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 11:58:22 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Pavel Machek <pavel@....cz>
CC:	Jan Beulich <JBeulich@...ell.com>, mingo@...e.hu,
	tglx@...utronix.de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86-64: don't leak kernel register values to 32-bit	processes

On 10/07/2009 02:44 AM, Pavel Machek wrote:
> Hi!
> 
>> While 32-bit processes can't directly access R8...R15, they can gain
>> access to these registers by temporarily switching themselves into
>> 64-bit mode. Therefore, registers not preserved anyway by called C
>> functions (i.e. R8...R11) must be cleared prior to returning to user
>> mode.
> 
> How can userspace "temporarily switch itself" to 64bit mode?
> 
> Such ability would lead to very interesting behaviour on 32-bit
> kernel, I'd say...

You can only do so with a 64-bit kernel, but on a 64-bit kernel you can
do it by executing a far jump to segment USER_CS (0x33).

	-hpa

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