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

[Jan Beulich]

>>> Pavel Machek <pavel@xxxxxx> 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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