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

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Message-ID: <CAGXu5j+Dh2jbAymcreOwX_T6=7FA8bOuwUFnwshdsVonAD5_EA@mail.gmail.com>
Date:	Mon, 21 Nov 2011 10:11:00 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Marco Stornelli <marco.stornelli@...il.com>
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

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

-- 
Kees Cook
ChromeOS Security
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