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Date:   Wed, 27 Mar 2019 18:59:48 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Jan Kara <jack@suse.cz>
Cc:     Mark Fasheh <mark@fasheh.com>, Dave Chinner <david@fromorbit.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        syzbot <syzbot+7a8ba368b47fdefca61e@syzkaller.appspotmail.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Jaegeuk Kim <jaegeuk@kernel.org>,
        Joel Becker <jlbec@evilplan.org>
Subject: Re: KASAN: use-after-free Read in path_lookupat
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On Wed, Mar 27, 2019 at 05:58:31PM +0100, Jan Kara wrote:
> On Tue 26-03-19 04:15:10, Al Viro wrote:
> > On Mon, Mar 25, 2019 at 08:18:25PM -0700, Mark Fasheh wrote:
> > 
> > > Hey Al,
> > > 
> > > It's been a while since I've looked at that bit of code but it looks like
> > > Ocfs2 is syncing the inode to disk and disposing of it's memory
> > > representation (which would include the cluster locks held) so that other
> > > nodes get a chance to delete the potentially orphaned inode. In Ocfs2 we
> > > won't delete an inode if it exists in another nodes cache.
> > 
> > Wait a sec - what's the reason for forcing that write_inode_now(); why
> > doesn't the normal mechanism work?  I'm afraid I still don't get it -
> > we do wait for writeback in evict_inode(), or the local filesystems
> > wouldn't work.
> 
> I'm just guessing here but they don't want an inode cached once its last
> dentry goes away (it makes cluster wide synchronization easier for them and
> they do play tricks with cluster lock on dentries).

Sure, but that's as simple as "return 1 from ->drop_inode()".

> There is some info in
> 513e2dae9422 "ocfs2: flush inode data to disk and free inode when i_count
> becomes zero" which adds this ocfs2_drop_inode() implementation. So when
> the last inode reference is dropped, they want to flush any dirty data to
> disk and evict the inode. But AFAICT they should be fine with flushing the
> inode from their ->evict_inode method. I_FREEING just stops the flusher
> thread from touching the inode but explicit writeback through
> write_inode_now(inode, 1) should go through just fine.

Umm...  Why is that write_inode_now() needed in either place?  I agree that
moving it to ->evict_inode() ought to be safe, but what makes it necessary
in the first place?  Put it another way, what dirties the data and/or
metadata without marking it dirty?