Re: [Xen-devel] [PATCH] x86/xen: Fix 32-bit PV guests's usage of kernel_stack

[Boris Ostrovsky]

On 04/09/2014 11:01 AM, Jan Beulich wrote:
> > > > On 09.04.14 at 16:41, <andrew.cooper3@xxxxxxxxxx> wrote:
> > The latter load however can easy fault; The arguments for %ds in
> > XSA-42/ CVE-2013-0228 applies to %{e,f,g}s as well.
> And it was only that latter operation that I pointed at.

We don't seem to reference %fs after the pop so doing the fixup (as 
David suggested) should be enough?

-boris


> > Furthermore, I am a little concerned about the performance impact of
> > this.  I would have thought that in most cases, %fs will already be
> > correct, at which point reloading it twice is a waste of time.
> Why would you expect %fs on the IRET path to commonly point to the
> kernel segment rather than whatever user mode wants/needs? Also, I'm
> not sure adding conditionals here wouldn't harm performance about as
> much as the save/load/restore. If anything I'd look into open coding
> GET_THREAD_INFO() without using %fs for this single case.
>
> Jan
>
>
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