linux-kernel - Re: [Xen-devel] Readonly GDT

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Date:	Wed, 10 Apr 2013 10:42:33 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	"Borislav Petkov" <borislav.petkov@....com>,
	"Kees Cook" <keescook@...omium.org>,
	"Will Drewry" <wad@...omium.org>,
	"Frederic Weisbecker" <fweisbec@...il.com>,
	"Steven Rostedt" <rostedt@...dmis.org>,
	"Eric Northup" <digitaleric@...gle.com>,
	"Julien Tinnes" <jln@...gle.com>,
	"Jeremy Fitzhardinge" <jeremy@...p.org>,
	"Alex Shi" <alex.shi@...el.com>,
	"Alexander Duyck" <alexander.h.duyck@...el.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
	"Ingo Molnar" <mingo@...hat.com>,
	"Marcelo Tosatti" <mtosatti@...hat.com>,
	"LKML" <linux-kernel@...r.kernel.org>,
	"Dan Rosenberg" <drosenberg@...curity.com>
Subject: Re: [Xen-devel] Readonly GDT

>>> On 10.04.13 at 02:43, "H. Peter Anvin" <hpa@...or.com> wrote:
> OK, thinking about the GDT here.
> 
> The GDT is quite small -- 256 bytes on i386, 128 bytes on x86-64.  As
> such, we probably don't want to allocate a full page to it for only
> that.  This means that in order to create a readonly mapping we have to
> pack GDTs from different CPUs together in the same pages, *or* we
> tolerate that other things on the same page gets reflected in the same
> mapping.

I think a read-only GDT is incompatible with exceptions delivered
through task gates (i.e. double fault on 32-bit), so I would assume
this needs to remain a 64-bit only thing.

> However, the packing solution has the advantage of reducing address
> space consumption which matters on 32 bits: even on i386 we can easily
> burn a megabyte of address space for 4096 processors, but burning 16
> megabytes starts to hurt.

Packing would have the additional benefit of Xen not needing to
become a special case in yet another area (because pages
containing live descriptor table entries need to be read-only for
PV guests, and need to consist of only descriptor table entries).

Jan

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