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=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,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 DD133C282DD for ; Wed, 8 Jan 2020 15:08:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id AF6B420692 for ; Wed, 8 Jan 2020 15:08:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728326AbgAHPI4 (ORCPT ); Wed, 8 Jan 2020 10:08:56 -0500 Received: from mx2.suse.de ([195.135.220.15]:60356 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727400AbgAHPIz (ORCPT ); Wed, 8 Jan 2020 10:08:55 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id B1F93AE35; Wed, 8 Jan 2020 15:08:52 +0000 (UTC) Received: by ds.suse.cz (Postfix, from userid 10065) id 2D89DDA791; Wed, 8 Jan 2020 16:08:42 +0100 (CET) Date: Wed, 8 Jan 2020 16:08:41 +0100 From: David Sterba To: Nikolay Borisov Cc: Josef Bacik , Qu Wenruo , linux-btrfs@vger.kernel.org, Zygo Blaxell , David Sterba Subject: Re: [PATCH v2] btrfs: relocation: Fix KASAN reports caused by extended reloc tree lifespan Message-ID: <20200108150841.GH3929@twin.jikos.cz> Reply-To: dsterba@suse.cz Mail-Followup-To: dsterba@suse.cz, Nikolay Borisov , Josef Bacik , Qu Wenruo , linux-btrfs@vger.kernel.org, Zygo Blaxell , David Sterba References: <20200108051200.8909-1-wqu@suse.com> <7482d2f3-f3a1-7dd9-6003-9042c1781207@toxicpanda.com> <2bfd87cf-2733-af0d-f33f-59e07c25d500@suse.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <2bfd87cf-2733-af0d-f33f-59e07c25d500@suse.com> User-Agent: Mutt/1.5.23.1-rc1 (2014-03-12) Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On Wed, Jan 08, 2020 at 05:03:35PM +0200, Nikolay Borisov wrote: > > > On 8.01.20 г. 16:55 ч., Josef Bacik wrote: > > On 1/8/20 12:12 AM, Qu Wenruo wrote: > >> [BUG] > >> There are several different KASAN reports for balance + snapshot > >> workloads. > >> Involved call paths include: > >> > >>     should_ignore_root+0x54/0xb0 [btrfs] > >>     build_backref_tree+0x11af/0x2280 [btrfs] > >>     relocate_tree_blocks+0x391/0xb80 [btrfs] > >>     relocate_block_group+0x3e5/0xa00 [btrfs] > >>     btrfs_relocate_block_group+0x240/0x4d0 [btrfs] > >>     btrfs_relocate_chunk+0x53/0xf0 [btrfs] > >>     btrfs_balance+0xc91/0x1840 [btrfs] > >>     btrfs_ioctl_balance+0x416/0x4e0 [btrfs] > >>     btrfs_ioctl+0x8af/0x3e60 [btrfs] > >>     do_vfs_ioctl+0x831/0xb10 > >>     ksys_ioctl+0x67/0x90 > >>     __x64_sys_ioctl+0x43/0x50 > >>     do_syscall_64+0x79/0xe0 > >>     entry_SYSCALL_64_after_hwframe+0x49/0xbe > >> > >>     create_reloc_root+0x9f/0x460 [btrfs] > >>     btrfs_reloc_post_snapshot+0xff/0x6c0 [btrfs] > >>     create_pending_snapshot+0xa9b/0x15f0 [btrfs] > >>     create_pending_snapshots+0x111/0x140 [btrfs] > >>     btrfs_commit_transaction+0x7a6/0x1360 [btrfs] > >>     btrfs_mksubvol+0x915/0x960 [btrfs] > >>     btrfs_ioctl_snap_create_transid+0x1d5/0x1e0 [btrfs] > >>     btrfs_ioctl_snap_create_v2+0x1d3/0x270 [btrfs] > >>     btrfs_ioctl+0x241b/0x3e60 [btrfs] > >>     do_vfs_ioctl+0x831/0xb10 > >>     ksys_ioctl+0x67/0x90 > >>     __x64_sys_ioctl+0x43/0x50 > >>     do_syscall_64+0x79/0xe0 > >>     entry_SYSCALL_64_after_hwframe+0x49/0xbe > >> > >>     btrfs_reloc_pre_snapshot+0x85/0xc0 [btrfs] > >>     create_pending_snapshot+0x209/0x15f0 [btrfs] > >>     create_pending_snapshots+0x111/0x140 [btrfs] > >>     btrfs_commit_transaction+0x7a6/0x1360 [btrfs] > >>     btrfs_mksubvol+0x915/0x960 [btrfs] > >>     btrfs_ioctl_snap_create_transid+0x1d5/0x1e0 [btrfs] > >>     btrfs_ioctl_snap_create_v2+0x1d3/0x270 [btrfs] > >>     btrfs_ioctl+0x241b/0x3e60 [btrfs] > >>     do_vfs_ioctl+0x831/0xb10 > >>     ksys_ioctl+0x67/0x90 > >>     __x64_sys_ioctl+0x43/0x50 > >>     do_syscall_64+0x79/0xe0 > >>     entry_SYSCALL_64_after_hwframe+0x49/0xbe > >> > >> [CAUSE] > >> All these call sites are only relying on root->reloc_root, which can > >> undergo btrfs_drop_snapshot(), and since we don't have real refcount > >> based protection to reloc roots, we can reach already dropped reloc > >> root, triggering KASAN. > >> > >> [FIX] > >> To avoid such access to unstable root->reloc_root, we should check > >> BTRFS_ROOT_DEAD_RELOC_TREE bit first. > >> > >> This patch introduces a new wrapper, have_reloc_root(), to do the proper > >> check for most callers who don't distinguish merged reloc tree and no > >> reloc tree. > >> > >> The only exception is should_ignore_root(), as merged reloc tree can be > >> ignored, while no reloc tree shouldn't. > >> > >> [CRITICAL SECTION ANALYSE] > >> Although test_bit()/set_bit()/clear_bit() doesn't imply a barrier, the > >> DEAD_RELOC_TREE bit has extra help from transaction as a higher level > >> barrier, the