linux-kernel - Re: [PATCH, RFC] x86-64: properly handle FPU code/data selectors

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Date:	Wed, 16 Oct 2013 08:50:54 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jan Beulich <JBeulich@...e.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	KVM list <kvm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Anvin <hpa@...or.com>
Subject: Re: [PATCH, RFC] x86-64: properly handle FPU code/data selectors

On Wed, Oct 16, 2013 at 8:36 AM, Jan Beulich <JBeulich@...e.com> wrote:
>
> In that case we use a 32-bit operand size [F]XRSTOR, and hence
> the upper halves get treated as selectors, and the offsets get
> zero-extended from the low halves, i.e. we preserve even more
> state for such a 64-bit environment now too (albeit I doubt any
> 64-bit code actually cares)

No, it does *not* preserve "more state".

It preserves *less* state, because the upper 32 bits of rip are now
corrupted. Any 64-bit application that actually looks at the FP
rip/rdp fields now get the WRONG VALUES.

The "upper bits zero" mode may be used just for JIT'ed code, for
example. It doesn't mean that you'd never have full 64-bit addresses,
so writing to the top half of the register *corrupts* that
information, because the top half bits are still relevant in general,
even if perhaps _one_ particular floating point exception happened
with the bits clear.

Now anybody looking at the FP state on the stack gets the wrong results.

More bits set is *not* "more state", when those bits are wrong.

              Linus
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