Re: general protection fault in fanotify_handle_event

[Jan Kara <jack@suse.cz>]

On Tue 23-04-19 21:05:22, Amir Goldstein wrote:
> On Tue, Apr 23, 2019 at 7:27 PM Jan Kara <jack@suse.cz> wrote:
> >
> > On Fri 19-04-19 12:33:02, Amir Goldstein wrote:
> > > On Thu, Apr 18, 2019 at 8:14 PM syzbot
> > > <syzbot+15927486a4f1bfcbaf91@syzkaller.appspotmail.com> wrote:
> > > >
> > > > syzbot has bisected this bug to:
> > > >
> > > > commit 77115225acc67d9ac4b15f04dd138006b9cd1ef2
> > > > Author: Amir Goldstein <amir73il@gmail.com>
> > > > Date:   Thu Jan 10 17:04:37 2019 +0000
> > > >
> > > >      fanotify: cache fsid in fsnotify_mark_connector
> > > >
> > > > bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=1627632d200000
> > > > start commit:   3f018f4a Add linux-next specific files for 20190418
> > > > git tree:       linux-next
> > > > final crash:    https://syzkaller.appspot.com/x/report.txt?x=1527632d200000
> > > > console output: https://syzkaller.appspot.com/x/log.txt?x=1127632d200000
> > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=faa7bdc352fc157e
> > > > dashboard link: https://syzkaller.appspot.com/bug?extid=15927486a4f1bfcbaf91
> > > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=155543d3200000
> > > >
> > > > Reported-by: syzbot+15927486a4f1bfcbaf91@syzkaller.appspotmail.com
> > > > Fixes: 77115225acc6 ("fanotify: cache fsid in fsnotify_mark_connector")
> > > >
> >
> 
> [...]
> 
> >
> > I'd prefer if we could make only fully initialized marks visible on
> > connector's list. Just to make things simple in the fast path of handling
> > events. So I'd just set mark->connector before adding mark to connector's
> > list and having smp_wmb() there to force ordering in
> > fsnotify_add_mark_list(). And we should use READ_ONCE() as a counter-part
> > to this in fanotify_get_fsid(). It is somewhat unfortunate that there are
> > three different ways how fsnotify_add_mark_list() can add mark to a list so
> > we may need to either repeat this in three different places or just use
> > some macro magic like:
> >
> > #define fanotify_attach_obj_list(where, mark, conn, head) \
> >         do { \
> >                 mark->connector = conn; \
> >                 smp_wmb(); \
> >                 hlist_add_##where##_rcu(&mark->obj_list, head); \
> >         } while (0)
> >
> > And I would not really worry about fsnotify_put_mark() - if it ever sees
> > mark->connector set, mark really is on the connector's list and
> > fsnotify_put_mark() does the right thing (i.e. locks connector->lock if
> > needed).
> >
> 
> Sounds good to me.
> 
> I was worried about fsnotify_put_mark() NOT seeing mark->connector set
> even though it is on the list (add mark; get lockless ref; remove
> mark; put lockless ref).
> Its subtle, not sure if possible.

Not possible for the cases it matters. fsnotify_put_mark() just decrements
the refcount. If someone is adding mark to the list, he is by definition
holding one reference so racing fsnotify_put_mark() cannot be dropping the
last one and so the result of the check doesn't really matter.

> I am assuming you will be sending the above patch to syzbot for testing.
> If you want me do help with anything please let me know.

OK, I'll write it and send it.

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