linux-kernel - Re: [PATCH] xen/blkback: do not leak mode property

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Date:	Wed, 05 Dec 2012 08:00:57 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Olaf Hering" <olaf@...fle.de>
Cc:	<xen-devel@...ts.xen.org>, <konrad.wilk@...cle.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] xen/blkback: do not leak mode property

>>> On 04.12.12 at 19:21, Olaf Hering <olaf@...fle.de> wrote:
> On Tue, Dec 04, Jan Beulich wrote:
> 
>> This looks necessary but insufficient - there's nothing really
>> preventing backend_changed() from being called more than once
>> for a given device (is simply the handler of xenbus watch). Hence
>> I think either that function needs to be guarded against multiple
>> execution (e.g. by removing the watch from that function itself,
>> if that's permitted by xenbus), or to properly deal with the
>> effects this has (including but probably not limited to the leaking
>> of be->mode).
> 
> If another watch does really trigger after the kfree(be) in
> xen_blkbk_remove(), wouldnt backend_changed access stale memory?
> So if that can really happen in practice, shouldnt the backend_watch be
> a separate allocation instead being contained within backend_info?
> 
> Looking at unregister_xenbus_watch, it clears removes the watch from the
> list, so that process_msg will not see it anymore.

That's not the scenario I was talking about: I'm concerned about
multiple calls to backend_changed() to similarly leak "mode" (and
possibly cause other bad stuff to happen) while the device is still
alive - after all it overwrites "mode" without checking what's in
there.

Jan

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