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Date: Tue, 22 Apr 2014 10:19:46 -0700
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Subject: Re: [PATCH] x86-64: espfix for 64-bit mode *PROTOTYPE*
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Andrew Lutomirski <amluto@gmail.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>, Borislav Petkov <bp@alien8.de>,
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On Tue, Apr 22, 2014 at 10:11 AM, Andrew Lutomirski <amluto@gmail.com> wrote:
>
>>
>> Anyway, if done correctly, this whole espfix should be totally free
>> for normal processes, since it should only trigger if SS is a LDT
>> entry (bit #2 set in the segment descriptor). So the normal fast-path
>> should just have a simple test for that.
>
> How?  Doesn't something still need to check whether SS is funny before
> doing iret?

Just test bit #2. Don't do anything else if it's clear, because you
should be done. You don't need to do anything special if it's clear,
because I don't *think* we have any 16-bit data segments in the GDT on
x86-64.

              Linus