linux-kernel - Re: [PATCH] fs: do not push freeing inode to b_dirty

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Date:   Mon, 14 Nov 2022 19:43:54 +0200
From:   Svyatoslav Feldsherov <feldsherov@...gle.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Lukas Czerner <lczerner@...hat.com>,
        "Theodore Ts'o" <tytso@....edu>,
        syzbot+6ba92bd00d5093f7e371@...kaller.appspotmail.com,
        oferz@...gle.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: do not push freeing inode to b_dirty_time list

Thank you for looking into this!

On Mon, Nov 14, 2022 at 12:46 PM Jan Kara <jack@...e.cz> wrote:
>
> On Sun 13-11-22 17:24:39, Svyatoslav Feldsherov wrote:
> > After commit cbfecb927f42 ("fs: record I_DIRTY_TIME even if inode
> > already has I_DIRTY_INODE") writeiback_single_inode can push inode with
> > I_DIRTY_TIME set to b_dirty_time list. In case of freeing inode with
> > I_DIRTY_TIME set this can happened after deletion of inode io_list at
> > evict. Stack trace is following.
> >
> > evict
> > fat_evict_inode
> > fat_truncate_blocks
> > fat_flush_inodes
> > writeback_inode
> > sync_inode_metadata
> > writeback_single_inode
> >
> > This will lead to use after free in flusher thread.
> >
> > Fixes: cbfecb927f42 ("fs: record I_DIRTY_TIME even if inode already has I_DIRTY_INODE")
> > Reported-by: syzbot+6ba92bd00d5093f7e371@...kaller.appspotmail.com
> > Signed-off-by: Svyatoslav Feldsherov <feldsherov@...gle.com>
>
> Thanks for the analysis! I was scratching my head over this syzbot report
> for a while and it didn't occur to me somebody could be calling
> writeback_single_inode() from the .evict callback.
>
> Also what contributes to the problem is that FAT calls
> sync_inode_metadata(inode, 0) so it is not marking this final flush as data
> integrity sync and so we happily leave the I_DIRTY_TIME bit set.
>
> > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> > index 443f83382b9b..31c93cbdb3fe 100644
> > --- a/fs/fs-writeback.c
> > +++ b/fs/fs-writeback.c
> > @@ -1718,7 +1718,7 @@ static int writeback_single_inode(struct inode *inode,
> >        */
> >       if (!(inode->i_state & I_DIRTY_ALL))
> >               inode_cgwb_move_to_attached(inode, wb);
> > -     else if (!(inode->i_state & I_SYNC_QUEUED)) {
> > +     else if (!(inode->i_state & (I_SYNC_QUEUED | I_FREEING))) {
> >               if ((inode->i_state & I_DIRTY))
> >                       redirty_tail_locked(inode, wb);
> >               else if (inode->i_state & I_DIRTY_TIME) {
>
> So even calling inode_cgwb_move_to_attached() is not safe when I_FREEING is
> already set. So I belive the I_FREEING bit check needs to be before this
> whole if block.

Agree, let me move the I_FREEING check before this if block.
The commit I am fixing didn't change this codepath, so I suspect there is an
implicit invariant which keeps inode_cgwb_move_to_attached call safe.
But I am 100% in favor of making I_FREEING check explicitly.

>
> I also think we should add some assertions into i_io_list handling
> functions to complain if I_FREEING bit is set to catch these problems
> earlier which means to be also more careful in __mark_inode_dirty(). But
> this is for a separate cleanup.
>
>                                                                 Honza

Sounds reasonable. Will look into that afterwards.

> --
> Jan Kara <jack@...e.com>
> SUSE Labs, CR

--
Slava