linux-kernel - Re: [PATCH v2 2/3] x86/ldt: Make modify

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Message-ID: <CALCETrVrXohCwO3g+cvKSPoXKHafc7SNNrht7rgJokYMHmZaoA@mail.gmail.com>
Date:	Tue, 21 Jul 2015 13:27:43 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Sasha Levin <sasha.levin@...cle.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>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [PATCH v2 2/3] x86/ldt: Make modify_ldt optional

On Tue, Jul 21, 2015 at 1:20 PM, Sasha Levin <sasha.levin@...cle.com> wrote:
> On 07/21/2015 03:59 PM, Andy Lutomirski wrote:
>> The modify_ldt syscall exposes a large attack surface and is
>> unnecessary for modern userspace.  Make it optional.
>
> Since this a "default y" option I think we need to make the
> implications of this a bit clearer.

Do you mean improving the help text?  To be clear, there's no change
on a non-EXPERT of default EXPERT kernel here.

>
> Do we know what userspace would break?

Some Wine and some DOSEMU most likely.  Also many of the exploits I've
written over the past year or two :)

>
> Maybe add a WARN_ONCE() in a stub syscall?
>

I think if we do that then we should do it for all the syscall disabling things.

--Andy
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