linux-kernel - Re: [Xen-devel] [PATCH] x86/xen: Fix 32-bit PV guests's usage of kernel

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Message-Id: <53457CDD0200007800007483@nat28.tlf.novell.com>
Date:	Wed, 09 Apr 2014 16:01:17 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Andrew Cooper" <andrew.cooper3@...rix.com>,
	"David Vrabel" <david.vrabel@...rix.com>
Cc:	<xen-devel@...ts.xenproject.org>, <boris.ostrovsky@...cle.com>,
	<srostedt@...hat.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH] x86/xen: Fix 32-bit PV guests's usage
 of kernel_stack

>>> On 09.04.14 at 16:41, <andrew.cooper3@...rix.com> wrote:
> The latter load however can easy fault; The arguments for %ds in 
> XSA-42/ CVE-2013-0228 applies to %{e,f,g}s as well.

And it was only that latter operation that I pointed at.

> Furthermore, I am a little concerned about the performance impact of
> this.  I would have thought that in most cases, %fs will already be
> correct, at which point reloading it twice is a waste of time.

Why would you expect %fs on the IRET path to commonly point to the
kernel segment rather than whatever user mode wants/needs? Also, I'm
not sure adding conditionals here wouldn't harm performance about as
much as the save/load/restore. If anything I'd look into open coding
GET_THREAD_INFO() without using %fs for this single case.

Jan

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