linux-kernel - Re: fanotify use after free.

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Date:	Tue, 28 Jan 2014 10:24:08 -0500
From:	Dave Jones <davej@...hat.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Jiri Kosina <jkosina@...e.cz>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: fanotify use after free.

On Tue, Jan 28, 2014 at 03:53:27PM +0100, Jan Kara wrote:
 > 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 :)

Yep, looks good to me too.  Thanks.

	Dave
 
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