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

From: Boris Ostrovsky
Date: Wed Apr 09 2014 - 11:35:26 EST


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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