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

[Jan Beulich]

>>> 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.

> 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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