linux-kernel - Re: [PATCH] Fix devmem_is_allowed for below 1MB accesses for an efi machine

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Message-ID: <506BC2A0.8060500@zytor.com>
Date:	Tue, 02 Oct 2012 21:44:16 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Matthew Garrett <mjg@...hat.com>
CC:	T Makphaibulchoke <tmac@...com>, tglx@...utronix.de,
	mingo@...hat.com, x86@...nel.org, akpm@...ux-foundation.org,
	yinghai@...nel.org, tiwai@...e.de, viro@...iv.linux.org.uk,
	aarcange@...hat.com, tony.luck@...el.com, mgorman@...e.de,
	weiyang@...ux.vnet.ibm.com, octavian.purdila@...el.com,
	paul.gortmaker@...driver.com, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] Fix devmem_is_allowed for below 1MB accesses for an efi
 machine

On 10/02/2012 09:31 PM, Matthew Garrett wrote:
> On Tue, Oct 02, 2012 at 02:50:09PM -0700, H. Peter Anvin wrote:
> 
>> That sounds like exactly the opposite of normal /dev/mem behavior... we
>> allow access to non-memory resources (which really could do anything if
>> misused), but not memory.
> 
> From arch/x86/mm/init.c:
> 
>  * On x86, access has to be given to the first megabyte of ram because that area
>  * contains bios code and data regions used by X and dosemu and similar apps.
> 
> Limiting this to just RAM would be safer than it currently is. I'm not 
> convinced that there's any good reason to allow *any* access down there 
> for EFI systems, though.
> 

Sorry, fail.

We *always* expose the I/O regions to /dev/mem.  That is what /dev/mem
*does*.  The above is an exception (which is really obsolete, too: we
should simply disallow access to anything which is treated as system
RAM, which doesn't include the BIOS regions in question; the only reason
we don't is that some versions of X take a checksum of the RAM in the
first megabyte as some kind of idiotic random seed.)

	-hpa
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