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From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Jan Beulich <jbeulich@suse.com>
Cc: "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	"Wei Liu" <wl@xen.org>, "Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH v2 2/2] x86/desc: Build boot_{, compat_}gdt[] in C
Date: Mon, 12 Aug 2019 11:36:01 +0100
Message-ID: <7c764dc0-9136-b937-feda-c9ce94fbef18@citrix.com> (raw)
In-Reply-To: <3b2ad6fb-d39f-e5b5-b35a-fc4ebfc695f3@suse.com>

On 12/08/2019 08:32, Jan Beulich wrote:
> On 09.08.2019 12:40, Jan Beulich wrote:
>> There is plenty more cleanup which can be done in the future.  As we are
>> 64-bit, there is no need for load_TR() to keep the TSS in sync
>> between the two
>> GDTs, which means it can drop all sgdt/lgdt instructions.
>
> I'm trying to figure what exactly you mean here. Are you suggesting
> we run with a TSS selector loaded whose descriptor's busy bit is
> clear? I agree this shouldn't cause issues in the 64-bit world, but
> it would still not feel right.

At a minimum, all the sgdt/lgdt can disappear because we're (AFAICT)
always on the native per-cpu GDT at this point.  (If not, I'm sure we
can arrange to be.)

As for running without a valid GDT reference, the CPU will function
fine, and it is a defence-in-depth strategy against Meltdown, seeing as
an attacker can no longer do sgdt; str to locate the TSS and find RSP0.

> Question is why they've retained the avail/busy distinction in the
> first place.

Easier than making any changes.

~Andrew

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Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-09 10:34 [Xen-devel] [PATCH v2 0/2] x86/boot: cleanup Jan Beulich
2019-08-09 10:38 ` [Xen-devel] [PATCH v2 1/2] x86: define a few selector values Jan Beulich
2019-08-09 11:50   ` Andrew Cooper
2019-08-09 12:35     ` Jan Beulich
2019-08-23  2:38   ` Tian, Kevin
2019-08-09 10:40 ` [Xen-devel] [PATCH v2 2/2] x86/desc: Build boot_{, compat_}gdt[] in C Jan Beulich
2019-08-09 12:19   ` Andrew Cooper
2019-08-09 12:43     ` Jan Beulich
2019-08-09 13:07       ` Andrew Cooper
2019-08-09 13:18         ` Jan Beulich
2019-08-09 15:25           ` Andrew Cooper
2019-08-12  7:32   ` Jan Beulich
2019-08-12 10:36     ` Andrew Cooper [this message]
2019-08-09 10:41 ` [Xen-devel] [PATCH v2 0/2] x86/boot: cleanup Jan Beulich
2019-08-09 12:39 ` [Xen-devel] [PATCH 3/2] x86/desc: Drop __HYPERVISOR_CS32 Andrew Cooper
2019-08-09 12:50   ` Jan Beulich
2019-08-09 15:36     ` Andrew Cooper
2019-08-09 15:52       ` Jan Beulich

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