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Date: Sat, 10 Oct 2009 15:27:07 +0200
From: Pavel Machek <pavel@....cz>
To: Jan Beulich <JBeulich@...ell.com>
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
On Thu 2009-10-08 12:41:27, Jan Beulich wrote:
> >>> 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).
I guess we should do just that -- 32-bit application temporary going
64-bit is asking for trouble. How will attached gdb behave? What if I
try the same code on 32-bit machine?
It seems only useful for malware...
Pavel
--
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(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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