Message-Id: <5165502902000078000CC022@nat28.tlf.novell.com>
Date: Wed, 10 Apr 2013 10:42:33 +0100
From: "Jan Beulich" <JBeulich@suse.com>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: "Borislav Petkov" <borislav.petkov@amd.com>,
        "Kees Cook" <keescook@chromium.org>, "Will Drewry" <wad@chromium.org>,
        "Frederic Weisbecker" <fweisbec@gmail.com>,
        "Steven Rostedt" <rostedt@goodmis.org>,
        "Eric Northup" <digitaleric@google.com>,
        "Julien Tinnes" <jln@google.com>,
        "Jeremy Fitzhardinge" <jeremy@goop.org>,
        "Alex Shi" <alex.shi@intel.com>,
        "Alexander Duyck" <alexander.h.duyck@intel.com>,
        "x86@kernel.org" <x86@kernel.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        "virtualization@lists.linux-foundation.org" 
	<virtualization@lists.linux-foundation.org>,
        "kernel-hardening@lists.openwall.com" 
	<kernel-hardening@lists.openwall.com>,
        "xen-devel@lists.xensource.com" <xen-devel@lists.xensource.com>,
        "Konrad Rzeszutek Wilk" <konrad.wilk@oracle.com>,
        "Ingo Molnar" <mingo@redhat.com>,
        "Marcelo Tosatti" <mtosatti@redhat.com>,
        "LKML" <linux-kernel@vger.kernel.org>,
        "Dan Rosenberg" <drosenberg@vsecurity.com>
Subject: Re: [Xen-devel] Readonly GDT
References: <20130408224328.GA17641@www.outflux.net>
 <51634935.9010905@zytor.com> <alpine.LFD.2.02.1304091122490.21884@ionos>
 <CAGXu5jKo=11RrsvctcZjP6dusAj659O_GsaA2J2qFq-6mOeRwQ@mail.gmail.com>
 <51645D6F.7070705@zytor.com>
 <CAGXu5jKkHKw8efdPOmt-Zi99m+uMKVGKpc_DC72_ztwxnRbRRQ@mail.gmail.com>
 <51646054.3090509@zytor.com>
 <CAGXu5jJNG4o+_-kk=A2AfSRwyQE0pgaQh7YpSjnD_fB-s7U+wA@mail.gmail.com>
 <CAG7+5M3==rndLLak9cZBKKdgQMmJMpSP_jpQd8HOVGp6LhUAgw@mail.gmail.com>
 <5164B5BD.5050702@zytor.com>
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>>> On 10.04.13 at 02:43, "H. Peter Anvin" <hpa@zytor.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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