From: Amir Goldstein <amir73il@gmail.com>
To: Jan Kara <jack@suse.cz>
Cc: linux-fsdevel <linux-fsdevel@vger.kernel.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
syzbot <syzbot+15927486a4f1bfcbaf91@syzkaller.appspotmail.com>
Subject: Re: general protection fault in fanotify_handle_event
Date: Fri, 19 Apr 2019 12:33:02 +0300 [thread overview]
Message-ID: <CAOQ4uxgbAQEok9AqnnVwqBrF=q0UBP6yQuvdG-WDVpHNk8Xi7A@mail.gmail.com> (raw)
In-Reply-To: <000000000000d97b380586d1205f@google.com>
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")
>
Jan,
It looks like lockless access to mark->connector is not safe as there is nothing
preventing a reader from seeing a mark on object list without seeing the
mark->connector assignment.
It made me wonder if (!mark->connector) check in fsnotify_put_mark() is safe.
I couldn't find any call site where that would be a problem, but perhaps
we should be more careful?
Anyway, it seems that fsnotify_put_mark() uses the non NULL mark->connector
as the indication that mark is on object list, so just assigning mark->connector
before adding to object list won't do.
Since a reference of mark is our guaranty that mark->connector is not
going away, I guess we could do opportunistic test for non NULL
mark->connector from lockless path, if that fails, we check again
with mark->lock held and if that fails something went wrong.
Another option is to teach fsnotify_first_mark() and fsnotify_next_mark()
to skip over marks with NULL mark->connector.
What do you think? Did I over complicate this?
Thanks,
Amir.
next prev parent reply other threads:[~2019-04-19 19:37 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-18 17:06 general protection fault in fanotify_handle_event syzbot
2019-04-18 17:14 ` syzbot
2019-04-19 9:33 ` Amir Goldstein [this message]
2019-04-23 16:27 ` Jan Kara
2019-04-23 18:05 ` Amir Goldstein
2019-04-24 11:46 ` Jan Kara
2019-04-26 10:55 ` Jan Kara
2019-04-26 11:13 ` syzbot
2019-04-29 2:10 ` Jan Kara
2019-04-29 5:36 ` syzbot
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