Re: [RFC] x86: xsave/xrstor support, ucontext_t extensions

From: Andi Kleen
Date: Tue May 20 2008 - 11:04:20 EST

H. Peter Anvin wrote:
> Andi Kleen wrote:
>>> Ok. CPU folks are planning to make some of the bytes at the end of
>>> fxsave
>>> image, SW usable.
>> Are they always zeroed in earlier CPUs though? If not that wouldn't
>> work 100% reliably because whatever cookie you put in could have been
>> there before by chance.
>> I don't see anything in the SDM guaranteeing zeroing.
> I'm pretty sure they weren't zeroed by the CPUs. If they weren't zeroed
> *by the kernel*, there might have been an information leak.

I don't think there is one. We never copy fxsave completely out of the
kernel. x86-64 does FXSAVE directly in/out user space, but the
only leak is what there was before.


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