lifespan of root::reloc_root and DEAD_RELOC_TREE bit are: > >> > >>     NULL: reloc_root is NULL    PTR: reloc_root is not NULL > >>     0: DEAD_RELOC_ROOT bit not set    DEAD: DEAD_RELOC_ROOT bit set > >> > >>     (NULL, 0)    Initial state         __ > >>       |                     /\ Section A > >>          btrfs_init_reloc_root()             \/ > >>       |                      __ > >>     (PTR, 0)     reloc_root initialized      /\ > >>            |                     | > >>     btrfs_update_reloc_root()         |  Section B > >>            |                     | > >>     (PTR, DEAD)  reloc_root has been merged  \/ > >>            |                     __ > >>     === btrfs_commit_transaction() ==================== > >>       |                     /\ > >>     clean_dirty_subvols()             | > >>       |                     |  Section C > >>     (NULL, DEAD) reloc_root cleanup starts   \/ > >>            |                     __ > >>     btrfs_drop_snapshot()             /\ > >>       |                     |  Section D > >>     (NULL, 0)    Back to initial state     \/ > >> > >> Very have_reloc_root() or test_bit(DEAD_RELOC_ROOT) caller has hold a > >> transaction handler, so none of such caller can cross transaction > >> boundary. > >> > >> In Section A, every caller just found no DEAD bit, and grab reloc_root. > >> > >> In the cross section A-B, caller may get no DEAD bit, but since > >> reloc_root is still completely valid thus accessing reloc_root is > >> completely safe. > >> > >> No test_bit() caller can cross the boundary of Section B and Section C. > >> > >> In Section C, every caller found the DEAD bit, so no one will access > >> reloc_root. > >> > >> In the cross section C-D, either caller gets the DEAD bit set, avoiding > >> access reloc_root no matter if it's safe or not. > >> Or caller get the DEAD bit cleared, then access reloc_root, which is > >> already NULL, nothing will be wrong. > >> > >> Here we need extra memory barrier in cross section C-D, to ensure > >> proper memory order between reloc_root and clear_bit(). > >> > >> In Section D, since no DEAD bit and no reloc_root, it's back to initial > >> state. > >> > >> With this lifespan, it should be clear only one memory barrier is > >> needed, between setting reloc_root to NULL and clearing DEAD_RELOC_ROOT > >> bit. > >> > >> Reported-by: Zygo Blaxell > >> Fixes: d2311e698578 ("btrfs: relocation: Delay reloc tree deletion > >> after merge_reloc_roots") > >> Suggested-by: David Sterba > >> Signed-off-by: Qu Wenruo > >> --- > >> Changelog: > >> v2: > >> - Add the [CRITICAL SECTION ANALYSE] part > >>    This gets me into the rabbit hole of memory ordering, but thanks for > >>    the help from David (initially mentioning the mb hell) and Nikolay > >>    (for the proper doc), finally I could explain clearly why only > >>    one mb is needed. > >> - Add comment for the only needed memory barrier. > >> --- > >>   fs/btrfs/relocation.c | 32 ++++++++++++++++++++++++++++---- > >>   1 file changed, 28 insertions(+), 4 deletions(-) > >> > >> diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c > >> index d897a8e5e430..17a2484f76a5 100644 > >> --- a/fs/btrfs/relocation.c > >> +++ b/fs/btrfs/relocation.c > >> @@ -517,6 +517,22 @@ static int update_backref_cache(struct > >> btrfs_trans_handle *trans, > >>       return 1; > >>   } > >>   +/* > >> + * Check if this subvolume tree has valid reloc(*) tree. > >> + * > >> + * *: Reloc tree after swap is considered dead, thus not considered > >> as valid. > >> + *    This is enough for most callers, as they don't distinguish dead > >> reloc > >> + *    root from no reloc root. > >> + *    But should_ignore_root() below is a special case. > >> + */ > >> +static bool have_reloc_root(struct btrfs_root *root) > >> +{ > >> +    if (test_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state)) > >> +        return false; > > > > You still need a smp_mb__after_atomic() here, test_bit is unordered. > > Nope, that won't do anything, since smp_mb__(After|before)_atomic only > orders RMW operations and test_bit is not an RMW operation. From > atomic_bitops.txt: > > > Non-RMW ops: > > > > test_bit() > > Furthermore from atomic_t.txt: > > The barriers: > > > > smp_mb__{before,after}_atomic() > > > > only apply to the RMW atomic ops and can be used to augment/upgrade the > > ordering inherent to the op. The way I read it is more like smp_rmb/smp_wmb, but for bits in this case, so the smp_mb__before/after_atomic was only a syntactic sugar to match that it's atomic bitops. I realize this could have caused some confusion, however I still think that some sort of barrier is needed.