linux-kernel - Re: [PATCH 2/2] x86/ldt: allow to disable modify

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Date:	Tue, 4 Aug 2015 08:00:54 +0200
From:	Willy Tarreau <w@....eu>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Andy Lutomirski <luto@...nel.org>,
	Kees Cook <keescook@...omium.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"security@...nel.org" <security@...nel.org>,
	X86 ML <x86@...nel.org>, Sasha Levin <sasha.levin@...cle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Jan Beulich <jbeulich@...e.com>,
	xen-devel <xen-devel@...ts.xen.org>
Subject: Re: [PATCH 2/2] x86/ldt: allow to disable modify_ldt at runtime

On Tue, Aug 04, 2015 at 05:54:51AM +0200, Borislav Petkov wrote:
> On Mon, Aug 03, 2015 at 11:45:24AM -0700, Andy Lutomirski wrote:
> > P.P.P.S.  Who thought that IRET faults unmasking NMIs made any sense
> > whatsoever when NMIs run on an IST stack?  Seriously, people?
> 
> What happened with asking Intel for a sane IRET-NG?
> 
> Should be relatively easy - take the current IRET microcode, get rid
> of the nasty crap, allocate a new opcode and done. Validation should
> actually have *less* to do and can reuse all current test cases.

Even easier, just add a few flags (probably 2 or 3 only) that IRET can
check to adjust its behaviour. Basically "don't re-enable NMIs yet",
maybe something to adjust the behaviour on bad CS/SS/SP/IP and a few
such things could possibly help. Maybe all of this could be summarized
as a single flag "I'm in a fault handler".

Willy

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