linux-kernel - Re: [PATCH 2/2] ramoops: remove module parameters

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Date:	Wed, 23 Nov 2011 17:40:25 +0100
From:	Marco Stornelli <marco.stornelli@...il.com>
To:	Kees Cook <keescook@...omium.org>
CC:	linux-kernel@...r.kernel.org,
	Chen Gong <gong.chen@...ux.intel.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>
Subject: Re: [PATCH 2/2] ramoops: remove module parameters

Il 22/11/2011 19:14, Kees Cook ha scritto:
> On Tue, Nov 22, 2011 at 9:23 AM, Marco Stornelli
> <marco.stornelli@...il.com>  wrote:
>> Il 21/11/2011 19:11, Kees Cook ha scritto:
>>>
>>> On Sat, Nov 19, 2011 at 1:25 AM, Marco Stornelli
>>> <marco.stornelli@...il.com>    wrote:
>>>>
>>>> Il 18/11/2011 20:31, Kees Cook ha scritto:
>>>>>
>>>>> The ramoops driver is intended to be used with platforms that define
>>>>> persistent memory regions. If memory regions were configurable with
>>>>> module parameters, it would be possible to read some RAM regions via
>>>>> the pstore interface without access to /dev/mem (which would result
>>>>> in a loss of kernel memory privacy when a system is built with
>>>>> STRICT_DEVMEM), so remove this ability completely.
>>>>>
>>>>
>>>> I don't like it very much. The loss of module parameters give us less
>>>> flexibility. The main goal of this driver is debug, so I think it should
>>>> be
>>>> fast to use. I mean it's not more possible reserve a memory region and
>>>> load
>>>> the module "on-the-fly", it needs a platform device, it's ok but I think
>>>> it's a little bit more complicated, (without talking about platforms
>>>> without
>>>> a device tree source).
>>>> I don't understand the problem of strict devmem. We shouldn't use kernel
>>>> memory region but only reserved ones and the driver doesn't use the
>>>> request_mem_region_exclusive, am I wrong?
>>>
>>> Hmmm, maybe I'm reading it backwards, but I think we want it to use
>>> ..._exclusive().
>>>
>>> int devmem_is_allowed(unsigned long pagenr)
>>> {
>>>          if (pagenr<= 256)
>>>                  return 1;
>>>          if (iomem_is_exclusive(pagenr<<    PAGE_SHIFT))
>>>                  return 0;
>>>          if (!page_is_ram(pagenr))
>>>                  return 1;
>>>          return 0;
>>> }
>>>
>>> If the region is exclusive, access is not allowed (return 0). ramoops
>>> currently uses request_mem_region() instead of
>>> request_mem_region_exclusive(). If we made that switch, I think I'd be
>>> happy. Would this create some problem I'm not seeing?
>>
>> I don't understand why we should use the exclusive version, to protect debug
>> data? You should provide a more valid reason to change, because the fact you
>> will be happier with this change is not enough for me :)
>
> I guess ..._exclusive() doesn't matter. My concern was that ramoops
> with the pstore interface and the module parameters could be used to
> bypass STRICT_DEVMEM if it were able to be loaded in some sensitive
> region of system memory. Perhaps the better approach would be to use a
> magic header so that uninitialized memory isn't visible? What do you
> think?
>
> -Kees
>

Sincerely, IMHO, if we consider the *debug* nature of this driver, it's 
sufficient a simple script (distributed with the kernel) to extract the 
all the information you need without touch the current implementation.

Marco
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