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

["H. Peter Anvin" <hpa@zytor.com>]

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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