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Message-ID: <4832E85B.4080303@firstfloor.org>
Date: Tue, 20 May 2008 17:03:55 +0200
From: Andi Kleen <andi@firstfloor.org>
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To: "H. Peter Anvin" <hpa@zytor.com>
CC: Suresh Siddha <suresh.b.siddha@intel.com>,
       Mikael Pettersson <mikpe@it.uu.se>, mingo@elte.hu, tglx@linutronix.de,
       torvalds@linux-foundation.org, akpm@linux-foundation.org,
       roland@redhat.com, drepper@redhat.com, Hongjiu.lu@intel.com,
       linux-kernel@vger.kernel.org, arjan@linux.intel.com,
       rmk+lkml@arm.linux.org.uk, dan@debian.org, asit.k.mallick@intel.com
Subject: Re: [RFC] x86: xsave/xrstor support, ucontext_t extensions
References: <20080513011030.GA31448@linux-os.sc.intel.com> <18477.35703.679574.760417@harpo.it.uu.se> <20080518013416.GB30034@linux-os.sc.intel.com> <18481.37905.297556.288317@harpo.it.uu.se> <20080520015723.GD30034@linux-os.sc.intel.com> <4832A173.6020203@firstfloor.org> <4832E672.3090702@zytor.com>
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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