linux-kernel - Re: [RFC] x86: xsave/xrstor support, ucontext

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Message-ID: <4832E85B.4080303@firstfloor.org>
Date:	Tue, 20 May 2008 17:03:55 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Suresh Siddha <suresh.b.siddha@...el.com>,
	Mikael Pettersson <mikpe@...uu.se>, mingo@...e.hu,
	tglx@...utronix.de, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, roland@...hat.com, drepper@...hat.com,
	Hongjiu.lu@...el.com, linux-kernel@...r.kernel.org,
	arjan@...ux.intel.com, rmk+lkml@....linux.org.uk, dan@...ian.org,
	asit.k.mallick@...el.com
Subject: Re: [RFC] x86: xsave/xrstor support, ucontext_t extensions

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.

-Andi

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