linux-kernel - Re: [PATCH v2 1/3] x86/ldt: Make modify

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Message-ID: <CALCETrW4o3Gq=yh54nD5d+n=HVS02c+WdcZ17yCUPhZfjtfSfA@mail.gmail.com>
Date:	Tue, 21 Jul 2015 19:12:40 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Brian Gerst <brgerst@...il.com>
Cc:	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"security@...nel.org" <security@...nel.org>,
	X86 ML <x86@...nel.org>, Borislav Petkov <bp@...en8.de>,
	Sasha Levin <sasha.levin@...cle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	stable <stable@...r.kernel.org>
Subject: Re: [PATCH v2 1/3] x86/ldt: Make modify_ldt synchronous

On Tue, Jul 21, 2015 at 7:01 PM, Brian Gerst <brgerst@...il.com> wrote:
> On Tue, Jul 21, 2015 at 3:59 PM, Andy Lutomirski <luto@...nel.org> wrote:
>> modify_ldt has questionable locking and does not synchronize
>> threads.  Improve it: redesign the locking and synchronize all
>> threads' LDTs using an IPI on all modifications.
>
> What does this fix?  I can see sending an IPI if the LDT is
> reallocated, but on every update seems unnecessary.
>

It prevents nastiness in which you're in user mode with an impossible
CS or SS, resulting in potentially interesting artifacts in
interrupts, NMIs, etc.

> --
> Brian Gerst



-- 
Andy Lutomirski
AMA Capital Management, LLC
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