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

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Message-ID: <506BC96D.10507@hp.com>
Date:	Tue, 02 Oct 2012 23:13:17 -0600
From:	Thavatchai Makphaibulchoke <thavatchai.makpahibulchoke@...com>
To:	Matthew Garrett <mjg@...hat.com>
CC:	"H. Peter Anvin" <hpa@...or.com>, 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

Thank you both for the comments.

Sounds like a better solution is to allow accesses to only I/O regions 
presented in the EFI memory map for physical addresses below 1 MB.

Do we need to worry about the X checksum in the first MB on an EFI system?

Thanks,
Mak.


On 10/02/2012 11:15 PM, Matthew Garrett wrote:
> On Tue, Oct 02, 2012 at 09:44:16PM -0700, H. Peter Anvin wrote:
> 
>> 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.)
> 
> Oh, right, got you. In that case I think we potentially need a 
> finer-grained check on EFI platforms - the EFI memory map is kind enough 
> to tell us the difference between unusable regions and io regions, and 
> we could avoid access to the unusable ones.
> 

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