linux-kernel - Re: [PATCH] x86-64: don't leak kernel register values to 32-bit processes

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Date:	Thu, 08 Oct 2009 12:41:27 +0100
From:	"Jan Beulich" <JBeulich@...ell.com>
To:	"Pavel Machek" <pavel@....cz>
Cc:	<mingo@...e.hu>, <tglx@...utronix.de>,
	<linux-kernel@...r.kernel.org>, <hpa@...or.com>
Subject: Re: [PATCH] x86-64: don't leak kernel register values to
	 32-bit processes

>>> Pavel Machek <pavel@....cz> 07.10.09 11:44 >>>
>How can userspace "temporarily switch itself" to 64bit mode?

By just determining (or guessing) the 64-bit user mode CS value, and far-
jumping/calling to an address with this CS as the selector.

>Such ability would lead to very interesting behaviour on 32-bit
>kernel, I'd say...

That won't work - you have to have a 64-bit kernel: EFER.LME and the L
bit of some user mode code segment descriptor must be set (or settable).

Consequently a 64-bit kernel could, if it wanted to, make it impossible for
user mode code to do such switching (and an example of this, where
security requires it, is 64-bit Xen disallowing 32-bit para-virtual guests
[kernel or user mode] to switch themselves into 64-bit mode).

Jan

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