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

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Date:	Tue, 22 Nov 2011 18:23:06 +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 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?
>
> -Kees
>

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 :)

Marco
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