Re: [PATCH, RFC] x86-64: properly handle FPU code/data selectors

[Linus Torvalds]

On Wed, Oct 16, 2013 at 5:00 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>
> The basic idea here is to either use a priori information on the
> intended state layout (in the case of 32-bit processes) or "sense" the
> proper layout (in the case of KVM guests) by inspecting the already
> saved FPU rip/rdp, and reading their actual values in a second save
> operation.

The rip/rdp thing looks very hacky. And *without* the rip/rdp thing, I
think the word-size always matches the TIF32 bit, right?

Why wouldn't the high bits be zero even in 64-bit mode? It would seem
to be a *major* bug if you are in 64-bit mode but (for example) try to
use the low 32-bit of virtual memory (ie something x32-like), and now
your patch decides to use the 32-bit layout.

As far as I can tell, you actually corrupt rid/rdp in that case
(because when you write the fcs thing, it overwrites the high bits of
rip, and fos overwrites the high bits of rdp). So now bits that
*should* be zero are not.

So you're basically trying to save some old state by corrupting new
state instead.

Am I overlooking something?

                Linus
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