linux-kernel - Re: 2.6.33-rc2: Xen/Guest switching to user mode with no user page tables

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Date:	Sun, 10 Jan 2010 16:49:51 +0300
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Ian Campbell <ijc@...lion.org.uk>, Brian Gerst <brgerst@...il.com>,
	Christian Kujau <lists@...dbynature.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: 2.6.33-rc2: Xen/Guest switching to user mode with no user page
	tables

On Sun, Jan 10, 2010 at 04:36:28PM +0300, Cyrill Gorcunov wrote:
...
> > 
> > With just the CS RPL fix below I see a GPF at kernel_thread_helper with
> > SS=3 (hence my hypothesis about NULL selectors and non-zero RPL above).
> > With both the SS and CS fixes things work fine.
> 
> any of CS,SS loaded with NULL descriptor should lead to #GP
> 

though SS with RPL=0 is allowed to be NULL descriptor in 64bit mode

> > 
> > Ian.
> > 
> > --- 
> > Subject: xen: 64 bit kernel RPL should be 0.
> > 
> ...
> 
> Good catch Ian! I've noted that Xen use it's own get_kernel_rpl
> while discussing this problem in a chat. But I must admit *I simply don't know*
> what Xen does, or how it works internally (neither I have will to learn it at
> moment :)
> 
> That said -- I'm happy if yor patch fixes problem (and it looks that
> get_kernel_rpl is guilty here indeed).
> 
> 	-- Cyrill

	-- Cyrill
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