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

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Message-ID: <CAMzpN2ga7M4PFuc4W7bTdvzeFp+8Th3Uh-+OBPMjc=E6ucym8Q@mail.gmail.com>
Date:	Tue, 21 Jul 2015 22:53:25 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
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 10:12 PM, Andy Lutomirski <luto@...capital.net> wrote:
> 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.

By impossible, do you mean a partially updated descriptor when the
interrupt occurs?  Would making sure that the descriptor is atomically
updated (using set_64bit()) fix that?

--
Brian Gerst
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