From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx2.suse.de ([195.135.220.15]:51180 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932762AbeFRNnk (ORCPT ); Mon, 18 Jun 2018 09:43:40 -0400 Subject: Re: general protection fault in find_device To: dsterba@suse.cz, clm@fb.com, dsterba@suse.com, jbacik@fb.com, linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com, anand.jain@oracle.com References: <000000000000e5da7e056ee43449@google.com> <45200a61-58c3-b7f0-d8e4-5108f8369ac5@suse.com> <20180618133228.GL24375@twin.jikos.cz> From: Nikolay Borisov Message-ID: Date: Mon, 18 Jun 2018 16:43:36 +0300 MIME-Version: 1.0 In-Reply-To: <20180618133228.GL24375@twin.jikos.cz> Content-Type: text/plain; charset=utf-8 Sender: linux-btrfs-owner@vger.kernel.org List-ID: On 18.06.2018 16:32, David Sterba wrote: > On Mon, Jun 18, 2018 at 10:03:18AM +0300, Nikolay Borisov wrote: >> So this suggests some inconsistency on fs_devices->devices list. On a >> quick look indeed it doesn't seem clear what the locking rules for this >> list are. In device_list_add in the !device case a device is added with >> fs_devices->device_list_Mutex held and using list_add_rcu. In the same >> function if we want to read the list ie invoke find_devices (because we >> have found an fsid) we are using plain list_for_each_entry (ie not the >> _rcu version and i don't see device_list_mutex being held while >> iterating the list). Additionally in btrfs_free_extra_devids the >> fs_devices->devices list is iterated with uuid_mutex being held and not >> device_list_mutex. In open_fs_devices we don't get any protection >> whatsoever while reading the list. > > The uuid_mutex or device_list_mutex is provided by a caller up the > stack. > >> Same thing in >> btrfs_find_next_active_device. If the list is supposed to be >> RCU-protected then the rules are: >> >> 1. There needs to be an out of band (ie not RCU) mutual exclusion of >> modifiers > > that's device_list_mutex for fs_devices::devices > >> 2. Iterating the list should use _rcu list primitives. >> >> Currently I don't see those 2 invariants being enforced in every code path. > > Where is it not enforced for example? Admittedly I didn't check the whole call chain but for example in find_device it's used "naked". Perhaps putting some lockdep_assert in various places dealing with fs_devices->devices list would help ? > > If the device_list_mutex is held, list traversal does not use > list_for_each_entry_rcu, otherwise it does (eg the DEV_INFO ioctl or > btrfs_show_devname). > > The problem that triggers this report is IMO in device_list_add that > uses the device list unprotected. Anand sent patches for that, but they > were titled as 'cleanups' so I skipped them for the merge window. > > Candidate fixes are: > > https://patchwork.kernel.org/patch/10437705/ > https://patchwork.kernel.org/patch/10437713/ Yep those 2 definitely look like fixing unlocked accesses to fs_devices->devices list >