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SubjectRe: [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 <> 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...
(cesky, pictures)

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