara wrote: > On Wed 19-02-20 10:30:47, Eric Biggers wrote: > > From: Eric Biggers > > > > If EXT4_EXTENTS_FL is set on an inode while ext4_writepages() is running > > on it, the following warning in ext4_add_complete_io() can be hit: > > > > WARNING: CPU: 1 PID: 0 at fs/ext4/page-io.c:234 ext4_put_io_end_defer+0xf0/0x120 > > > > Here's a minimal reproducer (not 100% reliable) (root isn't required): > > > > while true; do > > sync > > done & > > while true; do > > rm -f file > > touch file > > chattr -e file > > echo X >> file > > chattr +e file > > done > > > > The problem is that in ext4_writepages(), ext4_should_dioread_nolock() > > (which only returns true on extent-based files) is checked once to set > > the number of reserved journal credits, and also again later to select > > the flags for ext4_map_blocks() and copy the reserved journal handle to > > ext4_io_end::handle. But if EXT4_EXTENTS_FL is being concurrently set, > > the first check can see dioread_nolock disabled while the later one can > > see it enabled, causing the reserved handle to unexpectedly be NULL. > > > > Since changing EXT4_EXTENTS_FL is uncommon, and there may be other races > > related to doing so as well, fix this by synchronizing changing > > EXT4_EXTENTS_FL with ext4_writepages() via the existing > > s_writepages_rwsem (previously called s_journal_flag_rwsem). > > > > This was originally reported by syzbot without a reproducer at > > https://syzkaller.appspot.com/bug?extid=2202a584a00fffd19fbf, > > but now that dioread_nolock is the default I also started seeing this > > when running syzkaller locally. > > > > Reported-by: syzbot+2202a584a00fffd19fbf@syzkaller.appspotmail.com > > Fixes: 6b523df4fb5a ("ext4: use transaction reservation for extent conversion in ext4_end_io") > > Cc: stable@kernel.org > > Signed-off-by: Eric Biggers > > The patch looks good to me. You can add: > > Reviewed-by: Jan Kara Thanks, applied. - Ted