Message-ID: <5356AF9F.4010301@zytor.com>
Date: Tue, 22 Apr 2014 11:06:23 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Brian Gerst <brgerst@gmail.com>, Andrew Lutomirski <amluto@gmail.com>
CC: Linus Torvalds <torvalds@linux-foundation.org>,
        Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@linux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Alexander van Heukelum <heukelum@fastmail.fm>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Arjan van de Ven <arjan.van.de.ven@intel.com>,
        Alexandre Julliard <julliard@winehq.com>,
        Andi Kleen <andi@firstfloor.org>, Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] x86-64: espfix for 64-bit mode *PROTOTYPE*
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On 04/22/2014 11:03 AM, Brian Gerst wrote:
> 
> Maybe make the #GP handler check what the previous stack was at the start:
> 1) If we came from userspace, switch to the top of the process stack.
> 2) If the previous stack was not the espfix stack, switch back to that stack.
> 3) Switch to the top of the process stack (espfix case)
> 
> This leaves the IST available for any recursive faults.
> 

Do you actually know what the IST is?  If so, you should realize the
above is nonsense.

The *hardware* switches stack on an exception; if the vector is set up
as an IST, then we *always* switch to the IST stack, unconditionally.
If the vector is not, then we switch to the process stack if we came
from userspace.

That is the entry condition that we have to deal with.  The fact that
the switch to the IST is unconditional is what makes ISTs hard to deal with.

	-hpa

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