linux-kernel - Re: kfree a pointer "from the middle" causing protection faults

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Message-ID: <20150902160949.GA82121@unpythonic.net>
Date:	Wed, 2 Sep 2015 11:09:50 -0500
From:	Jeff Epler <jepler@...ythonic.net>
To:	Muni Sekhar <munisekharrms@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: kfree a pointer "from the middle" causing protection faults

On Wed, Sep 02, 2015 at 08:32:15PM +0530, Muni Sekhar wrote:
>  [ Please keep me in CC as I'm not subscribed to the list]
> 
> Hello,
> 
> 
> I am getting protection faults in different kernel modules if I try to
> free a pointer "from the middle" for example, look at the following
> code:
[..]

Most memory allocators require the pointer eventually passed to the
freeing function is the same pointer as the one returned from the
allocating function.  This is true for libc malloc/free, for instance.
As far as I know, it is true for the Linux allocators such as kzalloc.
The bug lies in whatever part of linux makes the invalid kfree call.

I have not found any documentation that kernel kzalloc/kfree allow
passing a pointer "from the middle".  For instance,
    These routines are used to dynamically request pointer-aligned chunks of
    memory, like malloc and free do in userspace
    https://www.kernel.org/doc/htmldocs/kernel-hacking/routines-kmalloc.html

If the faulty code that you allude to is in the Linux source then please
say what it is so that developers can fix it.  If it's an out of source
module or kernel patch then contact the supplier of that code.

Jeff
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