linux-kernel - Re: [PATCH RFC] x86: check for and defend against BIOS memory corruption

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Date:	Mon, 8 Sep 2008 20:14:50 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
cc:	Ingo Molnar <mingo@...e.hu>,
	Rafa? Mi?ecki <zajec5@...il.com>,
	Alan Jenkins <alan-jenkins@...fmail.co.uk>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC] x86: check for and defend against BIOS memory
 corruption

On Mon, 8 Sep 2008, Jeremy Fitzhardinge wrote:
> 
> Well, if we never want the direct map to be non-executable (which I

One too many negatives?

> think would be OK, since all the code is either core kernel or modules
> which are mapped elsewhere),

I had thought so until just before replying.

> then we can set NX on the level4 for the
> linear mapping which will make everything below non-executable.

That would be much the neatest answer: I hadn't realized
that inheritance (perhaps I'm still living in early-i386 days,
when IIRC there was a bug in inheriting WP from higher levels).

But then I stumbled across static_protections() in pageattr.c
(takes both addr and pfn, latter seems weird), whose BIOS_BEGIN
and BIOS_END seem to echo the ISA_START_ADDR and ISA_END_ADDR
used by is_ISA_range() in ioremap.c.

And peering at the pagetables I've got here for that area of the
direct map in 2.6.26 x86_64, yes, I'm missing NX from 0xc0000 to
0xfffff (presumably nothing tried to ioremap 0xa0000 to 0xbffff).

A simple answer might be to go the way you suggest, but remove
the special casing from pageattr.c and ioremap.c; but I fear
that will slow something down, or introduce further bugs.

Hugh
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