linux-kernel - Re: fanotify use after free.

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Message-ID: <alpine.LNX.2.00.1401281207140.3763@pobox.suse.cz>
Date:	Tue, 28 Jan 2014 12:07:51 +0100 (CET)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Jan Kara <jack@...e.cz>
cc:	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 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.

-- 
Jiri Kosina
SUSE Labs
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