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.
