Re: general protection fault in fanotify_handle_event

[Amir Goldstein]

On Tue, Apr 23, 2019 at 7:27 PM Jan Kara <jack@xxxxxxx> wrote:
>
> On Fri 19-04-19 12:33:02, Amir Goldstein wrote:
> > On Thu, Apr 18, 2019 at 8:14 PM syzbot
> > <syzbot+15927486a4f1bfcbaf91@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > syzbot has bisected this bug to:
> > >
> > > commit 77115225acc67d9ac4b15f04dd138006b9cd1ef2
> > > Author: Amir Goldstein <amir73il@xxxxxxxxx>
> > > 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > 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.

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.

Thanks,
Amir.