From: "Jan Beulich" <JBeulich@novell.com>
To: "Pavel Machek" <pavel@ucw.cz>
Cc: <mingo@elte.hu>, <tglx@linutronix.de>,
<linux-kernel@vger.kernel.org>, <hpa@zytor.com>
Subject: Re: [PATCH] x86-64: don't leak kernel register values to 32-bit processes
Date: Thu, 08 Oct 2009 12:41:27 +0100 [thread overview]
Message-ID: <4ACDEC070200007800018B57@vpn.id2.novell.com> (raw)
In-Reply-To: <20091007094248.GA1425@ucw.cz>
>>> Pavel Machek <pavel@ucw.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
next prev parent reply other threads:[~2009-10-08 11:42 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-09-30 10:22 [PATCH] x86-64: don't leak kernel register values to 32-bit processes Jan Beulich
2009-10-01 9:59 ` [tip:x86/urgent] x86: Don't leak 64-bit " tip-bot for Jan Beulich
2009-10-07 9:44 ` [PATCH] x86-64: don't leak " Pavel Machek
2009-10-08 11:41 ` Jan Beulich [this message]
2009-10-10 13:27 ` Pavel Machek
2009-10-08 18:58 ` H. Peter Anvin
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