Message-ID: <506BC96D.10507@hp.com>
Date: Tue, 02 Oct 2012 23:13:17 -0600
From: Thavatchai Makphaibulchoke <thavatchai.makpahibulchoke@hp.com>
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To: Matthew Garrett <mjg@redhat.com>
CC: "H. Peter Anvin" <hpa@zytor.com>, T Makphaibulchoke <tmac@hp.com>,
        tglx@linutronix.de, mingo@redhat.com, x86@kernel.org,
        akpm@linux-foundation.org, yinghai@kernel.org, tiwai@suse.de,
        viro@zeniv.linux.org.uk, aarcange@redhat.com, tony.luck@intel.com,
        mgorman@suse.de, weiyang@linux.vnet.ibm.com,
        octavian.purdila@intel.com, paul.gortmaker@windriver.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH] Fix devmem_is_allowed for below 1MB accesses for an efi
 machine
References: <1349213536-3436-1-git-send-email-tmac@hp.com> <506B6191.6080605@zytor.com> <20121003043116.GA26241@srcf.ucam.org> <506BC2A0.8060500@zytor.com> <20121003051522.GA27113@srcf.ucam.org>
In-Reply-To: <20121003051522.GA27113@srcf.ucam.org>
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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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