linux-kernel - Re: fanotify use after free.

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Date:	Tue, 28 Jan 2014 15:53:27 +0100
From:	Jan Kara <jack@...e.cz>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	Jan Kara <jack@...e.cz>, Dave Jones <davej@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: fanotify use after free.

On Tue 28-01-14 12:07:51, Jiri Kosina wrote:
> On Tue, 28 Jan 2014, Jan Kara wrote:
> 
> > >   2b:*	4d 8b 64 c6 08       	mov    0x8(%r14,%rax,8),%r12     <-- trapping instruction
> > > 
> > > R14 is 0x6b6b6b6b6b6b6c03, which looks like a use-after-free.
> >   Yup. But I'm somewhat puzzled by the trace. We crash when calling
> > fsnotify_destroy_event() from fanotify_handle_event(). The fsnotify code
> > has been called from do_sys_open() so the event was a 'FS_OPEN' which fails
> > the fsn_event->mask & FAN_ALL_PERM_EVENTS test.
> > 
> > Slapping my forehead, that's a really stupid bug. The event
> > fsnotify_add_notify_event() returns may be freed by the time we return
> > because we already dropped the notification mutex. And then fsn_event->mask
> > & FAN_ALL_PERM_EVENTS test will pass because FAN_ALL_PERM_EVENTS matches
> > with the poison pattern 0x6b6b6b6b. So yet another hacked up version of
> > fanotify fix is attached. And I have to seriously think about use counts
> > for fanotify version of that struct.
> 
> With the fixed version of the patch, all the fanotify-related oopses are 
> gone on my system.
  Thanks for testing. So now I have to come up with something mergeable :)

								Honza

-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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