Re: [PATCH v2] fsnotify: Avoid data race between fsnotify_recalc_mask() and fsnotify_object_watched()

[Jan Kara <jack@suse.cz>]

On Mon 15-07-24 10:22:03, Josef Bacik wrote:
> On Mon, Jul 15, 2024 at 03:04:10PM +0200, Jan Kara wrote:
> > When __fsnotify_recalc_mask() recomputes the mask on the watched object,
> > the compiler can "optimize" the code to perform partial updates to the
> > mask (including zeroing it at the beginning). Thus places checking
> > the object mask without conn->lock such as fsnotify_object_watched()
> > could see invalid states of the mask. Make sure the mask update is
> > performed by one memory store using WRITE_ONCE().
> > 
> > Reported-by: syzbot+701037856c25b143f1ad@syzkaller.appspotmail.com
> > Reported-by: Dmitry Vyukov <dvyukov@google.com>
> > Link: https://lore.kernel.org/all/CACT4Y+Zk0ohwwwHSD63U2-PQ=UuamXczr1mKBD6xtj2dyYKBvA@mail.gmail.com
> > Signed-off-by: Jan Kara <jack@suse.cz>
> 
> I'm still hazy on the rules here and what KCSAN expects, but if we're using
> READ_ONCE/WRITE_ONCE on a thing, do we have to use them everywhere we access
> that member?  Because there's a few accesses in include/linux/fsnotify_backend.h
> that were missed if so.  Thanks,

Indeed there are two accesses there that should be using READ_ONCE() as
well. I've missed those. Thanks for review!

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